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What was the purpose of all of the blinking lights on the early computers (think Univac and Eniac) of the 1950s & 1960s? What information did they convey?

I am almost 78 now but I started on computers in the early 60s. I was working for IBM at the System Development Corporation (SDC) in Santa Monica, California in 1965. I supported the AN/FSQ-32, which was the largest transistor (solid state) computer ever made and it weight 60.3 metric tons (132,960 lb). You could walk around inside the computer system. I had to learn every pulse within this system. It took me 9 months at 40 hours a week to learn the entire system so that I could trouble shoot any hardware problem.

I also worked on the IBM AN/FSQ-31 SAC Data Processing System at Offutt Air Force Base near Omaha, Nebraska. Both the Q31 and Q32 needed 24x7 support by electronic technicians or as I was called an IBM Field Engineer assigned to a job site. I did hardware support for the 8 years I supported these systems before going into programming full time. Below is a picture to what I saw for 8 years. Many of the lights show the binary value of each register with in the system. If you know how the system works, then you can look to see if the register has the correct binary information!

The man in the picture is touching a console that has a possibility of four instructions. You had to put your instruction in binary because each row of instructions had an on/off switch for a one or zero. Many times I would put an instruction (Like LDA for load accumulator) in the first row, then a branch back to that LDA instruction in the next row. I may be able to look at the blink lights and determine a problem because one bit was not able to change from a one to a zero. I often would put the computer in a loop using the switch console, then go find where I thought the problem was located and then use an oscilloscope to finish the trouble shooting.

 If I found the problem I may have to change out a "flip-flop" circuit so that the machine would now run correctly. Most of the time, the lights were used by the hardware field engineers for troubleshooting but some time by the systems programmers and very seldom by the application programmers. Note: about 30% of the hardware in the Q31 and Q32 was error checking hardware. This system used parity bits for all data transfers, therefore if any bit failed out 24, the parity bit would be in the wrong position and an error would be sent. You could set the computer so that as soon as the error occurred, the system would completely stop with the registers showing the status of the entire system. 

Again, if you know how the system worked, you could look at the registers and make a reasonable assessment.

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