I'm interfacing with a system using RS-232. To communicate, I use MAX2322 for TTL <-> RS-232 to interface with PIC microcontroller and PL2303 for USB <-> RS-232 to interface with PC.
All of the converters I've used, when not connected to the endpoint, have a symmetric signal on TX pin (+-7, +-6, +-10, depends on the converter). When I connect to the endpoint, the TX pin keeps the voltage for level 1 but the voltage for zero rise 2 or 3V (+7 -4, +6 -3, +10 -7).
That's not a problem (the system still works), but I'm trying to understand why that happend.
The communication uses 4 wires (GND, TX, RX and DTE) and exchange data in a 1200bps rate.
Can you help me understand why that happen?