Output Drive

Published Wednesday, August 1 2012

A couple of comments here and over on the Vintage Computer Forum prompted me to pull out my copy of Don Lancaster’s “TTL Cookbook” to verify my assumptions. As is so often the case, my assumptions were wrong.

The original TTL family used in the KD11-A, the venerable 7400 series, can drive a maximum load of 16mA per output, and consumes 1.6mA per input. The more modern (and cheap and easy to find) Low-Power Schottky 74LS00 parts on the other hand can drive a maximum load of only 8mA per output.

Well. Damn.

This doesn’t mean I can’t use 74LS00 parts as replacements, of course. But it does mean that I have to be very careful where I use them. I will have to be absolutely sure that I am driving NO MORE than 5 inputs from any output if I want to use an LS part.

The 74123 that I want to replace, for example, is safe. One of the outputs is driving three inputs, and the other output is driving only one.

The 7404 that I’ve already replaced, however, is no good. I will need to undo that fix immediately. One of the NOR gates is driving ten outputs! That’s the maximum you can drive from TTL to TTL, and twice as many as you can drive from LS TTL to TTL.

So, more delay while I get some original TTL parts. There’s a shop here in town that sells them, but I’ll need to take a couple of hours off work to drive there. The things we do for our hobbies!