chip pinout help

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Thu Aug 22 10:28:00 2002

On Thu, 22 Aug 2002, Jim Battle wrote:

> 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.

I think that the PMOS outputs are source followers with (usually external)
pulldown resistors, so that an 'on' PMOS output will be pulled up toward 5V,
giving a TTL high, and an off output will be pulled low by the pulldown
resistors giving a TTL low.

Old PMOS stuff is very tolerant of power supply variations, I remember using
old Nat Semi PMOS shift registers on a single 12v supply without any trouble,
though they were rated for +5,-12



>
> 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.


The inputs are referred to the 5V supply (and often require pullup resistors
on the TTL outputs for interfacing)

>
> 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 == frustum_at_pacbell.net
>
>

Peter Wallace
Received on Thu Aug 22 2002 - 10:28:00 BST