I fired up my unregistered copy of DaveCAD this weekend and this is the circuit that I'm going to try for direct cat control with the pi. I don't remember if I needed the extra diode on the rx line but I didn't want to take the chance blowing up the uart on the pi. Is loosely based on the usb adapter I already use. It allows for "shorting" the rx and tx lines together for radios that use a single line. It should work for ptt or squelch signals too for radios that need it.
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73 de KC0WBP
Tim KC0WBP