On Mar 21, 17:30, Philip.Belben_at_powertech.co.uk wrote:
> I would have thought a 2101 was far too small. These are 4kbit chips.
Correct :-)
> I agree 200ns probably isn't essential - my PET of that date uses 450ns
2114s.
Yes, I'm sure that's true. The 2114 PETs are rarer, but I can't see any
reason to use 200ns devices, except that's what MOS Technology (who were
largely owned by Commodore) were making.
> (FWIW there were FOUR motherboard designs for the early PETs, based on
all
> permutations of 2114 or 6550 RAM and 2316 or 6540 ROM.
I've got the schematics for all 4 types. They're almost identical except
for slight differences in the memory decoding.
> The thing that bothers me is that it says 3071 bytes free. This is
EXACTLY the
> number of bytes free you get on a 4K PET.
>
> My advice - Identify the suspect pair of chips (remember these are 4-bit
wide
> parts)
Oh, I did that to check that it was definitely the RAM and not anything
else, long before I posted my request. I could fix about anything else on
the PET, but I draw the line at grinding the top off to poke at individual
flip-flops in an IC :-) FWIW, the first job I had in
computers/IT/whateveryoucallit was looking after (and repairing at
component level) PETs and other micros, in 1981.
The PET determines the RAM size by reading and writing a byte in every
block and assumes it has found the top of RAM when it gets an error. It
was just coincidence that one of the 5th pair was the one to go. So I now
have a 7K PET, because I put the faulty IC in the top pair. It reports
6143 bytes free.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Tue Mar 21 2000 - 14:27:24 GMT