It really wasn't doing a whole lot, it bleeped, PAGE cleared the screen, and not much else
So I consulted the service manual, I initially looked at the keyboard, in theory in LOCAL mode or with TX/RX connected we should hear a bell similar to the one we hear at startup when CTRL + G is pressed, but alas nothing, checking the serial line on a scope, and sure enough, nothing!
My first point of call was the keyboard, whilst PAGE was doing something, this existed outside of the keyboard matrix and was a switch which connected straight to the minibus backplane which contained all the control electronics
Oh yeah - this machine is build before the advent of microprocessors, it's combinational logic boards on a passive backplane all the way down! A similar construction to contemporary minicomputers!
After spending hours looking at the keyboard to understand it, I was rather blown away, this keyboard uses a 7 bit counter (using a 4 bit counter IC with some flip flops to extend it), feeding the lower 4 bits into a 4-16 line decoder which pulsed each row, and the upper 3 bits into an 8-to-1 multiplexer which scanned each column, when a key is pressed and the decoder pulses the row whilst the multiplexer reads the row, the output of the multiplexer goes low, stopping the clock going into counter, and the 7 bit number on that counter - it's the ASCII value of the key you pressed!
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