Dot matrix? Pah!
Real mainframes used the old line printers, with individual sets of characters on a band that went round and hit the paper like a typewriter letter. From what I remember when we used them on an IBM 370/148 they did about 600 lines per minute. The technology was handy for doing multiple copies as we had lineflo paper with carbon paper interleaved.
A colleague had worked for the Gas Board as an operator some years before, and one of their people had realised that if you printed certain combinations of characters repeatedly you got different sounds from the printer as the bands vibrated. They then worked on the details, and soon had a set of cards ready which, when run, got the printer to play the National Anthem.
I believe that was the same colleague who had a problem one day: the bosses were bringing important guests to see their ultra-modern machine room, but complained afterwards that it had been very dull and boring - they were all just sitting around doing nothing. Of course they were actually in the middle of a long billing run, and the numbers were being crunched. So before the next official visit they prepared a program which they ran as soon as the visitors were spotted: it suspended the real job, and then started lights flashing, printer lids going up and down, tapes rewinding and unmounting etc etc. Not exactly productive, but it impressed the hell out of the visitors!
Ah, the joys of real computers!