> Yes, I'm sure that's true. The 2114 PETs are rarer, but I can't see any
If you used to service the things, I have to take your word for it, but I've
seen several with 2114s and I don't think I've seen any with 6550s. Oh well.
[...]
> 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
OK, so you are sure that it is the chip that's dead. I still think it's a fault
that affects the whole chip...
> 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.
I'm afraid I have a minor quibble here. To write a byte to every block would be
pointless as a memory diagnostic, although it would indeed find the top of RAM.
However, the PET actually writes two test patterns to EVERY BYTE. PETs I've
seen with dodgy RAM come up with really strange numbers of bytes free, not just
multiples of 256 minus 1. Early PETS like yours (and my first one) leave 36 in
each location when they've finished. Goodness knows why. Later PETS seem to be
using 01010101 and 10101010 as test patterns, since they leave 170 (decimal) in
each location.
(As an aside: vintage PET users, on old machines at least, will have come across
the corrupted links in the program listing, which instead of pointing to the
next line point somewhere silly. The most common crash then is a program
listing that just prints $ signs and doesn't respond to the stop key... Can't
remember what keyword 170 gives you on the newer machines, though.
When I was involved in developing an adventure game for the BBC micro, we typed
a lot of the text on an 8032 (can't remember why). 170 ended up as the token
for "HA" when we compressed it. Result - a corrupted address made the machine
laugh at us. No, this was not intentional!)
**********
Expansion box.
The old PETs don't AFAIK have the internal expansion connector - they have an
edge connector sticking out at one side. Very useful for toggling RESET with a
pair of tweezers!
That aside, on this edge connector are brought out most of the block select
lines from the 74154 I mentioned in my previous post. In particular, brought
out are lines 1 to 7, 9, A and B.
It takes only three four-input AND gates to re-encode any eight of these you
choose into three upper address lines for a 62256. On my PET I used 2 to 7, 9
and A, and got 32K main memory (including the 8K it already had) plus 8K of RAM
in ROM space, above the screen memory.
My suggestion is: disable the select to the upper 4K of memory within your PET,
and encode lines 1 to 7 and 9. This will give you 32K of main memory, several
spare 6550s, and 4K of RAM above the screen. Essentially, you will have a 4K
PET fully expanded.
Power, as always, comes from the second cassette port...
Note block select 0 is NOT present on the expansion connector, so you can't
readily disable all the internal RAM.
Philip.
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Received on Wed Mar 22 2000 - 08:52:04 GMT