Framing error if I disable transmitter after shift register is empty?

[Ioannis]

Since the output on that specific chip will remain as driven output, and if you insist on using this, a crapy solution maybe to add a diode at the output before the pin is connected to the bus.

The cathode to the PIC usart output and anode to the bus.

Like an OR connection of all the PICs that will be in parallel. Then a pull up will hold line to HIGH state.

This is subject to capacitances over the bus and the pull-up resistor may not be able to charge that capacitances fast enough but you can test it and see if your are OK with the speed you may obtain.

The chip that you found to be faulty, may have been hit by ESD charge. Are you familiar with ESD protection? Are you taking measures when you handle ESD sensitive components?

Ioannis