PROM Size on a 165

[Ludis Langens]

Mitch Louole wrote:
> 
> The schematic's I have seen on ECMGUY's site,
> show what looks like PROM pin 27 wired to A14 of the ECM bus.  (CS and OE
> are also reversed)  This does not make sense.  Pin 27 of the original 128
> PROM is /pgm pin, which should be tied high in the ECM.
> 
> I'm guessing the schematic was made from a later version ECM that takes 256
> PROM's.  (Hense the A14 connection.)

Thanks to Bruce, I have here a '165 so old it includes a white
Engineering Revision Jumper.  This old circuit board wires CPU line A14
to MEMCAL pin 4, which wires through to the EPROM's pin 27.  BTW, the
~CS and ~OE signals in a '165 class ECM are swapped from the way they
are in every other P4 I've checked.

> Does the MEMCAL not interface w/ A14 on the ECM board?  i.e. Does the MEMCAL
> tie PROM pin 27 high?  Can someone confirm that PROM pin 27 on a 165 ECM is
> held high all the time?

A 27128's ~PGM pin will be driven by A14.  While dumping out 27128
EPROMs, I've found that many of them ignore ~PGM when VPP isn't present.
 The rest treat ~PGM as an active high chip select.

As others have noted, you stand a good chance of finding a 27256 in a GM
'165 MEMCAL.  These EPROMs are usually all marked funny - I believe they
are 27256's with a bit error in the first half.

So - GM might very well have selected a specific vendor's 27128 that
works with ~PGM connected to A14.  Or they may have intentionally used
the 27256 rejects to avoid any problems.


ae2598 at wayne.edu wrote:
> 
> Is double-loading the thing to do, instead of just blanking out the second
> half?  I ran into the same situation at work, I took an image off a 128 to
> replicate, but only had 256's to use for the copies.  [...]  Anyway I
> took the 128 image, only write the first 1/2 of the chip, leaving the high
> side blank, and it seemed to work.

You need to put the '128 image into the _second_ half of the '256.  To
save programming time, put $FF's in the first half of the '256.  If
you're all thumbs, or don't have good software tools, you can put the
'128 image into both halves of the '256.  However, if you're really all
thumbs, this may confuse you even more when you go to edit the double image.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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