chip pinout help

From: Jim Battle <>
Date: Thu Aug 22 03:07:00 2002

At 12:35 AM 8/22/02 -0700, Eric Smith wrote:
>Jim Battle wrote:
> > 2) This one is a reverse job. The microcode board has quite a few mask
> > ROM chips. I don't care what the actual part number is, but I'm
> > hoping someone will say, "hey, that pinout is just like a ..." so that
> > I can find a prom programmer to read the thing. If I have to, I guess
> > I could build a parallel port dongle to read them. Here's what I
> > know:
> >
> > 24 pins
> > data out on pins 16-23
> > pin 1 is +5V
> > pins 2-8, 10, 11, 13, 15 are the address pins
> > pin 14 is either a chip select or output enable; active low
> > pins 12 and 24 are pulled up to +5; chip selects maybe?
> > pin 9 might be -15V (!)
> >
> > Embarrassingly, I can't find a connection to GND. unfortunately, I
> > don't know the backplane connections either, so I can't identify if
> > pin 9 is really -V or just an alternate ground (but I see a bypass cap
> > between the digital ground and this net, so it makes me think).
>PMOS parts typically ran on a single supply of around 12-15V. For TTL
>compatability, it was common to run them from +5V and -9V, with noground.
>+5V and -15V would effectively provide a 20V supply, which is more than
>was typical with PMOS, but not completely out of the question.

I had thought that maybe this was a possibility, and that input diodes
would clamp the output voltage to the digital ground, but I thought that
would be too hokey to be workable, and perhaps damaging to either the ROM
output or the clamp diodes on the input of the 75157 that the ROM is feeding.

Hmm, but then how is translation done on the inputs to the ROM? It is a
TTL device feeding the address, so the 0-5v swing (0.5v to 4+v) swing would
appear to be an input swing of +15 to +20.

I guess I'll have to tack down some wires so that I can look at the
voltages in an operating system...

> > One more thing, the manufacturer's logo and part prefix are "EA".
>"Electronic Arrays". They were pretty big in masked ROMs and keyboard
>encoders at one time. I'm not sure what eventually happened to them,
>but they were probably acquired by someone else.

Thanks for the great info. It looks like I'll have to build some kind of
tester myself to dump the ROMs.

Jim Battle ==
Received on Thu Aug 22 2002 - 03:07:00 BST

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