Today a memory of an old project that was a spectacular case of unfortunate timing came back to me. The British magazine Practical Electronics designed a desk calculator using TTL ICs. Unfortunately just as the 1st (or 2nd) part of 11 was published, MOS chips from Texas Instruments implementing an entire calculator came onto the market. And with better specs. The technology was obsolete even before it was published. PE decided to publish the project anyway for pedagogical benefit. Here is the cover of the July 1972 issue and the first 2 pages of that series:
The series has been archived here. It gets a mention in the EPE (Everyday Practical Electronics, two magazines having been merged sometime before) 50 year retrospective of 2014 (parts 1 and 2).
The component list is interesting. They weren't even Low power Schottky TTL packages, just standard TTL. Must have run a bit warm. 145 silicon diodes? Sounds like a microcode matrix.
I couldn't have built one anyway, it was far beyond my means at the time but it was interesting to read how the functions were implemented. I wonder if anybody other than the series author actually made one and wrote it up.
I needed to burn a 16kb (2kB) EPROM as part of a restoration project on an old piece of equipment. When I couldn't burn the EPROMs on my TL-866 programmer, I started investigating. Here's what I found out, summarised for your benefit.
Programming voltage
Practically all EPROMs of this era require 25V programming voltage. That's why my TL-866 couldn't do it, it failed on the first byte, it can only go up to 21V. I can understand why it's not supported, only a tiny fraction of chips require 25V programming. Maybe your programmer supports 25V. However some parts suffixed with A are 21V programmable so try that voltage first. I didn't have any A parts.
Pinouts
This article goes into detail about the differences. To summarise:
For the 2716, the Texas Instruments equivalent is 2516. The TI 2716 is a different beast, fortunately quite rare.
For the 2732, this time TI came out with the 2532, with a different pinout from the more widespread 2732. So TI 2532 != other 2732. Again my programmer cannot program 2532s, not just insufficient voltage but different algorithm too. If you need to only read them, say you want to put 2732 EPROMs in a machine using 2532 EPROMs, it's possible to make an adaptor that reroutes three pins. The article is only for subscribers, but if you click on the PDF image of the PCB, it will be obvious what they have done to the three pins 18, 20 and 21.
From the 2764 onwards, TI fell in line.
EEPROM substitutes
So I had no programmable EPROMs in my spares box, but fortunately I had a Xicor 2816 which is an EEPROM with pinout compatible with the 2716. It speeds up my development too as I don't need to go through the UV erase step.
28pin to 24 pin adaptor
If you can't get hold of a suitable chip or programmer, another way out is to build an adaptor using two IC sockets and patch wire so that you can use the much more widespread 28 pin EPROMs (2764 and above). Fortunately I don't have to do this (yet).
The series has been archived here. It gets a mention in the EPE (Everyday Practical Electronics, two magazines having been merged sometime before) 50 year retrospective of 2014 (parts 1 and 2).
