The hardware consists of one eight bit output port for the parallel data
(port 0B6h), one other output bit for a data strobe (bit 4
of port 0B5h) and one input bit as a ready signal (bit 3 of
port B5h).
To send a byte the printer driver outputs the character to the data port and then waits until the ready signal goes low. When the ready signal is low it strobes the data by setting the data strobe low for a few microseconds and then setting it high again (it is normaaly high when not in use). This completes the sending of a character.
The other bits of port 05Bh are used for various control
operations such as scanning the keyboard, and controlling remote control
relays. A variable PORTB5 which is at a fixed address
defines the current state of this port and the printer driver ensures
that all other bits of the port are maintained in their correct state.