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Questions about CCS 2422 Proms

pjh

Experienced Member
Joined
Apr 19, 2007
Messages
82
Location
Saraland, AL just north of Mobile
After acquiring a 3rd CCS 2422 FDC and checking it out, I was able to boot the CCS CP/M disk
that I have. I have two other 2422s that I couldn't get to work after chip testing
everything on those boards except the PROMS. PROMS have been a gray area for me but I
figured that these must be the problem on the first two boards. I listed the PROM chips on
each of the boards and this is what I have:

U22 U23
--------- ---------

1st 2422 IM5623CPE IM5623CPE

2nd 2422 MB7057 IM5623CPE

3rd 2422 IM5623CPE TB24S10N

Next I wanted to test each of these chips in the other boards. The 3rd board still worked
with each of the U23s , so I have three good U23 chips. The 3rd board fails with each of
the other U22 chips. The 1st and 2nd boards work with the 3rd board's U22 chip.

I then decided to delve a little deeper into this gray area of PROMs and bread-boarded a
little debounced single-step circuit to dump the contents of the chips out to a TIL311
hex display.

The U23 chips have 'F's in all of the memory locations except these:

0 1 2 3 4 5 6 7 8 9 A B C D E F
---+---------------------------------------------------------------
F0 D D D D D D C C F F F F F F F 7

The working U22 chip has this code:

0 1 2 3 4 5 6 7 8 9 A B C D E F
---+---------------------------------------------------------------
00 F F F F D F F F F F F F F F F F
10 F F F F F F F F F F F F F F F F
20 F F F F F F F F F F F F F F F F
30 B B B B 5 F F F F F F F F F F F
40 E F F F F F F F F F F F F F F F

Not the 'D' in location 04, the 'B's in locations 30-33, the '5' in location 34 and
the 'E' in location 40

Every other memory location in the U22 from 50 to FF has 'F's in it.

This brings up the questions that I have about the 1st board's IM5623CPE and the 2nd board's
MB7057.

Both the 1st board and the 3rd board have IM5623CPEs as the U22 chips. In fact, both '5623
chips have the exact same programmed contents. But the 1st board's '5623 fails to work and
the 3rd board's '5623 works. Is there a characteristic of Bipolar Proms where a chip could
be coded correctly, have the contents readable, but still fail?

The 2nd board's MB7057 is programmed with the correct data but in the wrong memory location,
so I haven't botherd to show its contents. However, I have a question about it. The MB7057
is an Open-collector chip. The '5623s are Tri-state. In what searching I have done on
Bipolar Proms, I seem to recall a mention in another forum that in some circuits, using either
Tri-state or Open-collector Bipolar Proms wouldn't matter. If the MB7057 had been
programmed correctly, would it have worked or would it have to have been an MB7052
Tri-state chip?
 

dave_m

Veteran Member
Joined
Feb 2, 2009
Messages
3,429
Location
Southern California, USA
It is possible that the set up time from address lines in to data out is out of spec slow. You can not tell that from your manual tester. Those old parts had high internal delays.

The open collector part would only work if the PROM output circuits have pull up resistors. Check the schematic.
 

pjh

Experienced Member
Joined
Apr 19, 2007
Messages
82
Location
Saraland, AL just north of Mobile
Thanks for the reply. I assume then that the 1st board's '5623 was an after-market purchase of a chip by a previous owner, in an attempt to duplicate an original Prom, only to find out that it didn't work.
I am hoping that I can program a 74S287 as a replacement.
 

Steph

Member
Joined
Nov 13, 2019
Messages
34
Location
Ottawa, Canada
Both the 1st board and the 3rd board have IM5623CPEs as the U22 chips. In fact, both '5623
chips have the exact same programmed contents. But the 1st board's '5623 fails to work and
the 3rd board's '5623 works. Is there a characteristic of Bipolar Proms where a chip could
be coded correctly, have the contents readable, but still fail?

It is possible that the set up time from address lines in to data out is out of spec slow.


Indeed, this is referred to as "read-access" speed.
( The time is takes to retrieve the data from the memory cells and push it on the data bus ).

Proms are significantly faster the eproms ( 30-50 usec for proms, 150-400 usec for eproms),
so they are often for used rapid access info like color palettes, sound frequencies,
computational values, etc..

[ Note: The SMALLER the read-access time, the FASTER the chip. So 30 usec is faster than 50 usec.
That's always been confusing. ]


But like the rest of us, things slow down with age.
It's not common, but proms and eproms do sometimes slow down to the point
where they can not access the data fast enough for the application.

The data would look fine on a programmer, but would not work in-circuit.

This is possibly what is happening.



The MB7057 is an Open-collector chip. The '5623s are Tri-state.
I seem to recall a mention in another forum that in some circuits, using either
Tri-state or Open-collector Bipolar Proms wouldn't matter.

I've read that too, but in my experience that only works
if you replace an open-collector with a tri-state. ...not the other way around.

As mentioned above, Open-Collectors need a pull-up resistor.
I would assume that a Tri-state prom would likely work in this situation
and not make use of the pull-up voltage.

However, putting an Open-collector prom instead of a Tri-state
would be missing the pull-up resistor, so it would not work.

My guess is that your MB7057 was used by mistake. You should probably be using a Tri-State prom.


Clear as mud ? ;-)

Cheers,
Steph
 

pjh

Experienced Member
Joined
Apr 19, 2007
Messages
82
Location
Saraland, AL just north of Mobile
Steph,

I did wonder if proms would deteriorate in performance over time but wasn't familiar enough with them to know off hand.

I can see what you mean by being able to put a Tri-state in place of an Open-collector but not the other way around.

Phillip
 
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