ID UV erasable PROMS used on an IBM PC board?

D. Resor organlists1 at
Sun Mar 20 22:05:00 CDT 2022

This is the CPU board out of a first generation 1982-83 IBM Electronic 85 Typewriter (Electronics Driven Selectric). 

I was looking to archive the software/firmware from these.  The machine was exposed to some dampness.  Corrosion has ensued on the interface connector between the CPU and driver boards.  

The next year, more energy efficient memory ICs were used.  Memory power failure back up only consisted of 3 AA batteries and could last up to one year.  The predecessor (this board) required 6 AA size Nicads and would only retain memory for a few hours.

IBM using their apparently very large stock of OLD aluminum covered ICs in as many products possible I guess.

Don Resor

-----Original Message-----
From: wrcooke at <wrcooke at> 
Sent: Sunday, March 20, 2022 7:13 PM
To: D. Resor <organlists1 at>
Subject: Re: ID UV erasable PROMS used on an IBM PC board?

> On 03/20/2022 8:59 PM D. Resor via cctalk <cctalk at> wrote:
> I cannot find a datasheet by any of the numbers silkscreened on these ICs.
> Could these be proprietary IBM P/N numbers?
> No need for a Dropbox account, close the login pop up and you can view 
> the image.
> Thanks
> Don Resor

More details would help.  What is the board?  Do you know at what address in the PC memory map they fit?
Based on the info you gave and the picture  I would bet $1 they are standard 2764 chips.  The 2764 was the first standard chip to have 28 pins.  The size of the die visible through the quartz window is consistent with 2764 (as opposed to 27128 or 27256) and the fact there is room for 3 which would give 24K total.  The PC didn't have a lot of places in the memory map that would allow more than 24K.  Three 27128s would be 48K (a LOT in those days) and the 27256 would be 96K.

I can't help with the part numbers.  But I doubt they are IBM proprietary.  The vast majority of chips used in the early PC line were standard from other companies.

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