Comment by birdsongs
2 hours ago
My fav job was an internship with a EW company that made these massive rackmount radar systems. My boss, the lead engineer, loved blue LED status lights. So the massive boards we made had hundreds on them. There were thousands total in the rack. GPIOs, busses, fpga signal outputs, all got an individual LED and mosfet.
It was crazy when it was running. You could visually debug a bus, if you wanted to.
My dad programmed minicomputers, which normally had LEDs on the front panel that showed the binary contents of the program counter, and maybe the current instruction and some other status indicators. I remember him saying that he could tell if his program was running normally by the patterns in the lights.