30 Nov
1998
30 Nov
'98
6:30 a.m.
Doug Spence wrote:
Large keyboard on a
machine that early is a new one on me!
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.
Indeed I have.
Tony Duell helped with one or two tips, I think. Such as,
65256 doesn't work, use 62256 instead. And the way of avoiding the need
for inverters if you use the right sort of gate when re-encoding your
address (but I may have failed to implement that).
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. > My own is the third of those, a 1978 revision (a
pity in a way - the
case
> has the old tape deck, the blue screen surround
and serial number
1000035,
one of the
very first)
Are you sure that 1000035 makes it one of the first? Perhaps it's one of
the first of that revision or something? My PETs are 0014090 (small keyboard) and 0020272
(large keyboard). Both
have the first motherboard variation, blue trim, and white screen.
Help. We need
the Anderson. Larry, where are you? Can you help on this?
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.
> The top 4 address lines are decoded on the mobo by
a 74154 to give block
> select lines. The block select lines 0 (bottom 4K of RAM), 8 (screen
> memory) C, D, E (I think) and F do _not_ appear on the expansion
connector.
> All others do. The rest of the address lines (0
to 11) are also
present.
Yup, you're right. Interestingly, Blocks 9, A, and B are listed as
"Expansion ROM" on my PET memory map... I had a dream a while back where
the university was throwing all kinds of neat old junk out, and I found a
horde of PET cartridges(!) that plugged into the side expansion port.
While I've never heard of such a thing, is there any reason a cartridge
of
that type couldn't have been a reality?
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...
> When I added a 62256 to my 8K PET, I encoded the
block select lines for
24K
> of RAM space and 8K of expansion ROM space (blocks
9 and A). Beware -
POKE
also fails
here, not just PEEK, if you're accessing this RAM from BASIC.
So you've actually already done something like this! Excellent! 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.
I think *all* of the 6550s in that machine are duds,
but I could move 4K
over from the working machine. (I've tried the dead ones in many, many
combinations but perhaps not all.)
Zog! You'll have to pull block select 0
from somewhere on the motherboard
then. You might as well take +5V from there while you're at it.
I'll try and dig out my RAM expansion board, and work out what it did.
Meanwhile, have fun!
Philip.