[Diy_efi] Re: What type masked ROMS on early motronic ecu's

[Phil Hunter]

Mike wrote:
> ... So how does one read a cdp1834 masked rom on a standard eprom
>  reader?

I made it to a local electronics surplus store just before they
closed Saturday, they have a library of old data books. The 1834
is "pin compatible" w/ the 2708 eprom, pins 19 & 21 are no-connects,
18 and 20 are active high chip selects.

Since the 2708 uses +12V, +5V, & -5V, I'd play it safe and read
it as a 2716 (+5 only) and ignore half the data. Might bend out
pins 19 & 21 so they don't make contact, sometimes N.C. pins are
connected internally. For the same reasons, use a 27C16 if you're
going to modify the code, may need to tie the N.C. pins high or low.

You might find useful info here: http://www.cosmacelf.com

Be sure to upload the image to the ftp site if you get it out
of the ROM.

Bruce wrote:

> It would seem you could watch how the 1802 reads them.
> A mask rom just has different control signals in read mode.
> Emulate that, or worst case capture the input address and
> data out on the fly as the 1802 runs.  Might be hard to get
> everything out that way.

The 1802 is fully static, so one might replace the clock w/
some kind of "one-shot" and step along w/ a push button one
instruction at a time.

The ROM might also be an 1833 which has some kind of internal
address latch to eliminate external decode logic, so single
stepping would be one way to figure that out.

Jeff wrote:
> ... has quite different '0' and '1' thresholds.  Try driving
> it with HC family logic, TTL won't work.

TTL can work. "classic" CMOS has a logic threshold at one half
of the power supply, if the supply is 5V then the threshold is
2.5V. "classic" TTL has a minimum logic high output of 2.4V,
adding a 4.7K pullup resistor is enough to have TTL drive CMOS.
I'd be surprised if a prom programmer wasn't designed to be
compatible w/ both TTL & CMOS EPROMs.

rgds,
philh
(digest)

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