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Mystery EPROM

M

mlaferriere

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May 19, 2016
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I found these two PROMs, on an archaeological expedition in my basement. I am thinking one of two possibility's, monitor roms from an X-80 (A Z-80 trainer made and used in house at Technical Career's Institute), or slim possibility out of an IBM or ITT ATW...? Any ideas? I have a reader here that I made back in the 80's for a TRS-80 Model I but neither one has been restored to the point I would apply power yet.

IMG_20160622_193148 A.jpg

Possibly 1987 and the version number...?
 
Good point on the memory, but have many manufacturers used EPROMS instead of roms? Perhaps custom test firmware for an early PC? As you said, a trainers monitor ROM would not need over 4K even with Tiny Basic.

I am trying to think about what and why I would have grabbed these from. I worked at Radio Shack from 86-87 and then 87 to 91 (or so) at MassMutuals PC Repair. We had IBM 5150's, XT's, AT's and then went mostly to PS2's. MassMutual also had HP terminals, VT102s, Wang PC's and a bit of other 'weird' stuff.

Other than those two places and times I'm not sure where else I could have come across these. Even if I saw these at a computer show I was usually pretty cheap and wouldn't buy something unless I had a immediate use for it.
 
These aren't EPROM's. EPROM's are UV erasable and have a quartz window over the die. They might be EEPROM's (electrically erasable) but they're typically named 28Cxxx instead of 27cxxx. I've seen this before, and I have a faint memory of seeing 27cX series EPROM dies, but in packages like that, effectively making them OTP rom.

If you dump them, see if you can write to them as "28c256". If so, they're EEPROM's. Don't try and write them as 27C256 EPROM's, that'd probably kill them.
 
Actually they are EPROM's - just "OTP" or one time programmable EPROM's. Less costly without the quartz window. They program like any other 27C256's, just no way to erase.

The 57C256 is also a 27C256 variant - just a very fast WSI part.

Christoffer said:
These aren't EPROM's. EPROM's are UV erasable and have a quartz window over the die. They might be EEPROM's (electrically erasable) but they're typically named 28Cxxx instead of 27cxxx. I've seen this before, and I have a faint memory of seeing 27cX series EPROM dies, but in packages like that, effectively making them OTP rom.

If you dump them, see if you can write to them as "28c256". If so, they're EEPROM's. Don't try and write them as 27C256 EPROM's, that'd probably kill them.
Click to expand...
 
They program, just like and other 27xxx parts. I said earlier that they were
OTP PROMs.
These are already programmed and can not be erased. ( at least
I assume so since they have labels indicating that they have been programmed )
These can be read on any programmer that programs 27C256 parts.
These are not like 28xxx parts and do not program like them.
Dwight
 
27C256 are 32kB. Quite big memory as for simple monitor program.
If for 8-bit computer, then probably with something else, but I think they are from some PC-clone or expansion card.
It is needed to read them, then analyze what is inside dump files. Look in hex, maybe interlace them to one binary if you see strings with even/odd characters missing.

There indeed were some electrically-erasable chips marked as 27C256 EPROMs, probably Winbond manufactured them, they had strange Vpp, but these chips in photo are OTP (one time programmable) chips.
 
I've found OTP ROMs of this type in specialised POS terminals based on Advantech micro-ATX. They hold BIOS plus proprietary extensions, supposedly to ensure security, but also to ensure that the user has to go back to the vendor for any modification or upgrades. Non-flashable BIOS.
 
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