Direct digital I/O ?

EM70..

About fifty years afo I made a battery portable tape recorder that used a DM71 (I think) as a recording level indicator - this was another use case for "magic eye" valves.

Dave

They were in the end cheaper than moving coil meters, and suited the high impedance valve circuits.

Once transistors arrived, MC VU meters were the obvious way to go, and stared to be churned out in millions

Then when LEDS turned up we used them as bar indicators.

Its all about what is cheap and does the job...

Not on the original 4080. Later machines had a lamp test, but only had a handfull of lamps indicating running, halted, reset, i/o, and so on, not things like register contents. That level of control moved to a serial (teletype) console.

These were minature bin-pin lamps, which were replacable from the front if you levered off the coloured cap. There was a special extraction tool which gripped the lamp and pulled it out from a deep recess. I never saw one of these in my 15 years there - we used the barrel of a ball-point pen which mostly worked, but would occasionally break the bulb. If you shorted the filament support wires, that instantly blew the 75 series driver chips which had no margin for any overcurrent, and that lamp then wouldn't work without a board repair.

On Sun, 3 Jul 2016 18:59:31 -0000 (UTC), Andrew Gabriel wrote in :

I think we had one somewhere -- can't remember which computer...

-- but a length of rubber or soft plastic tubing also worked often enough.