[Diy_efi] RE: looking for help with a prom N82S181

Ludis Langens ludis at cruzers.com
Wed Nov 10 05:54:36 GMT 2004

Steve Ravet wrote:
> There are a couple problems here.
> Pin 21 nCE1 (vpp):  The trace shows this signal toggling.  vpp=gnd is not listed in the datasheet (ST microelectronics) that I'm looking at.  vpp should always be either Vcc (5V) or Vpp (25V).  connecting it to ground may be OK, or it may keep it from working, or it may damage the device.  Assuming you don't want to program the chip in circuit, then wire this pin to Vcc.
> pin 22 nCE2 (/oe):  In 2716 read mode this pin is equivalent to the PROM nCE2 so this pin is OK.
> pin 23 CE3 (a10):  toggles continuously in your trace.  CE3=a10=Vcc is OK (will read from the upper half of the EPROM).  But CE3=a10=GND is not OK.  In this case the PROM would be tristated but the EPROM will drive data onto the output bus, potentially conflicting with another device.
> pin 24 CE4 (/ce):  seems to be permanently wired to Vcc to enable the PROM.  Vcc on this pin will permanently disable the EPROM.
> *IF* you can determine that CE4 is always driven high, ie connected to VCC, then a simple way to fix pins 23 and 24 is to wire pin 23 on the EPROM (a10) to ground, and to invert the signal at the pin 23 pad and then connect that to pin 24 of the EPROM (/ce)
> If CE4 is driven by logic then you still have to connect EPROM pin a10 to ground, then OR the pin 23 and pin 24 pad values, invert that, and connect to EPROM pin 24.

I suspect that Marcello simply needs to "rearrange" the four pins.  In
ECMs of this era, GM seemed to use a 74138 type 3to8 decoder to drive an
active low chip select.  That'll be on one of the active low chip select inputs.

A CPU address line or two might be brought over to the chip selects - to
allow for a bigger PROM or to provide extra decoding.  This can probably
be ignored.

The R/~W line might also be brought over to one of the active high chip
selects.  This way the PROM stays disabled during CPU writes to it's
address range.  You don't need to simulate this.

My instructions to Marcello are:  Pull out an ohm meter.  Find what pins
20 and 21 are wired to.  One of them almost certainly will connect to a
16 pin DIP package.  That will be your active low chip select.  Connect
it to pin 18 or 20 of your EPROM.  Connect the other three pins of your
EPROM to appropriate logic levels.

BTW, consider using a 2732 or 2532 - you may need to do less pin swapping.

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