OSI Computer (Ohio Scientific Inc.)

OSI C1P, Samson 65, Octopus 65, EC-65, EC-65K

About the OHIO computer

The OHIO computer was well known in the 80’s. Among other “home computers” such as Apple II, Commodore C64 and Sinclair ZX80, the OHIO computer was a favourite builders object. Mostly in the states but also copied by home-brewers in Europe. The original machine contained the a 6502 micro-processor and software was loaded by means of an audio tape. The original machine (see picture below) had an Easter egg: hit “L” at the boot menu and you got Bill Gates name and some Microsoft details.

Elektuur, a Dutch Electronics magazine, published a number of articles based on the OHIO computer. The nicknames became EC-65(K) and Octopus in Holland and Samson-65 in Germany.

The design was based on the original OHIO, but evolved to a much more sophisticated machine. The design consisted of several building blocks such as, CPU, VDU, (D)RAM and FDU boards.

Even a Z-80 board was designed the be able to run CP/M software in a Client/Host configuration.

The magazines where published around 1985.


CPU PSIO-RTC Extension cards
  • 6502 or 65C02
  • Clock frequency 4/2/1 MHz
  • ROM 2 or 4 Kbyte
  • Buffered Data & Address bus
  • 64Kbyte direct addressable
  • DMA
  • 2* PIA 6520 / 6820
  • Real Time Clock MC 146818
  • ACIA 6551
  • Centronics port
  • 2* 8 Bit for Keyboard connection
  • (D)RAM – 64Kbyte memory card(s)
  • FCU – 360/720 Kb Floppy controller
  • VDU – Composite Video controller
  • EPROM – EPROM programmer
  • AX25 – Packet Radio Controller


My first version of this famous machine including TV-set as CRT!.

Below you will find the Block Diagram of the “Cloned” OHIO Computer. (Unfortunately in Dutch) but the essence is illustrated clearly. A versatile system based on a 8 Bit bus and expandable within the limits of the 6502 uPC chip-set.

Further devlopments

I ended up with basically two operating systems in my machine. The Host 6502 assembler running a home-brew DOS operating system and the Z80 Client running CP/M.

HOST – Rockwell 65C02

CLIENT – Zilog Z80

Most of the software was created using Assembler language and dedicated Compilers.

Assembler language

An example of the software I wrote for HAM usage can be found here.