As I recall, the NCR Century 200 computer I worked on had a row of bulbs (not LEDs) on the front panel. It think there were around 16, and the operator manual stated that if there was an error, the computer would stop and the error code could be determined by examining the "number" in the bit patterns of the lights.Thankfully my employer had also invested in a state-of-the-art teletype (the sort that you put a typewriter ribbon in, and you put fanfold paper through with holes punched along the side), and the teletype would helpfully spit out a more informative message, like:Code: [Select]5100 INOPERATIVEAlthough you usually could operate "by ear" as if the job was printing a huge number of statements on the line printer (the fast printer), which made a terrific racket, and the printer suddenly stopped, you would then hear the teletype pathetically typing away in the background "5-1-0-0-I-N-O-P-E-R-A-T-I-V-E" which took about 5 seconds.
They were precision devices, and if they went wrong, well the print-out was hard to read.
I spent a lot of time repairing and working on those large 132 column line printers.
I think for simple reasons of space [line printers] only had upper-case letters
Hmm. I wonder what a modern high-speed printer looks like these days?
Printronix rings a bell. I think you are right, they had a line of dots which buzzed away as the paper passed them.As far as I have seen (and this was a little while back) they1 have very fast laser printers. There was also a technology, whose name eludes me2, where they put toner on the page but just used very high pressure to fuse it to the page. (Looks like the technology is called Ionography).1. High-speed production houses, like banks, insurance companies.2. Miltope, maybe.Also I believe they print "packing information" onto the side of each page of the letter (look at the bar code on the edge of a recent statement). This tells the envelope-insertion equipment how many pages go into one envelope (eg. for a multi-page bank statement).