Replacing 6550s

From: Doug Spence <>
Date: Sun Dec 6 08:45:51 1998

On Mon, 30 Nov 1998 wrote:

> Doug Spence wrote:
> >> The original PET came with four different motherboard variations, viz:
> >>
> >> RAM = 6550, ROM = 6540
> >
> > Both of my PETs are of this type, but one has the small keyboard and
> > internal tape drive, and the other has the big keyboard.
> Large keyboard on a machine that early is a new one on me!

I don't know that machine's entire history, so I suppose it's *possible*
that the top half doesn't match the bottom half. Maybe I should inspect
the hinge very carefully. :)

It does have a blue-trimmed 9" white display, though, and I can't imagine
anyone going to the bother of removing the Sphynx's head to change its

The front sticker is no longer on it. Apparently it now lives on a guitar
owned by the bother of this PET, two owners ago.

This particular PET also has upgrade ROMs, not the originals.

Apparently this particular PET was 'the very first personal computer used
at Air Canada'.

> Blue trim was dropped fairly early - both the machines we had at school
> were black trim - as was the rebadged cassette deck with the lift-the-lid
> eject mechanism, the latter being replaced by the C2N. I had always
> assummed that 1000035 meant the 35th machine with the 220-240V power
> supply.

Well, it still could mean that. I don't know when Commodore started
making the things in Europe as compared to North America. Canada probably
got a lot of the first PETs because Commodore was such a big company in
Canada before the personal computer era.

> > Actually, IIRC my small-keyboard PET uses little rubber cups. But I
> > suppose there may be springs as well. The keyboard didn't work when I got
> > it, so I had to disassemble it and wipe the circuit board clean. I never
> > disassembled it beyond pulling the circuit board off.
> No rubber cups. Rubber cups or domes always in my experience give some
> sort of mechanical hysteresis when you press them. All PET keyboards I've
> used are smooth until they hit the stop. Small keyboard had little black
> rubber pads set into the plastic mouldings of the keys.

I think there are rubber cups of some sort, but they may be soft
protective things rather than what provides the 'bounce'. I just tried
both PET keyboards and compared the feel to other keyboards, and you're
right, they are smooth. Potential 'rubber cup' keyboards: CoCo 1, Amiga
3000. Weird undefinable keyboard that mushes/springs at the bottom: Atari
130XE. :)

> Only the lack of power. Flying lead from cartridge to 2nd cassette port is
> the usual solution AFAIK (it's what I did on my RAM expansion). No, I've
> never heard of ROM cartridges like this but I've met other things I think.
> ROM expansion usually went inside...

Where would you put ROM expansion? There's no empty sockets in this model

> > Why do POKE and PEEK fail there? Was that done on purpose or is it just
> > the result of something lame like using a signed value to represent
> > addresses?
> No, it's software. It was a feature that was supposed to prevent
> inquisitive geeks disassembling the BASIC ROM between $C000 and (I think)
> $E7FF. The OS ROMs, above $F000, were peekable, though, as was the I/O
> space in the E block. You could of course peek and poke the screen, $8000
> - $83E7 inclusive.

An inquisitive geek wouldn't be stopped long by that! Bill should have
known better. :)

> I'll try and dig out my RAM expansion board, and work out what it did.
> Meanwhile, have fun!

No fun until all my assignments and exams are done. :/

> Philip.

Doug Spence
Received on Sun Dec 06 1998 - 08:45:51 GMT

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