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Sunday, August 14, 2011

My IBM 5150, the "three dollar computer"

One day long ago, I came home to find that a friend had spotted a "good deal" at a flea market and had dropped off a computer for me. It was an original PC, circa 1981, with an ominous sticker on top: "Does not work, but case is usable - $3.00". I didn't know what to expect but decided to take a look. What I found wound up costing far more than that.

Later, I opened it up and saw what awaited me. There was a case, a motherboard with an assortment of socketed chips, a CGA card and finally an AST SixPackPlus board (serial, parallel, battery-backed clock/calendar chip, game port, and a bunch of RAM). That's it. By itself, it was unusable. For starters, it didn't have a power supply!

During the weeks and months which followed, I started figuring it out. First, I stole the power supply from my dad's computer (yep, the one where I did the UART swap) and tried it in the "new-to-me" old machine. Nothing happened. I worried that maybe I had fried our power supply, but upon returning it to our real machine, it was fine. Something else was going on.

I'm not sure how I happened across it, but at some point while staring at the innards of this beast, I noticed something unusual. Right next to the 8088 in pictures I had found in books, there was normally an open socket. On my machine, there was a chip living there! Oh, hey, cool, I thought. Someone added the 8087 math coprocessor.

Then I looked a little more closely. It was an 80287, or the math coprocessor for the 80286! I have no idea how it found its way into this poor little machine, but an AT-era math chip had no business being in an original PC, even if they fit in the same socket. I pulled it out and again stole the power supply back from the main machine.

It powered up! It actually started doing stuff. Then it said PARITY ERROR and threw some kind of numeric code. Another friend of the family pointed me at documentation about all of those DIP switches on the board, and how to decode a RAM error. Happily enough, it pointed me at a given location, and I was able to find some replacements down at EPO, that same place where I had found the UART some years before.

Upon replacing that chip, the message changed. Another one was toast. So I replaced that. I think I wound up replacing three of them, and then things really got interesting. I was given the gift of ROM BASIC. That's pretty cool, considering this box had no floppy drive, no hard drive, and no cassette drive (yep, it had a port for that). It fell all the way back to the ROM and gave me a wacky little place to poke at things.

At this point I could justify the purchase of an actual power supply, so I did that and stopped stealing the one from our big machine. Now I could play with this thing and do dumb BASIC tricks like sounds, colors, and rudimentary pulse-width-modulation of the cassette relay to amuse myself -- click click click! But, I wanted more.

A friend let me borrow a MFM + floppy controller board, and I paired that with an old ST-251 given to me the year before by some neighbors when they upgraded. I also found a full-height 5.25" floppy appropriate to that machine (so it would fill the huge gaping hole in front) and I was on my way.

Not long after that, I started playing with networks, and dropped an old borrowed 3c503 thinnet card into it. That and our first computer constituted my first real Ethernet configuration. Once I picked up Netware Lite, I needed to name these things.

Naturally, I called this machine "3dollar". Never mind the fact that I had to buy a power supply, RAM chips, an Ethernet card, a MFM controller, and plowed a whole bunch of time into it. That was what it said on the sticker, and that's what I used. Besides, it was a good excuse to tell a story any time someone saw that sticker.

"300 dollar computer" just didn't have the same ring.