Thx,
Yes, It should be High level, because on Rx and Tx lines constant High always when no signal transmission (silent). Phototransistor probably work well for your application, because your solution use only 9,6 Bauds speed. Try to use fast speed photodiode and BC847 transistor for example. This will give you much better rise fronts for short pulses. Best phototransistors have couple-tens uS for rise up, while photodiodes have 5-100 nS