28 Nov
1998
28 Nov
'98
12:20 p.m.
On Mon, 23 Nov 1998 Philip.Belben(a)pgen.com 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.
RAM = 6550, ROM = 2316
RAM = 2114, ROM = 6540
RAM = 2114, ROM = 2316
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.
I have circuits for all four, if you need help.
All had the "chicklet" keyboard. Not rubber keys in this case, Cameron.
Square plastic keys in a very small qwerty-but-no-offsets arrangement.
Little helical springs for key return; conductive rubber pads onto
interdigitated PCB tracks for contacts.
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.
All but the last few machines of the production run
had blue-white phosphor
for the screens.
I prefer it to green phosphor. A PET without a white display doesn't
really feel like a PET to me. But the first computer I ever used was a
PET with a white display.
BTW, all those of you who never used number pads for
lack of comma keys, on
the small keyboard PETs the numbers were ONLY on the number pad. Top row
was punctuation only...
This applies to my large keyboard PET as well. Numbers on the keypad
only.
The position of the keys on the two keyboards is a little different. The
small keyboard's top row is:
! " # $ % ' & \ ( ) <-
On the large keyboard it is:
@ ! " # $ % ' & \ ( ) <- [ ]
Actually, there are quite a few keys that moved. Everything between the
SHIFTs and SPACE had to be moved elsewhere for the large keyboard.
But I digress.
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?
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?
If you've got an 8K PET with some dud 6550s, try
and swap them around until
you have 4K of good memory, and remove the second 4K. Then map 28K of RAM
space and 4K of ROM space to your memory expansion chip.
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.)
The machine the good 6550s are in right now is the machine that originally
had the dud chips. Every piece of RAM in that box was dead, including the
mystery chips (I still haven't identified what they are) in the RAM
expansion daughterboard. Or at least, so it seems.
The machine has been working without problems with the 6550s that I
pulled from the small-keyboard model for a couple of year, so whatever
happened to the memory seems to have been a one-time event.
If you've got a 4K PET with some dud 6550s,
you'll have to find block
select 0 from somewhere on the mobo.
I wouldn't mind replacing the 6550s entirely. I'll keep them in a bag, as
Ethan suggested, but actually use the machines with modern SRAM.
WARNING. +5V does NOT appear on the expansion
connector. Most people get
it from the second cassette port, but it won't drive anything big. 62256 +
a little TTL is fine though.
The RAM expansion daughterboard I've got actually pulls power directly
from the PET's power supply, and then feeds it to
the motherboard.
I guess the lack of power on the connector kills the "PET ROM cartridge"
idea. :)
Hope this helps. My explanations tend to confuse
people...
Not this time! Thanks. :)
Philip.
--
Doug Spence
ds_spenc(a)alcor.concordia.ca
http://alcor.concordia.ca/~ds_spenc/