New subject: PET 2001 oddity - solved!

17 Jan 2003

On Jan 17, 18:18, Adrian Vickers wrote:
...

 Unfortunately, the PET seems to have developed an odd fault: It won't take
...
  a BASIC program, and some keywords seem to be
knackered... 
...
  So.... I figure the BASIC ROM has become slightly
corrupted, OR I've got a
...
  flakey memory chip which reads OK but doesn't
write properly. The question
...
  is, how to find out? 
Swap some of the RAM chips around and see if it makes a difference.  If it
does, particularly if it fixes it, swap them back -- it might just be a bad
socket contact.  Be careful with the RAM chips: if you have the type of PET
I think you do, they're MOS Technology 6550, aka unobtainium, and unlike
any other RAM chip.

...
  1) Can the BASIC ROM be swapped with one from, say, a
3032 or 4016? In
 fact, which one IS the BASIC ROM? 
No.  BASIC (and also the rest of the code, whether you call it a kernel or
a monitor, or "stuff") is spread over several chips.  And in the early
PETs, the ROMs too are MOS Technology specials, and the different versions
of PETs had different ROMs.  In fact, there was an upgrade for the
originals, because they didn't handle the IEEE routines properly, which
made it impossible to use disks properly (amongst other things).

...
  2) If it's a dodgy memory chip, what's the
best way of isolating it? I have
...
  an oscilloscope, but nada skill in this sort of
thing.
 3) If, as my money is on, it's the BASIC ROM, can it be replaced with an
 EPROM - if so, there's a whole gamut of additional questions to follow....

If it's a later unit with 24-pin 2332 mask ROMs, then a TMS2532 EPROM can
be used (not a 2732, nor other 2532s that don't have the TMS prefix).  If
it's got 28-pin MOS Technology MPS6540 ROMs, you'd need a carrier to
shuffle some signals, at the very least, and possibly some logic to handle
the multiple select lines.  The good news, though, is that I have a
chicklet-keyboard 2001-N as well, and if necessary, I could probably do a
ROM dump for you (though IIRC it used to be on the 'net somewhere).  I
wouldn't need to move more than a few hundreweight of stuff to get at it
;-)

-- 
Pete						Peter Turnbull
						Network Manager
						University of York