Doug wrote: << It is possible that Berkeley got the idea from working on the Univac (in the late 40's, when it was being designed by EMCC). I suspect that the earlier relay machines also used lights to help debug faulty relays, but I'm really looking for when lights were first used for output and switches used for input, similar to the way a bunch of the mini front panels worked. (It may be that Simon was the first -- I dunno.) Just a few weeks ago I looked at the Harvard Mark I, and I know very well the Zuse machines. No lights! John