Sunday, 19 June 2011

Computer Museum (2)


Fun though it was to dig out my old Sharp PC-1211 for the previous article in this series, it was a little disheartening to realise that any attempt to procure its toxic little mercury batteries would likely land me on a terrorist watchlist for the rest of my life. So it was a happy surprise to be reminded that its successor, the PC-1500, takes four bog standard AA cells. Here it is, recently emerged from the loft, all powered up and asking for permission to erase its now random memory contents, the dream debris of its (almost three decade) nap.

This 1983 purchase was funded through the usual channels. In other words, I initially went halfers with my pal Brian. Then once I'd accumulated enough buroo cheques, I did him over like Eric Cartman for full ownership. Honestly, sometimes I wonder how my friends ever did put up with my behaviour. But the time between striking this partnership and its ultimate betrayal was a golden age; the PC-1500 turned out to be a hacker's wet dream.

Peek And Poke

We discovered various ways to abuse this BASIC programmable pocket computer, forcing it to interpret pre-crafted code memory contents as data, and vice-versa. This first revealed that the BASIC ROM recognised several keywords not mentioned in the official user's manual. Not just any keywords, but the holy trinity of PEEK, POKE, and the almighty CALL. Soon we had discovered enough single machine code I/O instructions to discern that the processor was very Z80-like in its architecture, at which point Troy fell, and shit was lost.

Soon armed with the full processor instruction set, we started writing super fast Moon Landers, Star Treks, Snakes, Space Invaders... and of course my own personal favourite, Son of the Revenge of Complex Arithmetic III. Having figured out the display hardware too, we had what felt like unlimited graphics power. Although the monochrome LCD was just 156×7 pixels, it was much cleaner and sharper looking than that of the PC-1211, which had used an ugly yellow-green filter to protect its fragile, almost still prototypical, vampiric liquid crystals, from damage by daylight.

Actually on second thoughts, I think Dominoes was my favourite, for several reasons. It was the first game I wrote using the full power of the machine, and the one that paid off the initial purchase costs. The domino images were pixel-perfect, and the game let me introduce my dad to computers (no mean feat for a geek in the early 80s), because its UI was so friendly: to play the 3-4 domino, you just typed 34. Finally, the AI was terrific; it made a truly formidable opponent. How did I achieve this level of awe? I programmed it to look at your hand.

Technology Caught Evolving

So here's what my PC-1500 looks like inside. Notice the two 0.1" pitch chips, the ones labelled TC5514P, sticking up like sore thumbs on an otherwise 0.05" pitch surface mount 2-layer board with through plating. Those are 1K by 4 bit static RAMs. The big LH5801 on the bottom board is the CMOS static 8-bit CPU, its LH5811 neighbour the peripheral I/O controller (an unwritten law said these always had to be named +10 higher than the corresponding CPU part number). The whole machine, like all such pioneers, screams a thoroughgoing compromise of new and old technology; 6V performance versus 130mW C-MOS battery life.

No Peripheral Vision

We never did fork out for the audio cassette interface and printer. This might seem unbelievable now, but it's true: the ritual and preamble to playing a computer game involved an hour or so of typing it all back in again. From your own notebooks, or from multipage magazine listings. Quite often this was the point at which games evolved, as you'd notice some possible improvement, or identify a great extension, each time you laboriously re-typed the now familiar code.

That was bad enough for BASIC code. But now we had to enter machine code and hexadecimal data, all without the aid of an assembler. Not something you want to have to do even once, but we did it every day. More than the lack of program memory, it was this tiresome drudgery that taught us only ever to write optimum code, first time.

I did design, and build into one of those pale blue Marshalls Electronics project boxes, a DC power supply to use with the PC-1500. Sadly I never really trusted my own crowbar overvoltage protection circuit enough to use it for more than a few minutes at a time. Eventually binned it just last year.

Previously: Computer Museum (1)

Update (June 25): the day after I power it up, the old PC-1500 begins haemorrhaging from the top right corner of the display. Click the photo on the right to sleuth the evidence. What the hell is this? So I dismantle it...

◀ Inside there's this 10cm dark red opaque plastic block, soft yet brittle, seemingly stuck to the top of the display with strawberry jam. That's it on the right hand side, while the trail of blood can now be seen on the PCB, display, bezel, and other components of the casing. This is going to take a lot of solvent, and a few drums of cotton buds...

Half a bottle of meths and 100 cotton buds later: burp. Well that was an epic, what a fiddly stripdown, clean and rebuild. But the operation, the sticky gunky gluey plastic blockectomy, has been a success, and the wee beastie is back in pristine working condition. As for the thing I removed, not been able to find any reference to that in the online PC-1500 reference material. I suspect it may have been a thermal mass.

The late seventies belonged to the bright, power hungry, red 7-segment LED wristwatch and calculator. Arriving in their New Romantic mullets, the first LCD replacements were quite temperamental, by which I mean, temperature sensitive. They needed thermistor circuits to stabilize their viewing angle. My guess is that the soft plastic strip I've removed and discarded was designed to average out fluctuations in the sensed temperature, e.g. to stop the display from fading when the device was held in warm hands.

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