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I want to tee into an existing keyboard so that I can enter from say MS Excel. Many, many moons ago I had something working, and it used I think 7400 he hardware is what I am looking for now. Excel was not used in this case. When a sensor went high it messaged a previous set up decade counter,(for the order sequence required, which sent the 'momentary' into the tee of keyboard. Then waited for another sensor to be triggered.

No micro was used. Just a piece of veroboard with decade counter, 7400? battery, and possibly optos between 7400 and keyboard.
Now at 76 my brain is clouded in fog and I cannot find the said board or remember the components. But it was quite simple.
Thankyou

 

Keyboards have a USB interface these days. I don't think a handful of MML will be much help.

 

I cannot figure out what you are trying to do.
You have a PC keyboard and you want output from Excel to activate (mimic) a key press? That doesn't make much sense to me.

Perhaps you should ignore the perceived solution for now and give us the big picture of what you are attempting to do.

 

Oops. The keyboard is in a vintage model train controller called Hornby Zero 1. (no pc involved etc)