22 Aug
2002
22 Aug
'02
3:07 a.m.
At 12:35 AM 8/22/02 -0700, Eric Smith wrote:
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(a)pacbell.net
Jim Battle wrote:
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...
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. 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. 
22 Aug
22 Aug
10:28 a.m.
On Thu, 22 Aug 2002, Jim Battle wrote:
At 12:35 AM 8/22/02 -0700, Eric Smith wrote:
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
Jim Battle wrote:
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. 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.
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...
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(a)pacbell.net
Peter Wallace
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. 
8456
days inactive
8456
days old
2 comments
3 participants
participants (3)
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eric＠brouhaha.com -
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pcw＠mesanet.com