Professor Mark Csele

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.

Like contemporaries (including the Apple and the Commodore machines), OSI systems used the low-cost 6502 processor. Created as a low-cost response to Motorola’s 6800 processor, and commercially available (in production quantities) in 1976, the 6502 could use all readily-available and inexpensive 6800 peripheral chips (as OSI did to save cost). The 6502 achieved low costs through a number of design features including the fabrication in NMOS technology and use of “zero-page” memory which allowed reduction of the number of on-chip registers to six. The 6502, in a design reminiscent of the PDP-8 processor, allows fast access to the first 256 bytes of RAM from most instructions (making the 6502 almost RISC-like). The next 256 bytes (0x100 through 0x1FF) are prewired as the stack. All 6502 systems, therefore, require at least 512 bytes of RAM located in the lowest memory page and unlike the contemporary Z-80 processor, programs cannot simply be located at address zero (the 6502 actually fetches a vector for the start program from the highest two bytes – a ROM must be located there).

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.

This chart showing the OSI product line was taken from the 1980 catalog. It offers insight into systems available at the time and basic features of all available systems.

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.

If you are into the history of OSI, or other late 1970 / early 1980 systems, you might enjoy some of the podcasts at Floppy Days. The OSI was featured on episode 27 with some commentary provided by myself.

Operating Systems

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.

The ‘native’ O/S appears to be entirely track-driven: an application could save or load a program or data from the disk via track number however the creation of a directory and access to files by name requires the use of utility programs. Built-in functions in the O/S include the ability to invoke the BASIC interpreter, assembler, or extended monitor; initialize (format) a disk, load and save files from/to memory, and select an active drive letter.

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.

OSI Hardware

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.

CPU 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
    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.

505 CPU Board

  • TOP VIEW of the 505 CPU Board
  • 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.

510 CIIIP CPU Board

with Z80, 6800, and 6502 CPUs

560Z Multi-CPU Board

Memory Boards

520 SRAM Board

525 SRAM Board

527 24K SRAM Board (fourty-eight 2114 chips)

420 Older 2102 SRAM Board

    • TOP VIEWof the 420 Memory Board with 4K Installed
    • 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).

530 DRAM Board

450 EPROM Board

455 SRAM Board

I/O Boards:

430 Audio Cassette I/O Board

440 Video Board

540 Video Board

550 16 port 6850 Serial Port 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.
  • TOP VIEW of the 470 Floppy Controller Board
  • SIDE VIEW of the 470 Floppy Controller Board

542 Keyboard

Backplane Boards

580 Backplane board (8 slot)

495 Prototyping card

I have scanned a listing of all cards available for the OSI 48 line bus. These cards were designed for use with 500 series systems like the above) but could also be used with a 600 board with a 610 expander installed.

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.