this is why I am looking for the MPP 1000e pinouts. The serial driver has already been written. Very low speed but I don't care about that.
Just a remark, it will not be easy to bit-bang the pia to generate or read serial data.
I had to wire my ESP8266 to joystick-port-1 to get it from a weird 135kbaud uart setting back to life.
Needed to disable NMI/IRQ, kill antic and start sending/shifting on the exact same scan-line.
I used ROL to shift bytes out, were TX was bit-0.
Otherwise, the outgoing bits were not evenly spaced.
But I guess on much lower baud-rates, this is less critical.
Bytes need to be send out most significant bit first, start-bit is zero.
success with this little project :-)