Navigation Bar Home Page

Computer History Part Three. The OSI-400 Thru TI-99

After leaving the Navy, and landing a good civilian job, I decided I had to get into the world of home and personal computers. After a short search, I found my first personal computer from the friend of a friend. This friends' friend was a computer hobbyist who had most of the bits for a spare OSI-400 system for sale. I bought the computer (in boxes), along with a lot of documentation, and went home. The next few weeks were consumed by building a cabinet, putting the computer back together again, reading the documentation, finding a terminal, and trying to make it all do something. Well, I did eventually. The OSI-400 (Ohio Scientific) was a 6502 machine. It used a plug-in bus, consisting of a backplane with rows of molex style pins that the boards plugged into. Before I was done, I had a processor board, four memory boards each with 4k of ram made up of 2102 chips, and an I/O board. The I/O allowed me to use a tape recorder for program storage. The computer connected to a keyboard, monitor, and a Teletype ASR-33. (Which I acquired through work). I had Tiny basic on tape, and a couple other programming tools. I spent a lot of time with this system learning basic, and playing. But I never did anything really useful with it.

For this page, I dug my OSI out of it's dark corner. These are pics of it upon recovery. I don't think the Blue board belongs!

I am not going to try to revive this computer at this time, but I will try to document it here. Speaking of documentation. Somewhere I have an entire box of documentation on this, as well as some software on cassette. Some of it is on fanfold, and some of it is hand written from the original OSI company principals. I can't find it now, but the search is ON! Although I am mostly pretty organized, I have one room in my basement full of old computer stuff, books, and software - Tons of software. That room is a disaster area. Writing this history column might just nudge me towards to organizing it. Especially since I need a lot of stuff down there for this.

Below are some photos and short descriptions of the cards. Many of them have been heavily modified. Why, I don't remember (if it was me experimenting, modifications, or corrections). The chips all date to 1976/1977. All the boards plugged into a 48 pin backplane using 'molex' style connectors. I built the case, and I originally had lights and data switches on the front (just like real computers). I remember the Keyboard (not OSI) as being a kit from somewhere. The power supply I have with it is 5v and Plus/minus 12. Some of the chips have been 'borrowed' over the years. I also thought I had a couple more boards somewhere (RAM?). Ah Well. Click on an image for full size.


This is the Backplane with all the boards plugged into it. In my case, I had an aluminum bar for the tops of the boards to anchor to with a small L bracket. The backplane is totally passive. It has some buss terminating or pullup resistors at one end. The wires on the left are for the power supply. The Coax on the right was the video from the 440 Video board. The connectors were 4 groups of 12 Molex male pins on the backplane, with mating female connectors on the boards.

This is the 400 CPU. It is a rev B and the foil pattern says 12/75. The processor in the upper left is dated Nov 1977. There are a number of empty sockets on this board, including a 24 pin and a 40 pin. I don't remember what went there. I bet one was a UART and the other a PIA? It looks like it has 8T26 buffers for data to the buss, and uses 7417 hex buffers for the address. The only clock I can find is an LM555 based one with a trim pot. There is no place for a xtal. I will trace it out when I have time. I know at one time I had an ASR-33 terminal hooked up, and I think it was to the CPU Board. I really wish my camera had manual focus for these shots.

This is the 420 4k RAM Board. I have two of these for a total of 8k (I remembered I had like 16k). Each board uses 32 1kx8 2102 ram chips for a total of 4k by 8. It has extra socket space to add 4 more bits to it for a 4kx12 bit, which I find made it compatible with the 560Z PDP-8. I also didn't remember these had a provision for battery backup. (I learned both of those from the website linked to on the page, and here: Marks OSI board Index). Some of the 2102s are dated 1977 and some 1978. Both the boards are marked Rev C. Looking at ram today, it is an even bigger shocker how far we have come!

Here is the 430 Serial Board. It is copyright on the board as 1977, Rev B. This board did all kinds of things. It has an AMI S1883 UART. I know this did the cassette I/O and had serial ports, According to the above website it also has provisions for digital I/O and an 8 Channel A/D converter. Those are obviously not populated on this board, but there IS a lot of unused circuitry!

This is the 440 Video Board. I think that is all it does. (That's all! With discrete logic, no VLSI here!). If I recall it had a very limited display. The above web site says 32 columns, 4 colors. That sounds about right! Again, this uses totally discrete 74xx series ttl logic. Even the output appears to be 7403 quad Nors. This WAS in the Don Lancaster TV Typewriter era. Remember Don? This board appears to be a REV B. Some of the chips are dated back to 1975. The output cable was separate. This board is highly modified. I don't remember why, but it looks like some of the hookup wire I may of used at the time (Purple wires). Interestingly enough, it again, does not appear to have any crystals. How it maintained sync, let alone color phase, I don't know. but, I will someday find the Docs!



Here is a Web site with a list of OSI Boards and descriptions:
Marks Ohio Scientific Board Index

Here is also another really well done website with info on OSI and superboards from an experimenter:
Professor Mark Csele's Ohio Scientific Page. This is a great site, well done, and interesting. Check it out for other topics also.

Deciding I needed a real (read commercial) computer, I came upon an ad for the TI-99 which was on sale for, I think $200.00. I don't know which came first. My decision, or seeing the ad, but either way, I was soon the proud owner of a shiny, new TI-99/4A computer. Hooked it up the day I brought it home, and I was in business. For something. I had a lot of fun with the TI. I really liked the keyboard, and it had a very nice display (32 columns of Black & White.). I created a lot of basic programs to solve math problems - trig solvers and the like. But, when I finally got over the cassette recorder, and lack of a printer connection, I decided to expand. Sticker shock does not adequately describe my reaction to the prices for TI-99 peripherals. And this was long before Ebay. I simply couldn't afford to even start to expand, which required the expansion box - a four figure ($1400.00) box! Well made, with a floppy drive 32k memory and a serial port! Not enough bang for the buck! You think maybe this is why the TI-99 failed? I truly believe the TI-99 would have enjoyed much more success if the expansion wasn't so highly priced. It had a lot of potential. And while there were cheaper solutions available, at that time, the PEB (Peripheral Expansion Box) was by far the most usable.

The TI-99/4A was an interesting and relatively powerful computer for it's day. Based on the TMS99000 processor, it was a 16 bit machine in an 8 bit world. 3 MHZ and 16k ram. It had a nice version of basic, and a crisp display albeit only 32 columns. The keyboard, although 2/3 size, was outstanding to use! Storage for the basic unit was via your own cassette recorder, and it had a joystick port. Other ports were a cartridge port on the front, and the expansion port on the right side.


An interesting sidenote. Shortly before the demise of the TI-99, Radio Shack started selling some of the parts as a "special purchase" (Prompting many rumors). I remember the Processors, video modulators, power packs, a couple other ICs, and the Keyboards. Here is a picture of a TI-99 keyboard (From the beige plastic version). Still in the original Radio Shack package (Cat # 277-1017) Price - $2.95.

My first TI-99/4A was the metal one pictured here. They later released a cheaper beige plastic one that also had cheaper internals. I also have one of those from somewhere. The TI was OK, I had fun, and learned some more programming, but, I wanted to play with hardware, and HAVE some hardware. It was time to move on.


Fortunately, I was about to find a new friend. The VIC-20.

On to part four. Commodore.