I've tried 2 more things: A PC with an FTDI USB-RS232 dongle works flawlessly with the TMA-1,

On the oher hand, an Arduino with a pair of RS232-TTL converters and the [Link: arduino.cc] sketch shows similar binary nonsense as a reply to commands that the TMA-1 apparently receives correctly, but, while the length of the response is consistent, the exact data returned varies, unlike what the RFX9600 does. That sketch also reflects data 1:1 if I short pin 2 and 3 on the far end.