Vintage Ohio Scientific Computers
This machine was near-and-dear to me as it was my first real computer, an Ohio Scientific Superboard II. In 1979 I got my machine, a 6502-based single-board computer with a whopping 8K of RAM and BASIC in ROM. The board was completely self-contained, requiring only a 5V, 3A power supply to run (I built my own using a 78H05 regulator).
History of OSI Systems
Ohio Scientific (OSI) started business in 1975 and through the late 1970's and early 1980's produced a series of 6502-based machines called the "Challenger" series.
The earliest catalog I could locate was the spring 1978 issue. That year, OSI offered several models based on a single-board CPU, the model 500 card. This card featured 8K BASIC-in-ROM, 4K of RAM on board, and a serial port interface. In the most basic offering, the C2-0 Model 500 Board ($298), the user was required to add a power supply and a serial terminal for a basic stand-alone system. The C-28K 500 Superkit ($398) consisted of the basic model 500 CPU board, again with 4K RAM and 8K BASIC-in-ROM, along with a 480 8-slot backplane and 440 video board (with 32 by 32 character display) allowing use with a video monitor instead of a terminal. It was an unpackaged, "open" system in which the user added a power supply and an external ASCII parallel keyboard for a complete system. Finally the C2-1 500-1 Terminal Add-On ($498) version was basically a simple model 500 CPU board in a case with power supply and standard EIA (RS-232) connectors intended as a "local" computer to run short programs locally.
FYI: the prices quoted are in US dollars from US catalogs. In 1980, the exchange rate for Canadian dollars was less than favourable at about 1.17 so later figures quoted, in Canadian dollars, will always seem higher.
As well as the systems based on a standalone CPU board, OSI offered more complete cassette-based Challenger-II systems. These early machines were housed in blue and black cases (unlike later machines housed in beige ad brown cases).
Offerings from the Spring 1978 catalog included:
- C2-8S Challenger II ($545)
A CPU board with 8K BASIC-in-ROM, 4K RAM, and a serial port in a large cabinet with power supply and an 8-slot backplane.
- C2-4P Challenger IIP ($598)
A complete system with a CPU with 4K RAM (a model 502 card), video board (the enhanced 540 board with a 32 by 64 character display) and cassette interface. Housed with a power supply and a 4-slot backplane (two slots occupied in the original unit) in a small case with integral keyboard on top of which the monitor usually sat. This is the Challenger 2 - 4P system outlined on this page.
- C2-8P Challenger II ($825)
Same as the above but with an 8-slot backplane (two slots occupied) and housed in a larger case allowing more expansion than the 4P system. Had a separate keyboard attached via ribbon cable. This is the Challenger 2 - 8P system housed in the large blue-and-black box outlined on this page.
The Challenger-II was also offered with various floppy drive options. All floppy-based systems were housed in two larger black-and-blue cases, one for the CPU with an 8-slot backplane and separate keyboard and a second case for the floppy drive(s) which had white fronts. Configurations included: A 16K RAM system with a single 8-inch drive (250K) and a serial port (C2-S1S) for $1990, A similar system except with a video card (C2-S1V) listed for $2490, and a 32K RAM system with a dual floppy drive (C2-S2S) for $3090. Note that many systems utilized serial terminals for user interface in the same manner as contemporary mainframe machines did ... many larger OSI systems were indeed designed to use terminals (the front of the 1978 catalog shows a Challenger II system with two eight inch floppy drives and a Hazeltine 1500 terminal in an office setting).
Finally, the Challenger-III system was also available in 1978 with a triple-CPU board (6502, 6800, and Z80) and a dual floppy drive for $3590. Options for the Challenger-III system included a 74 Megabyte hard drive for $6000 (drive only) - a hard drive was an almost unheard-of option in 1978 and something always associated with a large mainframe computer, not a "personal" machine. Having a Z80 processor, this system could run CP/M!
By 1979, OSI offered three types of systems including the new low-cost Challenger I, as well as Challenger II and III systems.
The new Challenger-I systems for 1979:
- Superboard II ($279)
Also called a model 600 board, and new for 1979, this was a complete entry-level system entirely on a single board with keyboard, 8K BASIC-in-ROM, video (24 by 24 characters displayed), and 4K RAM (expandable to 8K). The user needed to supply a 5V/3A power supply and a video monitor for a complete system which includes cassette interface on board.
- C1P 4K ($349)
A packaged version of the superboard (with 4K RAM) in a metal case with power supply.
- C1P MF ($1259)
The same as above but coupled with a 610 expansion board (with 12K RAM on the 610 board plus the 8K on the superboard for a system total of 20K), floppy controller (part of the 610 board), and a mini-floppy (5.25") drive in a second box connected via ribbon cable. Shipped with OS-65D 3.0 Operating system.
Updated Challenger-II systems for 1979:
- C2-4P ($599)
A cassette-based system housed in a small case with integral keyboard (usually with the monitor on top) and 4-slot backplane. Had 4K of memory and BASIC-in-ROM on the 502 CPU card and a video card leaving two slots free.
- C2-4P MF ($1533)
A mini-floppy version of the above again with a 4-slot backplane. Uses a model 505 CPU with integral floppy controller and hence had a separate RAM card with 20K of system RAM (as well as the usual video card). This system did not have BASIC-in-ROM on the CPU card (it was run from disk).
- C2-8P ($799)
Similar to the C2-4P, a cassette-based system housed in a larger case with separate keyboard and 8-slot backplane allowing more expansion. Had 4K of memory on the CPU card and a video card.
- C2-8P DF ($2597)
A dual eight-inch floppy system consisting of two boxes: a CPU box as above (but with 32K of RAM) and a separate box housing two eight-inch floppy drives.
As well as these systems, listed in the spring 1979 catalog, OSI also offered several Challenger-III systems with up to 48K RAM and several odd versions of the "regular" Challenger lines above including an inexpensive C1P system with a single-sided mini-floppy and 12K of RAM (running P-DOS for "Pico-DOS" since 12K is not enough RAM to run OS-65D) and a C2-4P MF system with colour video, sound, and AC remote control I/O from the factory.
In 1980 OSI changed both the look of their machines, adopting a beige-and-brown colour scheme and renamed their product line from the "C2-4P" to the "C4P" and "C2-8P" to "C8P": none of their computers carried the "Challenger-II" designation anymore (which, of course, created a great deal of confusion amongst owners). The C1P and C4P models had beige metal cases with oiled walnut wood sides.
Offerings were similar to those in the 1979 catalog including the Superboard, Challenger C1P and C1P MF models, Challenger C4P and C4P MF models, and Challenger C8P and C8P DF models. Some models featured more RAM from the factory including the C4P and C8P with 8K, and the C4P MF with 24K. The basic C1P MF model was still shipped with 12K RAM - expansion to 20K on the 610 board allowed use of the newest OS-65D version 3.1.
Not mentioned in their catalogs, but apparently available, were special "OEM" version with deep cases holding both eight-inch floppy drives as well as an 8-slot backplane in the rear.
There are two types of OSI systems on this page, the Superboard/Challenger 1P machine covered below and 500-series (Challenger 2) hardware, which used the OSI 48-line bus, covered further down on the page.
Series 600 Hardware (Challenger 1P / Superboard)
The OSI series of machine were relatively inexpensive machines that ran BASIC. At the time (late 70's to early 80's) there were a limited number of choices available to hobbyists looking for a computer: there were several systems which were 'machine code only' (usually featuring a keypad and seven-segment displays), the Apple-II and Commodore PET systems which did everything (including running BASIC) but was particularly expensive at over $1000, and the OSI systems which ran Microsoft BASIC from ROM. BASIC-in-ROM was a great feature since it allowed a novice to learn programming without having to invest heavily in a system that featured floppy disks.
The machine itself was shipped, ungloriously, wrapped in aluminum foil. Manuals were photocopies. It cost about $400 CDN at the time (circa 1979), with the upgrade for 4K more of 2114 SRAM costing an additional $100 or so (in 1979 ... the prices of RAM were quite high then). The advertisement above is circa 1981 (from Electronics Today magazine, April 1981) from the place I bought it from, Arkon Electronics on Queen St. in Toronto. The place later sold Apple and is now no longer there. Click on the small ad above to see the full ad which features pricing of OSI systems as well as other electronics.
Memory, as I said, was expensive at the time. Click Here to see an advertisement for RAM from Exceltronix in Toronto. The reason I had this Ad at all was that I purchased 8K of 2114 a year or so after I got the machine and built my own RAM expansion board (giving me a total of 16K). Two 2114 chips were required for 1K of RAM and so 16 chips were required for 8K of RAM.
These units were shipped in two forms, one the single-board Superboard (covered below), and the packaged Challenger 1P - the identical board in a nice plastic case with a power supply - as pictured in this 1981 Ad from Popular Electronics aimed not at the hobbyist, but the 'serious' computer user.
My original board was a revision A or B but the board I was given was a revision E
(made in 1980) so enhancements were made along the way.
Visible here above the keyboard is the model and revision number. To the upper-right
the 40-pin chip is the 6502 CPU itself and to the left of that the on-board BASIC
which came as four ROMs, each 2K*8 in size.
The circuitry was quite simple by today's standards and most chips were 'normal'
TTL, except for the data bus buffers which were 8T28's (and difficult to obtain at that).
The board contained the keyboard, CPU, ROM, RAM (up to 8K on board), a video
section for connection to a monitor or TV via an RF modulator, and cassette I/O.
Expansion was provided via a 40-pin IC socket. A jumper allowed this board to connect to a 610 expansion board which also allows connection to an OSI bus (rare, though, to expand a C1P system in this way). The 610 board featured RAM (up to 24K more) and a floppy-disk controller. In my case (i.e. since the expansion board probably cost as much as the original machine), I built my own expansion boards in which the OSI expansion connector was connected to several 44-pin card-edge sockets. I had built an I/O port board for the unit as well as a RAM expansion unit (for 16K of total RAM).
Complete schematics for a Revision B (circa 1979) board and diagrams for the Ohio Scientific Model 600 Board:
- Pages 1 and 2 showing the CPU, buffers, and address decoding circuitry
- Pages 3 and 4 showing memory (RAM and ROM)
- Pages 5 and 6 showing memory (ROM) and the ACIA (serial port) used for the cassette
- Pages 7 and 8 showing cassette interface and video RAM circuitry
- Pages 9 and 10 showing clock circuitry and video character generator ROM
- Pages 11 and 12 showing keyboard and related circuitry
- Page 13 showing Keyboard and optional DAC components
- Chip Locations on the PCB
- Connections exteral hookup of the power supply, video, and cassette
- Memory Map detailing the placement of memory and peripherals in the 64K address space
A few pinouts and detailed photos ...
- Connector J2 photograph with pinout. Pin 7 connects to the tape recorder MIC, Pin 10 to the tape recorder EAR, and Pin 12 is composite video output. Pin 11 is usually unused. The two potentiometers in the photo are used for the tape system.
- RS-232 Option shown here installed. Signals are available on J3 beside the power leads. See schematic page 6 for the circuit.
- IO Connector to the 610 expander board (40 pin). The open 16-pin sockets are for 8T28 bidirectional buffers.
- ROMS All for BASIC Roms, the monitor Rom, and the ACIA beside the CPU - here on the left side of the photograph.
- Video RAM for black&white video. Open sockets are for an extra RAM and buffer chip for colour video. These are not shown on the schematic which is for an earlier revision B board (the photo is of a revision D board).
While my original board was a revision B (schematics above), my 'new' boards are revision D - functionally similar to the C1P series II boards. These boards feature a connector (J3) for connection of a 4-pole, 3-position switch allowing connection of the on-board serial port to the cassette, a modem, or a printer. This connector must be installed in order for the cassette to operate. Other features of the rev D board include provisions for colour video and a delayed power-on reset where the BREAK key must be pressed for two seconds to effect a reset.
- J3 Wiring showing the switch required for the cassette, modem, and printer
- Sheet 1 of 5 showing the CPU and the Monitor ROM (including 2716 jumpers)
- Sheet 2 of 5 showing RAM and ROM wiring including 2716 jumpers
- Sheet 3 of 5 showing video clocking, RS232 drivers, and cassette circuitry
- Sheet 4 of 5 showing video circuitry
- Sheet 5 of 5 showing the keyboard
- Chip Placement for the rev D board
Using 2716 ROMs
A popular modification, but one I never did to my machine, was to upgrade the SYN600 monitor PROM to a third-party one such as CEGMON. CEGMON gave the user the ability to backspace, for example, while the original monitor lacked this feature. Today, a number of collectors must use 2716 ROMs to replace missing ones on the board (I have one unit like that). When using 2716 ROMs instead of the factory masked 2316 ROMs one must jumper the board as follows:
- Character Generator ROM: Cut traces currently between pin 18 and +5V and between pin 20 and +5V. Run a wire from each pin (18 and 20) to ground (pin 12 on the ROM).
- Monitor ROM: Immediately below the ROM is a jumper - Follow the trace from pin 20 to this jumper. Cut the trace and jumper as shown in the photo below (cut the trace shown, add the trace shown in yellow).
- BASIC ROMs: Follow pin 20 from each ROM and cut the traces before they reach the output of the 74LS04 inverters (U16). Run wires from pin 20 of each ROM directly to the outputs from the 74LS138 decoder (U73) - pins 12-15 as shown in the photo. While the original masked ROMs use active-high selects, 2716 EPROMs require active-low select lines.
One contact on the internet suggested the use of MP3 files to store old tape images and load them back into the machine but a better solution is to use the RS-232 port to load and store BASIC programs to a PC running a terminal emulator package such as Procomm or Hyperterminal. This is facilitated by the fact that typing 'LOAD' or 'SAVE' on the machine simply sends data going to the video display to the ACIA and accepts data arriving at the ACIA as if it were typed on the keyboard - this is one of the stranger features of an OSI C1P system an reminiscent of old PDP-8 teletype-based systems. For a PDP-8 running FOCAL, for example, the user 'saves' programs by listing them to the teletype but first setting the paper-tape punch ON so that received characters are punched to the tape. Similarly, the PDP-8 could not 'tell' whether a user was entering a program manually or via a paper-tape being read by the teletype so simply loading a paper-tape into the teletype and setting the punch to 'read' mode caused characters to be sent to the PDP-8 and the program to be entered.
So, typing 'SAVE' on an OSI C1P system simply routed all text destined for the screen to the serial port as well (ditto for the 'LOAD' command). In that respect, simply replacing the FSK demodulator for the cassette tape system with an RS-232 level converter allows the Superboard to load programs from that port. Note that earlier 500-series systems often used an RS-232 serial connection instead of a video display and keyboard - video display was an added board and an added option.
Revision D boards often come with the RS-232 option already installed, all that is required is a switch on J3 allowing the user to select cassette, RS-232, or modem operation. Revision B boards have wiring on the PC board for the RS-232 level converters however these must be installed by the user. A series of jumpers configures the input to the ACIA to come from the cassette tape's demodulator circuitry or from the RS-232 level converter (useful for connecting, say, a modem or for the output line, a serial printer which I did on my original system).
The RS-232 interface (for a revision 'B' board) requires the addition of two transistors as shown in the schematic - Q1 is a 2N3906 PNP and Q2 a 2N3904 NPN. As well, several resistors and a diode are required to be installed. Follow the circuit diagram, the photograph, and the top-down placement diagram for details. Note the unusual position of the transistors - leads must be bent in an unusual manner (with the base pin out front) in order to be mounted on the board. Otherwise, assembly is not critical.
Circuit Details ...
Aside from addition of the circuit components, one jumper (a PC board trace) must be cut. The diagrams show two 'W10' jumpers. One, only two wires, is broken to allow full-spec'd RS-232 voltage levels by applying -5 volts to pin 7 of J3. This is normally not required, however, so this jumper may be left alone. In this case, non-standard RS-232 levels of 0 and 5V are used instead of the true bipolar signal - so long as the RS-232 cable is short, most PC's handle the signal properly.
The actual 'W10' jumper which must be cut is located on the bottom side of the board and detailed in the photograph. With this jumper removed, even cassette operation requires a jumper on J3.
Connector J3 Wiring (for Cassette Tape operation):
- Jumper J3 Pin 5 to J3 Pin 6
- Jumper J3 Pin 4 to J3 Pin 6
- J3 Pin 2 (RS-232 Out) to DB-9 Pin 2 (Transmit to PC)
- J3 Pin 3 (RS-232 In) to DB-9 Pin 3 (Receive from PC)
- Board Ground to DB-9 Pin 5 (Ground)
After modification, the system should be tested to ensure the cassette operates then the jumper switched and the RS-232 connection tested. On the PC, run a terminal emulator such as Procomm or Hyperterminal and set it for 300,N,8,1 (300 baud being the normal baud rate of the tape system - this can be changed later). Type 'SAVE' on the OSI. Now, anything typed on the OSI keyboard should be echoed to the PC via the serial port, including program listings (this is, of course, how programs are saved to the PC, by capturing incoming data on the terminal emulator program to a text file).
To load a program from the PC into the OSI, type 'LOAD' on the OSI keyboard. When something is typed on the PC, it will now be entered into the OSI as if it were typed locally. In this manner program files stored in a text file can be uploaded to the OSI. This method will be considerably more reliable than the use of cassette tapes (or MP3 audio files) - even the original tape system had a high error rate and unless everything was just right, loaded programs frequently had an error or two which required the user to manually correct the offending program line - programs ALWAYS came with a full listing allowing the user to correct minor errors which occurred during loading from tape.
Software and System OperationUpon boot-up, the user was presented with a prompt for 'D/C/W/M ?' which allowed you to boot from disk, cold boot, warm boot (and preserve memory contents), or enter the monitor to debug machine-code programs. Few systems had disk drives since floppy disk drives alone sold for $650.00 in 1980, let alone the controller! Don't believe me ... look at one of the advertisements above.
Perhaps the most interesting section of circuitry on the system was the video display which used a meagre 1K of RAM. Video circuitry in the OSI switched the address and data lines of the video RAM from the 6502's bus to an internal clock generator composed of binary counters. When a character was retrieved from the video RAM it was converted to a dot pattern by the character generator ROM (U41 in the schematics) which was output as an analog signal representing video levels (black or white). Although the video RAM could store 1024 characters and the actual video RAM was mapped as 32 lines of 32 characters, the actual display was 24 characters across by 24 lines - peripheral characters were not visible and used as a 'guard band' to prevent off-screen characters. Later revisions of the OSI board included a socket for a third 2114 video RAM which stored colour information. Further add-on boards (available from third-party manufacturers) even allowed bitmapped graphics with resolutions up to 256 by 256 pixels! (This, incidentally, would require at least 8K of video RAM for a monochrome display where each pixel could only be 'on' or 'off' ... more than most OSI systems were shipped with). RAM, in those days, was a precious commodity and were the most expensive chips in the machine.
And as a huge bonus the character generator supported ASCII characters as well as a large number of graphics characters for games. Character zero, for example, was a race-car so where you'd normally drop a character 0x41 into a video memory location to make an "A" appear there (e.g. "POKE 53381,65" would make an "A" appear in the upper left corner), a character zero could cause a race-car to appear. Similarly, characters 248 through 255 depicted a tank in one of eight possible orientations 45-degrees apart, as used in the "tank for two" game (below, left). Some gaming graphics were large and required two side-by-side characters including contemporary Star-Wars fighters, Starship Enterprises (Character codes 9 and 10), and ships and submarines. The machine lacked enough memory to allow true bitmapped pixels.
Software was supplied on cassette tape which loaded at 300 baud. You could actually
watch the data as it was loaded into the machine which took an average of 3-5 minutes
One predominant software supplier for OSI systems was 'Aardvark Services'.
Aardvark's ad from Micro magazine, August 1981 details the myriad of software available for an OSI system at the time, most of it on cassette tape.
Anyone heard of them since?
Early Eighties Computer Hardware:
The early eighties were a time of homebrew computers. Unlike today, where even an amateur can buy a peripheral or upgrade and install it, to upgrade a computer in the early eighties required most users to become quite fluent in hardware (and to own a soldering iron). It was common for users to make extensive modifications to their machines as illustrated below.
Micro magazine was dedicated to the 6502 and 6809 crowd. Featured articles in this issue
(Aug 1981) included the making of a coprocessor board to do complex mathematics (yes,
a real Intel math coprocessor attached to an OSI CIIP!),
adding I/O to the OSI system, and numerous software articles most using machine code.
The magazine covered 6502 systems like the Apple, OSI, and Rockwell AIM-65.
Surprisingly, the magazine was not Apple-centric (as this was the most popular 6502
system at the time) - likely because it was a bit of a 'hardware hacker' magazine.
OVERCLOCKING was popular even with machines such as these. One of the modifications I
made to my original machine was a 'GT' option which increased the clock frequency from
1MHz to 2MHz. I found the original documentation on this and scanned it for your
"GT MODIFICATION FOR 600 BOARD
The CIP or Superboard II as delivered by OSI has a primary clock
frequency of 3.932l60MHZ. This frequency is a multiple of 60 (the
video vertical sync frequency) and also 15,360 (the video horizontal
sync frequency). One cycle of this primary frequency corresponds
to a single dot, or 1/256th of a horizontal scan line on the monitor.
The clock signal fed to pin 37 of the 6502 is the primary frequency
divided by 4, or .98304MHZ. This is within 2% of the nominal lMHZ
which most vendors advertise, for 6502 based systems (KIM, SYM, AIM, APPLE,
Fortunately for us, a clock frequency of 1.96608 MHZ (approx 2 MHZ)
is available in a very convenient spot. With a short length (1" or so)
of small diameter insulated wire, a sharp instrument to cut a trace, and
two simple soldered connections, you too, can be running your 600 Board
at twice normal speed."
The instruction sheet goes on to describe in detail how to connect the clock,
then it warns you as follows:
"Both the basic proms and the monitor prom have fast enough access
to run at the 2MHZ rate. There could be a problem with 2114
memory chips, especially on the CIP because of the enlosure. Chips get a
little warmer when you box them up and don t circulate the air. I have one
chip in my system that gets flaky after about an hour. I'll either replace
it or put in a fan"
A diagram showing how the modification was made to the board (original)
... Some things just haven't changes in over 20 years: computer types are
ALWAYS trying to push their chips past the limit!
Floppy Disk Drives
Ohio Scientific computers employed unique floppy drives which provided separated data and clock signals. In the beginning (1976), OSI used eight-inch drives made by GSI with a 470 controller board which was originally designed to use such a drive. Later eight-inch drives shipped by OSI, made by Siemens and Shugart, also featured data separators on the actual drive . Such drives use pin 30 as clock (separated) and pin 34 as data whereas 'industry standard' drives feature a single, combined, output on pin 30 only. These drives also used AC drive motors which ran continually - no motor control was required.
When OSI went from an eight-inch to a 5.25" drive for their smaller machines they chose to ship drives made by Micro Peripherals (MPI) which also have internal data separators. Like their eight-inch predecessors these drives also ran continually and OSI made no provision for drive motor control. Note that not all MPI drives feature an internal data separator. Model 52 drives, for example, feature a connector on the controller board (J5) which is not normally populated - except for an OSI drive where J5 connects to a data separator daughter board.
Modern drives, which provide combined clock and data on pin 30 of the 34-pin card-edge connector, require an external data separator circuit. One interesting article I've found regarding the interfacing of a 610 board to an actual drive is from the Aardvark Journal, Feb. 1982 in an article entitled "OSI AND THE SHUGART SA400 BY STANLEY WINDES" which described the changes required to add a standard Shugart SA-400 floppy drive to an OSI system.
The author writes: "The first thing that became apparent upon examining the OSI hardware was that the Data Separator was not on the 610 Board and it was not part of the SA400 electronics either. After much searching and head scratching I concluded that since the OSI system never misses inserting the clock pulse before each data pulse that is written on the disk, the Data Separator would be easy and simple to design. Some of the disk writing systems withhold some of the clock pulses written on the disk. This makes the data separator much more difficult. The data separator I came up with, and found to work very well, is shown in figure 3."
I have a disk-drive working now with two types of controllers. The first controller, a third-party disk controller built from plans featured in Elektor magazine in the early 1980s, is connected to an OSI 600 board with 12K of 2114 RAM (4K of which is piggybacked on top of the original 8K chips on the motherboard itself). The drive is an original MPI model-52 5.25" drive, but one without the internal data separator (so any drive like a Shugart SA-400 could be used here). The controller is almost fully compatible with the OSI 470, 505, and 610 controllers (although the 600 board has been modified to use the controller) and the system is currently booting HEXDOS (which, when loaded onto the 12K machine shows 8K of free RAM avalable). Like the original OSI floppy system the motor runs continually and it was necessary to jumper the drive to be selected continually.
HEXDOS is a simpler operating system than OSI's standard OS-65D - it is only one track long (occupying only track zero) and uses OSI's Monitor and Basic ROMs to save memory. A 'normal' OSI configuration using a floppy disk was a 24K machine (the extra memory on a 610 expander board) running OS-65D. OS-65D was a complete O/S having it's own BASIC so that BASIC-in-ROM was not required (larger disk-based OSI systems often had only a boot PROM installed).
The disk drive/controller had become 'flaky' and boots once every twenty-five tries. On many attempts the drive appears to boot, displays reduced memory (the O/S consuming some 3.5K), then 'hangs'. The controller is calibrated (the board has a data separator circuit on it) but the problem still exists - in time this problem, believed to be simple bad wiring, shall be repaired as well.
The next step was to get the system running with a 610 controller. An external data separator was built according to the article above. Input to the separator is from pin 30 of the disk drive and output connects to pins 10 and 11 on J2 of the 610 board. The controller was calibrated by adjusting the potentiometer to give a 6ms pulse (active high) from the Q output of the 74121 (pin 1). A cable was built as follows which connects the original OSI 610 board (connector J3 - pin 24 is the outermost pin near the edge of the board) to the floppy-disk drive 34-pin connector as follows:
|OSI-610 J3 Pin||Function||FDD Connector Pin|
|3||HSelect Drive 1||10|
|10||Receive Clock||From Data Separator|
|11||Receive Data||From Data Separator|
|12||Ground||All ODD Pins|
|13||Ground||All ODD Pins|
|18||Select Drive 2||12|
The system works well with the original MPI-52 drive running HEXDOS. Since the 610 has 12K of RAM on it alone, and the 600 board 8K, the system has 20K and so can boot OS-65D DOS. Aside from testing a few odd 5.25" drives, I plan on tackling the 'blue box' described below. I'd like to use be able to copy disks from eight-inch format to 5.25" format as well. In theory, the controller should work with both drives with only minor changes (the data separator needs to be calibrated differently, and the drive controller itself set for the new drive - 8" disks have higher capacity and higher data rates than 5.25" drives storing a whopping 2800 bytes per track instead of only 2000 bytes for a 5.25" drive!
A reader from Florida had a problem recently in using a newer model 52 MPI drive in place of an older model 51. Apparently the 51 has a daughterboard (the data separator) but the 52 does not. He writes the following on changes that must be made to the model 52 drive:
1. Daughterboard connector
2. pin 5 and 9 of the 74LS123
CUT the connection between pin 1 (NAND ) and the 74LS123."
When I examined a old model 51 board I found that trace is cut (right at pin 1 of the NAND, bottom of the board), whereas on the 52 it is intact.
Hopefully that helps someone else trying to make their floppy system work!
My Original C1P System
My superboard was originally used as a single, open board for which I built an external memory board and I/O interface on 44-pin card edge boards. The memory board features 8K of 2114 RAM chips stacked together giving the system a total of 16K. Later, Dad built a wooden box for the system (which I used for some time) and finally I built the system into a PDP-8/A case as pictured to the left. The OSI 48-pin bus was connected to the PDP-8 backframe and I built cards to suit.
In order to fit into the case the keyboard was separated from the single board - it was connected via a large cable (I literally cut the printed-circuit board with a jigsaw). I/O and memory cards were then built which fit into the backframe. The cards pictured here include two parallel I/O interfaces, a sound/voice card featuring an 'SN' chip, and a 24K memory card using newer 2K-by-8 memory chips.
OSI 500-series (Challenger II) Hardware
Prior to (and along with) building the Challenger 1P / Superboard machine, OSI produced several models of computer in the form of multiple boards which plug into a bus. The Challenger 2 models consisted of various configurations from a single board (using a serial port to communicate with a terminal and with on-board RAM) to a system featuring separate cards for the CPU, RAM, video, and floppy-disk interface. OSI went even further to produce a Challenger 3 series which featured three CPUs (6502, 6800, and Z-80) and even a model with a Winchester (hard) drive: a rarity in any personal computer system in the late 1970's.
There is also a confusing bit of nomenclature here as OSI often labelled their Challenger 2 systems as "Challenger 4P" and "Challenger 8P" when, in fact, they were really simply "Challenger 2 - 4P" or "- 8P" ... this was evident when one looked at the front of a box branded "Challenger 8P" to find the model number "C2-8P". So, you'll often see people refer to an "8P" but really, it's a Challenger 2 - 8P".
This 1978 Ad from Popular Electronics is an early advertisement for 500 series systems including the 500 single-board and C2 systems based on the model 500 CPU board coupled with a video card. Pricing is in $US but at the time the exchange rate for $CDN to $US was almost 1:1 anyway.
Challenger 2 - 4P
The smallest Challenger 2 system is the 4P, circa 1977. This system is composed of separate boards including a 502 CPU board, a 540 video card, and a keyboard all mounted in a thin case. With a newer graphics card than older systems, featuring a whopping 2K of video RAM, a 64-by-32 character display was supported. The system featured a four-slot backplane (hence the "4P" designation) of which only two cards were installed in the basic (cassette-based) system allowing further expansion with RAM and floppy-controller cards. The "2P" label identifies this as an older unit (originally branded "2P-4", later "4P").
The system was twice the cost of the superboard but featured three-times the video capability and used the OSI standard 48-line bus allowing easy expansion via plug-in cards. You'll note the 502 CPU card, shown here as the top card in the backplane, features five ROMS (BASIC as well as the monitor, just like the superboard) and 8K of RAM in the form of sixteen 2114 chips. The CPU board also contains a 6850 ACIA used for the cassette interface. The later superboard, then, is really just a 502 board with an integral video section as well as integral keyboard (with a significant cost reduction in the process).
I had received this system in late 2008. It featured the usual bugs expected with an older machine (including power supply issues in the form of failed electrolytic capacitors) as well as a failure in one (or more) of the upper 4K of RAM chips. The on-board RAM was clearly expanded after production as the upper 4K are manufactured by a different company than the lower 4K (which was presumably installed by OSI). Removing the upper 4K of RAM, the system boots! The user is presented with the customary "C/W/M ?" boot prompt and, upon booting to on-board BASIC, a 64-character display appears ... perhaps the best enhancement of such a system over the Superboard. A simple POKE command allows the user to switch between the default 64-character wide display and a 32-character wide mode ... presumably for software compatibility with many games.
You'll note the lack of a disk boot "D" option on start-up. The 502 CPU board had on-card BASIC ROMs as well as 8K of RAM but lacked a floppy controller - it was intended as a cassette-based system only. Upgrades to include a floppy disk interface were, of course, possible by adding a 470 floppy controller card and more RAM in the form of another card (as well, the monitor ROM must be changed - most owners at this stage would have upgraded to CEGMON anyway which gave the Disk-boot option). This would have populated all four slots. As well as the cassette-based 4P configuration above, OSI shipped a system pre-configured for floppy disk in the form of the "4PMF" version for "mini-floppy". That version used a 505 CPU which lacks on-card RAM but features a disk controller on the CPU card. Such a system usually consisted of a 505 CPU, 527 memory board, and 540 video card leaving one slot remaining for expansion.
Challenger 2 - 8P
The next largest system is the Ohio Scientific C2-8P as pictured to the left. The one pictured to the right is how a full-blown system would have appeared (including eight inch disk drives). This older machine (several years before the Superboard II above), packaged in a big blue box, features separate boards for each function like the 4P above however includes a larger expansion box and an 8-slot backplane allowing greater expansion. The 8P was intended as a machine for research, educational, or small business use. I have not tested this unit yet, but intend to work on it as a 'long term' restoration project.
This Challenger-2 box consists of the following:
- 580 Backframe (8 slot)
- 500 CPU (with 32 2102 SRAM chips for a total of 4K of memory on the CPU board) - circa 1977.
- 540 Video Card (with parallel keyboard connector)
- 550 Serial Card (Sixteen 6850 serial ports of which 4 are currently installed)
- 'Micro Interface' memory (24 6116's = 48K SRAM) and printer board - circa 1981
The model 500 CPU card is seen here plugged into the backplane. The CPU is the large chip near the top left and the empty sockets are for optional ROMs (for languages such as BASIC). Also visible are the thirty-two 2102 memory chips at the bottom of the board - totalling a whopping 4K! The 500 CPU board was originally used in older "Challenger 2" and superkit systems and was replaced, in future 8P systems, with the 505 CPU ... this is an odd system using an older CPU card but with an updated memory card using the newest (in 1981) 2K memory chips! As well as the cards in this machine I also have a few spare cards for the unit including a 505 CPU card plus one populated and one unpopulated model 470 floppy controller card (with parallel printer port).
It is interesting to note that the system includes a sixteen port serial card (useful for multi-user systems) and a third-party, non-OSI built memory board.
More Photos of the C2P System ...
- A TOP VIEW of the C2P System
- REAR VIEW of the C2P System showing the RS-232 Connectors. The reason for a larger expansion case becomes obvious when considering the "rats nest" of I/O wiring accompanying expansion boards installed.
Challenger 2P - OEM
A unique machine, this Challenger 2 system is housed in a single, elongated (deep) box - physically it resembles the 8P (below) however the box is twice as deep. It features two 8" floppy drives and has no video card: only a serial port is provided for connection to a serial terminal. The system consists of an 8-slot backplane mounted immediately behind the floppy drives with a 505 CPU, 535 memory card, and a 470 card (used only as a parallel printer port in this system). The 505 CPU has an integrated floppy controller, a single serial port, and a single ROM socket which bootstraps from the disk (the boot options with OS-65 are "H/D/M" since the system could accomodate both floppy disks as well as a hard drive).
The system had minor bugs when powered-up, primarily a short of the 24V supply used to power the floppy drives. The units has two physical power supply modules: one provides +5V to both all cards and the floppy drives, as well as +15V and -12V to the backplane and the second provides +24V for the floppy drives alone. The problem turned-out to be two dead capacitors on the Siemens FDD100-8D drives which were essentially a dead short across the 24V line (dead electrolytic capacitors are unfortunately common on old equipment). Replacement of these capacitors was straightforward, the control circuit board was re-mounted on extra spacers to keep the drive pulley from touching the board (as it did the first time it was powered), and the system booted perfectly .... the system runs OS-65U and OS-65D.
Challenger 2 - 8P DFOn a family vacation to Halifax I picked-up this beautifully-maintained Challenger 2 - 8P Dual floppy machine. Circa 1979, the unit has a 505 CPU and three 16K memory boards - five of the eight slots on the backframe are occupied. Impressively complete, the system had all disks and manuals.
You can see the Insides of the system as well including the three 520 memory cards near the front of the unit, the 505 CPU at the rearmost slot, and the 540 video/interface board below that.
The system features two eight-inch drives (Siemens FDD 100-8 drives) in a box the same size as the CPU and connected via a large ribbon cable. When booted, the screen prompts "H/D/M ?" ... unlike the single-board system this one does not have BASIC-in-ROM on the CPU board (hence the lack of the 'C' cold-start option). BASIC is run from the disk.
The system was is great shape but has the usual problems for a system thirty years old, namely dirty drive mechanicals and loose connectors and chips (chip creep). After a quick cleaning the power supplies were disconnected and output voltages verified to ensure the old supplies would not toast the boards (Old supplies sometimes have old capacitors which fail causing the output voltage to go well beyond that expected - I've seen an old 5V supply producing 7V). Supplies were reconnected and the system booted. At first, the screen showed random characters but after some 'wiggling' of connectors the system was made to boot, once, from the eight-inch floppy drive and was able to perform a 'DIR' as seen on the screen shot to the left. Booting is not reliable yet so a bit more work is likely required on the system - either the drive mechanicals are marginal or perhaps simply all that is required is a recalibration of the electronics (the 470 controller has several analog potentiometers). Since I now have a working 8" drive on the OEM machine, diagnostics and calibration should be trivial.
Challenger 3P - OEM
This perhaps one of the most interesting systems ever built: a floppy-disk based computer featuring three separate processors allowing the user (or even a program) to choose which processor to use from a selection of popular late-1970's processors including the 6502, 6800, and the Z-80. When using the Z-80, for example, the user could run the CP/M operating system (one of the most popular at that time).
I received this system from Tom MacDonald, a friend who works with instrumentation and computer hardware. The system resembles most others offered in the late 1970's by Ohio Scientific: several printed-circuit cards together on a backplane which holds up to eight of which one card is the CPU, and several others memory and I/O including disk interface. The system features two eight-inch floppy drives, the most popular size at the time and with storage capacities far in excess of the very newest 5.25-inch "mini-floppy" drives. This system, a C3-OEM, lacks a video display (becoming popular with "personal" computers) having a serial port instead through which the user interacts with the machine. The terminal could be either a video terminal, connected via an RS-232 serial interface, or a teletype terminal connected via a 20mA current loop (the CPU had both interfaces).
A "typical" high-end machine of the time, the most unusual thing about this particular system is the multi-processor CPU board.
As received the configuration was as follows: A 510 CPU board (with a 6502, 6800, and Z80 processor), a 470 floppy disk controller, a 522 48K memory card, another 527 memory card with only 8K populated, and a second 470 card used only as a Centronics printer interface. While a "standard" C3 system used different RAM boards (usually three 520 boards containing 16K each), this particular system featured a single 522 memory board utilizing unusual 421D 1K*8 SRAM chips in an 22-pin 0.4" wide DIP package. In addition to the 48K RAM installed (as expected), this system also featured a separate 527 memory card with only 8K installed. This RAM is required to increase the system RAM to 56K to run CP/M or the highest-level of the OS-65U (6502-based) operating system used for multi-user usage. It is mapped at 0xD000 through 0xEFFF.
As usual with an old system, it is best to _not_ trust the power supplies so all cards were first removed and the supplies tested separately. This system features three power supplies: A large Deltron MPS-2 supply which powers cards in the backplane (Supplying +5V, +12V, and -9V), a smaller +5V supply for the floppy disk drives, and a third small +24V supply also for the floppy drives. After the supplies were verified, configuration of the boards was checked before each was reinstalled.
Examination of the CPU card shows a few apparent modifications including the addition of an AND gate on the adjacent 527 memory board which served to disable data bus transceivers prohibiting access to the on-board 6810 memory in the region F0xx as well as the usual FCxx and F7xx ranges (FCxx is used for serial I/O and F7xx for a PIA and so nothing else must be permitted to access these memory areas). Why the region at F0xx must be omitted as well is a mystery. The fact that this CPU board has this extra RAM (128 bytes, normally located at F2xx) indicates it was the "deluxe" version of the 510 board with software-selectable processor (allowing a program to change processors) instead of the simplified version which used a three-position switch. The extra RAM is used to allow a 6800 or 6502 to call and execute programs using a different processor. This RAM is relocatable and under program control can be located at FF00 allowing it to contain restart and interrupt vectors (in place of a normal monitor EPROM). During a switch from one processor to another, this RAM area could be reprogrammed to point to the location of a new program. As for the after-market modification made, it was odd that the 74LS08 gate used was placed on a separate, adjacent, board and connected via three long wires however the 510 board is "stuffed" and lacks the usual prototyping area that most OSI cards have (i.e. a DIP outline in copper allowing addition of circuitry).
Well, the machine was reassembled and works perfectly! OS-65D runs well on the system and in one of the most amazing displays of late 1970's technology, runs CP/M! Take a look below at a capture of a session:
H/D/M?D 49K CP/M Rev. 2.24 for Ohio Scientific C3 Copyright (C) 1980, 81 by Lifeboat Associates A>DIR A: PIP COM : STAT COM : LOAD COM : DDT COM A: XSUB COM : MEMR COM : COPY COM : OSIGEN COM A: FORMAT COM : OSILNKS ASM : WFBIOS COM : ED COM A: SUBMIT COM : ASM COM : DUMP COM : DUMP ASM A>STAT *.* Recs Bytes Ext Acc 64 8k 1 R/W A:ASM.COM 24 3k 1 R/W A:COPY.COM 38 5k 1 R/W A:DDT.COM 33 5k 1 R/W A:DUMP.ASM 3 1k 1 R/W A:DUMP.COM 52 7k 1 R/W A:ED.COM 3 1k 1 R/W A:FORMAT.COM 14 2k 1 R/W A:LOAD.COM 15 2k 1 R/W A:MEMR.COM 8 1k 1 R/W A:OSIGEN.COM 46 6k 1 R/W A:OSILNKS.ASM 58 8k 1 R/W A:PIP.COM 41 6k 1 R/W A:STAT.COM 10 2k 1 R/W A:SUBMIT.COM 20 3k 1 R/W A:WFBIOS.COM 6 1k 1 R/W A:XSUB.COM Bytes Remaining On A: 177k
CP/M is the predecessor to PC-DOS and so, not surprisingly, is easy to use for someone well acquainted with DOS. The actual O/S lacks many of the usual DOS commands such as COPY, for which the "PIP.COM" program is used (it is a generic copying utility). A formatted disk may be made bootable using the "OSIGEN.COM" command which builds a specific version of CP/M for this machine: the main addition being that the 510 CPU boots in "6502" mode which then boots the Z-80 processor to run CP/M.
Of course, most OSI machines ran the OS-65 operating system. This is a relatively primitive track-based O/S providing only the most basic of operations. Although it features a basic command interpreter for "native mode" commands such as "INIT" to format a disk the majority of commands, including external commands, are written in BASIC which is built into the O/S. On boot-up, the O/S runs the BASIC interpreter and executes a BASIC program named "BEXEC*". Being track-based, the O/S lacks a true directory ... in the original versions the user was required to access files by track number directly. A "CALL" command (one of the most fundamental of this O/S) loaded tracks from the disk to memory where they could be executed via the BASIC interpreter or the assembler, also part of the O/S. A corresponding SAVE command writes data in a block of memory to the disk at a specific track and sector.
Disk organization is simple: for an eight-inch disk there are 77 tracks (numbered 0 through 76) and each track can store up to twelve pages of 256 bytes each (total disk capacity is hence 231K). Sectoring is soft and accomplished in software - each track can be broken-up into one or more sectors and each sector can contain any number of pages (the maximum usage of a track being a single sector of twelve pages). A header on each track tells the O/S the track number, and each track also has at least one sector header identifying the sector number and the number of pages in that sector (if more than one sector exists on a track, subsequent sectors with their own headers will be found on that same track - sectoring does consume disk space, though, and so if more than one sector exists per track a maximum of eleven pages can be accomodated per track).
A named directory was possible, however, via a file stored on the ninth track which stored filenames and the starting track. To use the named directory, the user needed to CREATE a file using a utility - this inserted a record into the directory - and the external DIR command could be used to read this file to the screen. The actual code for the "DIR" command was written in BASIC and is shown below:
10 REM DIRECTORY UTILITY FOR OS-65D VERSION 3.2 NMHZ 20 REM 30 NF=0 40 PN=11897 50 DEF FNA(X)=10*INT(X/16)+X-16*INT(X/16) 80 DV=1 : Y=1 : X=PEEK(8994) 90 IF X=<Y THEN 110 100 DV=DV+1 : Y=Y+Y : GOTO 90 110 PRINT "LIST ON LINEPRINTER INSTEAD OF DEVICE #";DV; 120 INPUT A$ 130 IF MID$(A$,1,1)="Y" THEN DV=4 10000 REM 10010 REM PRINT A DIRECTORY OUT 10020 REM 10030 PRINT #DV : PRINT #DV,"OS-65D VERSION 3.2 NMHZ" 10035 PRINT #DV," -- DIRECTORY --" : PRINT #DV 10040 PRINT #DV,"FILE NAME TRACK RANGE" 10050 PRINT #DV,"------------------------" 10060 DISK ! "CALL 2E79=08,1" 10070 GOSUB 11000 10080 DISK ! "CALL 2E79=08,2" 10090 GOSUB 11000 10130 PRINT #DV : PRINT #DV,NF;"ENTRIES FREE OUT OF 64" : PRINT #DV 10140 END 11000 REM 11010 REM READ DIRECTORY OUT OF BUFFER INTO ARRAYS 11020 REM 11040 FOR I=PN TO PN+248 STEP 8 11050 IF PEEK(I)=35 THEN NF=NF+1 : GOTO 11130 11060 N$="" 11070 FOR J=I TO I+5 11080 N$=N$+CHR$(PEEK(J)) 11090 NEXT J 11100 PRINT #DV,N$;TAB(12);FNA(PEEK(I+6));TAB(16);"-"; 11110 PRINT #DV,TAB(17);FNA(PEEK(I+7)) 11130 NEXT I 11140 RETURN
The utility is quite simple and works using a CALL command (an internal OS-65 command) to read the contents of track 08 into memory at location 0x2E79 (11897 decimal). The contents of the memory at this location are then displayed directly on the screen. Similar commands exist to delete and rename files. An example output of this command is shown below:
OS-65D VERSION 3.2 NMHZ -- DIRECTORY -- FILE NAME TRACK RANGE ------------------------ OS65D3 0 - 8 BEXEC* 9 - 9 CHANGE 10 - 10 CREATE 13 - 14 DELETE 15 - 15 DIR 16 - 16 DIRSRT 17 - 17 RANLST 18 - 19 RENAME 20 - 20 SECDIR 21 - 21 SEQLST 22 - 23 TRACE 24 - 24 ASAMPL 27 - 27 COLORS 28 - 28 C-ASM1 29 - 29 C-ASM2 30 - 30 48 ENTRIES FREE OUT OF 64
Debugging a 500-series system ...
A contact on the web recently had an OSI system with various problems and I have made a series of notes regarding debugging a C2/C4/C8 system:
Check the power supply first to ensure the voltages are correct - I have seen an old '5V' power supply putting-out over 7 volts! After that, power-up the system and check for video output: lack of video output might indicate a problem either with the 540 video card or with the CPU card. Checking the CPU card is simple enough: First, though, determine which CPU is in use. The common CPUs available were the 502 (for a 'BASIC in ROM' system like a C4P) and the 505 (for a floppy-based system since the 505 has an on-card disk interface). Aside from the disk interface, the biggest difference is the clock - the 502 has no internal clock. With a 502 board the CPU derives its clock from the OSI bus which is supplied from the 540 video card (which must be installed in this case), with a 505 board an on-card 4MHz crystal is divided by two sections of a 7476 JK flip-flop for the CPU. Either way, a clock must appear at pin 37 on the 6502 chip (phase-0) before anything else will function. With the CPU functioning, the address bus should show a great deal of "random" activity which may be seen on a scope or logic probe.
If the CPU card is running, check the video card - specifically check the waveform of the control lines on the 2114 RAM chips used as video RAM (the two 2114's closest to the character generator ROM) on the 540 video card. Using a logic-analyzer, or a scope, monitor the /WE (pin 10 on the 2114) and /CS (pin 8) lines as the RESET key is pressed. During a RESET (pushing the key on the front panel), one should see a series of write operations to the RAM ... this is how the OSI clears the screen to remove the random 'garbage' which appears on power-up (random RAM contents, displayed as ASCII characters). Note that address lines are always active since they are also used for read operations during video display. As well as a reset, pressing a key on boot-up such as "M" (to enter the monitor) will cause a write to the RAM and in the monitor pressing a key such as "0" or "1" will then cause another write. Verify this and it will assure you that the CPU is running properly.
In the above logic-analyzer output, channel 1 is wired to /WE pin, channel 2 to /CS pin, and channels 3 and 4 to address lines A0 and A1 respectively. When the CPU is running normally, no write operations occur (channel 2 was triggered here since channel 1 is always high). Only read operations take place during data display and, as expected, there is a good bit of activity on the 2114 chip address lines.
It might be noted that the video RAM must be relatively fast: it must be rated at 400ns or faster (i.e. the chips in these sockets will not work if they have a "-5" suffix as this indicates a 500ns chip). Use of slower chips may not allow proper data read/write and so strange patterns may appear on the video screen.
Floppy-based systems usually ran the OS65D system, for most applications, or OS65U for more demanding applications (it featured extensions for timesharing 16 terminals as well as extensions for database management). Both systems were centered around BASIC although an assembler and extended monitor were included allowing the writing of machine-code programs for the system. When the system is booted the kernel loads and immediately runs a BASIC program named 'BEXEC*' on the floppy disk. Usually, the user is either presented with a menu from which the user selects an operation or for development purposes the user may select to run BASIC and is presented with a BASIC prompt. Typing EXIT in BASIC brings the user to the actual O/S command interpreter, usually with a familiar "A" prompt. From the command interpreter the user could execute simple disk operations as well as run BASIC, and assembler, or an extended monitor (and possibly other languages).
Oddly, most utility program included with OS-65D were written in BASIC. Utilities included:
- CREATE a utility which creates a new named file, required before one could simply use it to save a BASIC program (i.e. you CREATEd the empty file first then could save your program or data into it). Without using CREATE first, the user could not use filenames, only access files by track number.
- DELETE used to delete a named file from the disk
- DIR yes, even a simple directory listing required the execution of a program which does this.
If this all looks primitive, remember that OS-65D was created in 1976 and PC-DOS would not appear until quite some time down the road. Original PC's came with BASIC-in-ROM and a cassette port as well ... floppy disks were still expensive in the early 80's and it was not intended that all machines would have drives.
Below is a listing of various OSI cards allowing identification of these should you come across them. It was compiled from various OSI literature. Included are photos of the boards in my collection and schematics for a few select boards.
- 500 C2P CPU Board (6502) The 500 board could be used as a complete 'stand-alone' board featuring 8K BASIC in ROM and 4K of RAM on board or as the CPU board in a larger system. All that was required for stand-alone was an ASCII RS-232 terminal as well as a power supply. It was also available as a 'SuperKit' with this 500 board, a 440 video board, and an 8-slot backplane for these and more cards. For the SuperKit, ROMs were configured to use video and a user-supplied ASCII parallel keyboard instead of the RS-232 terminal (the RS-232 option was not installed on the 500 board for this version).
- 502 CPU Board TOP VIEW of the 502 CPU Board from a C2P
- 505 CPU Board TOP VIEW of the 505 CPU Board
- 510 CIIIP CPU Board with Z80, 6800, and 6502 CPUs
- 560Z Multi-CPU Board
- 520 SRAM Board
- 525 SRAM Board
- 527 24K SRAM Board (fourty-eight 2114 chips)
- 420 Older 2102 SRAM Board TOP VIEW of the 420 Memory Board with 4K Installed
- 530 DRAM Board
- 450 EPROM Board
- 455 SRAM Board
- 430 Audio Cassette I/O Board
- 440 Video Board
- 540 Video Board TOP VIEW of the 540 Video Board
- 550 16 port 6850 Serial Port Board TOP VIEW of the 550 Serial Board
- 470 Floppy Controller with Parallel port Although intended for use as a floppy disk controller, the board was often used as a generic parallel port interface for printers simply by populating only a portion of the card.
- 542 Keyboard Seen on top of the system, this was a simple matrix keyboard scanned under software control.
- 580 Backplane board (8 slot)
- 495 Prototyping card
An updated 500 CPU, this board has similar features to the 500 however features 8K of 2114 RAM on-board. The board includes BASIC and MONitor ROMs for cassette-based configurations.
SIDE VIEW of the 505 CPU Board
This CPU was used on 8P and possibly 4P systems - it features an on-board floppy controller, serial port for a console, and a single ROM socket (and so was always used with a floppy disk system). The board is part of a 3-board set (including a video card and a memory card) to make a complete system. The video card could be omitted if an RS-232 terminal were used for I/O but a memory card is certainly required since the 505 has no provisions for RAM.
Schematics - Page 1 showing the CPU
Schematics - Page 2 showing the single ROM and decoder logic
Schematics - Page 3 showing the integrated floppy controller
Schematics - Page 4 showing the serial UART and baud rate generator
Schematics - Page 5 detailing signal names used on the diagrams
Schematics - Page 6 a top view to identify components
Note that although the circuitry appears to be identical to my 505 board the component placement (see Page 6) is completely different than my photograph reveals. Apparently there were different revisions of this board.
SIDE VIEW of the 420 Memory Board with 4K Installed
This board could also be wired for 12-bit wide RAM , presumably to accomodate the Harris 6100 PDP-8 CPU when the 460Z CPU expander was used (the PDP-8 being one of the most popular CPUs at the time. Running the 6100 would ensure the availability of a huge range of software).
SIDE VIEW of the 540 Video Board
The 540 board could accomodate black&white as well as colour video. It featured a keyboard connector in the form of a 16-pin DIP socket to which a model 542 keyboard was connected
Schematics - Page 1 showing the address decoder
Schematics - Page 2 showing video memory (all 2K of it)
Schematics - Page 3 display counters
Schematics - Page 4 keyboard buffer
Schematics - Page 5 colour memory
Schematics - Page 6 colour output
Schematics - Page 7 AC control (option)
Schematics - Page 8 I/O connector pinout
SIDE VIEW of the 550 Serial Board
TOP VIEW of the 470 Floppy Controller Board
SIDE VIEW of the 470 Floppy Controller Board
Schematics - Page 1 showing latching and buffering logic
Schematics - Page 2 showing wiring and noise (DAC) logic
Schematics - Page 3 showing the key layout on the 8-by-8 matrix
Schematics - Page 4 showing component placement on the PCB
Have a look at the specs and pricing for an OSI system as seen in
Popular Electronics in
1978. Shown are various configurations as well as many I/O boards available
for the OSI.
With Thanks ...
I must conclude with thanks to a few people who have "donated to the cause" including Greg Swick, a colleague of mine, who gave me a 600 board and a bunch of 500 boards. This rekindled my love for these old OSI machines in the first place. Scott Gregory, a buddy of mine who gave me the blue Challenger II seen on this page. Richard Parsons who sent me a number of interesting boards, O/S disks, and documents related to the OSI 500 and 600 series of computers. There's a wealth of information in the Aardvark and TOSIE journals I received including information of connecting disk drives. John Walker, who sent me a 600 and 610 board as well as some excellent docs (including a Sam's Photofact servicing manual) on the system. John Horemans for sending 600/610 circuit data. Jerry Travis who sent me a 600 and 610 board as well as two MPI drives. Russell Cooper from Australia who sent me a 600 and 610 board and some nice docs on OS-65D. Charlie Wainwright from Halifax who gave me the beautifully preserved Challenger 8P unit and docs (we picked-up the unit while on a family trip out East in 2006). Dan Batholic from MN who gave me a C2-OEM and a load of docs. This interesting machine has two 8" drives and was designed to run from a serial port (it has no video card installed) like a C3. Doug Pelletier from MI who gave me a C2-4P unit (a small black and blue unit with integral keyboard) with original docs. Tom MacDonald, a colleague, who got me the Challenger 3 system.
... and a number of other people I've had e-mail contact with who have sent me scans of various documents on OSI boards.