I told you powering from the PC could give problems....
A quick explanation: RTS means the PC is sending, so the xmtr is not allowed to send. Hence a RTS at logic O when the PC waits for an incoming signal.
With the RS-232 protocol, tne signal voltage is inverted, so we get a positive voltage on RTS in our case (when FMS is started). The diode (and resistor in series) prevents from bringing a negative voltage to the components.
and, as it is a Zener, limits to its nominal voltage. I 'd put a 4.7 rather than a 5.6V.
You may try with 'rts logic value powering' for more explanations.
Other remark: this topic had its place in "PIC interface" and not "Serialport -PPM- interface".
