Electronics > Repair

Vintage TMS2764 EPROMs acting strange... Schrödinger's EPROM?

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SilverSolder:
I took two TMS2764 EPROMs out of a piece of equipment, erased them, and programmed them with the latest firmware (with a MiniPro programmer).

So the weird thing is - it verifies 100% OK in the programmer...  I am able to read the code out and compare on the PC, 100% the same.  But the EPROMs don't work in the actual circuit!?!

I tried with some Intel EPROMS instead and they work fine.   All of the 7 TI EPROMs that I have are behaving the same way as described above...


I am struggling to understand how the programmer can read the EPROMS, but the CPU in the equipment they came from cannot?!?   :-//

MK14:
If it was a poor quality programmer, e.g. the proper programming voltages were too low, or not enough time was used, to properly program in the bits. I suppose, the programmer casually (relatively slowly) reading/verifying back the bits, might see the correct values.
But a relatively high speed processor (slow, by today's processors), needing rapid from the EPROMS (slow by today's standards, probably) data accesses, might be responding too slowly, because the programming has been done too faintly.

N.B. Above is speculation, there are other possibilities.

Have you covered the EPROM windows ?
Sometimes they can be sensitive to visible light, and stop working properly, without opaque labels on them. Ideal labels, seem to have aluminium (?) strips inside them, for near perfect no light leakage.

BrokenYugo:
Maybe the burn profile is wrong, check datasheet vs programmer voltages? I've heard of programming them twice to really make sure the gates are charged fully, maybe try that. 

MK14:
Also, the access times can vary between different chips (datasheets). Are the 'new' ones fast enough ?

SilverSolder:

The voltages and timing all checked out vs. the datasheet. 

I got them working by programming them multiple times with the same data, thanks for the ideas.

Maybe these devices are just getting old and cranky?  They are 30 - 40 years old, after all...

They seem to behave more like analog than digital devices!  :D

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