Blinky lights WAS: The SR71 Blackbird was designed ENTIRELY with slide rules. fwd 714

Beasts like the B6700 had a large panel of light used for debugging purposes. It showed the Burroughs B when the system was idle, and the letters D-U-M-P when it was taking a memory dump. Although I never saw it myself it was known the (British) Police National Computer Unit system displayed a picture of a pig when idle.

I operated the less powerful 'medium systems' which also had some lights, such as the comparison registers which showed if a comparison had tested higher, the same, or lower. When sorts were running the relative brightnesses of these lights gave experienced operators a feel for how much longer the task would take.

The medium systems also had a Nixie display (Burroughs patented the Nixie) which displayed numbers. It was needed when typing in the sequence to tell the machine where to get its boot instructions (usually from cards) where the operator actually typed in an operation and operand and told the machine to execute it. Also if the machine halted due to a fault then you could see at a glance from the 300094 shown that it had just executed the 'Branch Communicate' instruction to halt itself.

The later Burroughs Small Systems had a really nice trick, in that you could set the machine to show you the system CPU usage, individual program CPU usage, memory usage, IO channel usage and so on from a 'bar graph' of the lights (i.e. all on = 100%, left-most half = 50% and so on) or from the relative brightnesses of lights, where fully-on = 100%.