The OSI Replica Project

When I was at the Maker Faire in San Mateo in May 2012, I met Vince Briel of It was the high point of the day and made the Maker Faire even more memorable than it already was. When I introduced myself, he immediately knew who I was. He knew about Propeddle project and he told me what he was working on: a replica for the OSI 600 “Superboard II”.

I had to admit that I’d never heard of such a thing, but now I know that the Superboard II was the base for the Ohio Scientific Challenger computers. I’ve seen those before but never worked with them. Ohio Scientific was present at the legendary 1977 West Coast Computer Faire where the “big three” (TRS-80, the Apple-II and Commodore PET) were also announced. While the page, linked above, dates the OSI in 1978, this report by David Ahl mentions the Challenger being demonstrated at the show where the other computers were merely announced.

Vince told me at the Maker Faire how he had the 6502 connected to a Propeller to emulate video, and we discussed my PETSCII driver for Propeddle for a while. We talked about using the RDY line to hold the 6502 while the Propeller was working, and how it wasn’t needed for Propeddle. We also talked about connecting the Propeller to 5V system and I told him Propeddle uses the WDC 65C02S which is able to work on 3.3V like the Propeller.

A few months ago, Vince and I started exchanging emails about the OSI, and I started learning about the system. It’s actually a fairly simple 6502 system with RAM, ROM, video, a keyboard and a 6850 ACIA. It doesn’t use any interrupts, the keyboard is basically a switch matrix with an address decoder. The ROM has an early version of Microsoft Basic.

The video hardware on the OSI is a bit weird. It uses a 256-character font in monochrome without inverse video, and the memory is organized as 32×32 characters (1KB total). However, the characters are so wide that on a CRT monitor the leftmost and rightmost characters are invisible, so the editor only uses 25 columns and 25 rows.  Vince asked me for help to implement that in his video driver so I did. And things went on from there.

The 6502 has to access memory inside the Propeller to display video, and I contributed some code to do this. Vince then proceeded to add more code to let the 6502 access the ROM too. I designed a schematic to map a number of areas in the upper 32K of the 6502’s address range into the Propeller as “virtual memory” so that the ROM could be stored in a contiguous area of hub memory. That way it takes fewer instructions to decode memory operations. I also modified the PS/2 keyboard driver for the Propeller that Chip Gracey (of Parallax) wrote, so that it can be used to emulate the polled keyboard of the OSI.

By this time, Vince decided that this project (which he had been working on for a while) is now finally in a stage where he’s confident that he can finish it. You can see the work in progress on his discussion list here.

It’s been an absolute honor and pleasure to work with Vince, on a project that’s almost (but not quite) completely unlike Propeddle 🙂

I hope to see him again at the next Bay Area Maker Faire in May 2013. He promised he’ll bring his prototype and I can’t wait to see it and play with it. I taught Vince some things that I know from the Propeddle project, and I learned some important things that I will use for the continuation of Propeddle. It will definitely be possible to make an OSI emulator with Propeddle, but it will not be the same as “the Briel thing”. 🙂