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Computer startup procedure
Just powering a CPU puts it in some kind of "zombie state". The CPU needs to be reseted at least once to start normal operation.
The z80 needs to see an activated (on low) signal on it's reset pin. however, this operation isn't instantaneous. Documentation tells us that the reset signal should be present for at least 3 full CPU cycles to work.
The terminal used to talk to this computer is a dumb terminal that was once used to connect to an old french "teleinformatique" service over POT lines called "Minitel". In it's version 1B, the Minitel terminal exposes on it's back a (more or less) RS232 compatible port. The minitel operate either at 1200 or 4800 baud 7BIT ASCII with even parity. Upon booting up the terminal, the minitel is in an incompatible configuration for us, as it expect to connect to the network via it's internal modem and to the landline.
A series of keystrokes needs to be executed on the device to put in in an useful configuration mode:
- FCT + T; A : for putting it in "informatique" mode. This turns the minitel into a computer terminal
- FTC + T; E : for disableing the local echo. By default the minitel writes to the screen whatever you type on the keyboard. We want it to only display the computer's output
- FTC + P; 4 : for enableing the 4800 baud mode. This will be required as our serial interface will be hardwirered for 4800 baud operations.