1024x8 bit BPROM & EPROM question

[Ludis Langens]

Louis Baiani wrote:
> 
> Compared to the original 1kx8 BPROM found in my ECM (84 vehicles), do
> you think a standard EPROM (1kx8 2708) would work?  You're talking
> probably 450ms access time compared to 50ms with the actual blown fuses
> in a BPROM.  I wonder if the ECM cares enough to really read this that
> quickly or is it picky and that's why GM used BPROM.  Has anyone ever
> suceeded in using a standard EPROM (if it's a fast EPROM, then that's
> different).  Any feedback would be appreciated.

I assume you mean 50ns and 450ns, not milliseconds.

Here's why GM used bipolar PROMs in the earliest ECMs.  Back in 1980
(when GM started using PROMs), the three supply voltage TMS2716 was king
of the hill.  Single supply EPROMs as well as larger EPROMs were still
in the "advance information" and "sampling" stages.  That was especially
true a year or two earlier when GM had to commit to a design. 
Automotive temperature grades and speeds faster than 450 ns were likely
unavailable in MOS EPROMs.  Further, EPROMs were expensive - they were
meant for prototyping and extremely small production runs.


Scot Sealander wrote:
> 
> Louis Baiani wrote:
> 
> > Anyone know where to get these?  ECG's 74S478 is not longer available I
> > guess.  I wonder if a PROM/UV prom or ERPOM would work instead.  They
> > refer to it as a DIL24 package I guess.
> 
> If you put the program in 4 times, it should work in a 4kx8 EPROM, you
> know, the 2732 series of EPROM's used in the C3 ECMs.  This is four
> times as big, but it will probably still work.  I don't know what your
> application does with the N/A pins that are used on the '32, so load it
> four times.  One of the four ought to show up in the right place.

Won't work.  There are reports that a Texas Instruments TMS2532 might
work.  Lets analyze the situation.  There are four pins of interest. 
The twenty other pins are the same in all the 24 pin PROMs/EPROMs of interest.

Here's how a CrossFire ECM connects the four pins:

  21  A10 (from CPU)
  20  ~CS (from 74138 decoder)
  19  Unknown, possibly R/~W, possibly 5 volts
  18  ROMEN (from edge card, normally pulled to 5 volts)

Here's what these pins do on a 1KByte PROM:

  21  ~E1
  20  ~E2
  19  E3
  18  E4

So, during a normal PROM read cycle, 18 and 19 are both high, 20 is low
for addresses from 0x3000 through 0x37ff, and 21 contains the 0x0400
address bit.  This means the PROM responds to addresses from 0x3000
through 0x33ff - exactly the used range.  Data that I have suggests that
2KByte PROMs use pin 21 as their additional address input.  This means
that GM designed the PCB for future upgrades.

Now lets look at how 24 pin EPROMs use these four pins.  I've omitted
the 2758 because it is just a half tested 2716.

      2732  2716  TMS2532  TMS2516  TMS2508  TMS2716  TMS2708
  21  A11   VPP   VPP      VPP      VPP      -5 V     -5 V
  20  ~OE   ~OE   ~OE      ~OE      ~OE      A10      ~CS
  19  A10   A10   A10      A10      NC       12 V     12 V
  18  ~CE   ~CE   A11      ~CE      ~CE      ~CS      PGM

>From this table, it can be seen that the Intel pinout devices will never
be enabled in GM's circuit.  The smaller 5 volt only Texas Instruments
pinout devices also won't enable.  And your crazy to even consider using
a tripple supply EPROM.

Of the 24 pin EPROM choices, only the TMS2532 might work as is.  The
only problematic pin is 21 (VPP).  It is supposed to be held at 5 volts
during normal EPROM operation.  I have a data sheet saying it may draw
as much as 12 ma of current.  Perhaps this isn't an absolute
requirement.  In any case, the PROM image would have to placed in the
upper quarter of this EPROM.


74S478 isn't the only part number of a 1KByte PROM.  These are all
equivalent:

  MMI 6381
  Fairchild 93451
  Harris 7681
  Intel 3628
  Intersil 5618
  Motorola MCM7681
  National DM87S228
  Raytheon 29631
  Signetics N82S181
  Texas Instruments TBP28S86
  Texas Instruments SN74S478

It should be possible to find small numbers of some of these chips.

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


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