Replacing 6550s

From: Philip.Belben_at_pgen.com <(Philip.Belben_at_pgen.com)>
Date: Mon Nov 23 10:47:34 1998

>> Hi all!
>>
>> Thanks to everyone who supplied information regarding the differences
>> between the 6550 and 2114 (especially Ethan who supplied pinouts).
>
> You're welcome.
>
>> I'm wondering if it would be possible to replace the RAM in an old PET
>> with a modern SRAM by use of some kind of plug-in adapter.
>
> Sure thing.
>
>> An example of what I'm thinking of is the SIMMfonie or AmiFast
Zip-to-SIMM
>> adapters for the Amiga 3000. Could something like this be built that
>> would plug into the 16 22-pin sockets on the PET motherboard, and lead
to
>> a single modern SRAM? Possibly with the addition of a couple of clip-on
>> leads to get two more address lines to the RAM adapter so that the PET
>> could have 32K.
>
> There is that big edge connector on the side of the static PETs (it was
> two seperate pin connectors on later PETs). All the signals you need for
> RAM expansion are there. For maximum preservation, I'd consider pulling
> all the 6550's out of the board and sticking them in a bag. Then, wire
> up a 6264 (or 62256!) to a small board after finding a suitable
connector.
> I expect the pinout to be on ftp.funet.fi, but if it isn't, I can dig
> out my PET schematics. If I still had my old 4K PET, I'd build one, too.
>
> You shouldn't have to build a clip-on, but if you did, there's nothing
> to keep it from working. You can get all the data bits from two sockets,
> and all the address bits from one socket, plus the additional bits
> from somewhere else. Remember that part of the logic in the PETs decodes
> the A8-A11 lines to chip selects. You need the original address bits,
> not the decoded ones.
Right. Here goes. Answers to one or two other people's questions may be
embedded here...

The original PET came with four different motherboard variations, viz:

RAM = 6550, ROM = 6540
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)

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.

All but the last few machines of the production run had blue-white phosphor
for the screens.

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...

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.

It is quite easy to re-encode these for addressing a large RAM chip
(62256). You need three 4-input NAND gates - two 7420 chips will do it.
(NB you are actually ORing block selects, but these are active low, so you
use NAND.)

(Also NB it is a long time since I looked at this. You may want to use AND
gates to keep things active low when you do chip select with the 4th gate)

The 4th NAND gate can combine chip select and clock and things - copy a
circuit from the motherboard (yes I'll send you the schematics if you want
- e-mail me privately).

If you're only attaching 8K, you need merely combine 2 block selects to get
chip select (a single AND gate), and use one of those for the top address
line A12.

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.

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.

If you've got a 4K PET with some dud 6550s, you'll have to find block
select 0 from somewhere on the mobo.

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.

Hope this helps. My explanations tend to confuse people...

Philip.
Received on Mon Nov 23 1998 - 10:47:34 GMT