I hadn't even looked at the schematic other than to identify you were using a transistor pair to drive the output, etc. It's hard to read, is that rail voltage 6.2 or 8.2 volts? I should have a 7806 and maybe even a 7808 3 pin regulator in my parts bin.

Regardless, if this bus is electrically similar to the older 8192 baud ALDL, I have to wonder (and will experiment with) a simple resistor and diode arrangement as I used on my data logger.