Advantech ADAM-5511 Manual page 55

COM2: The COM2 port is dedicated as an RS-485 interface. Screw
terminals DATA- and DATA+ are used for making the COM2 RS-485
connections. Users have to prepare an ADAM-4520 RS232/485
converter for the linkage with PC.
COM3: This is a spare RS-232 port for advanced users which require
more than two ports in the system. But please do not send any
commands from the PC during the self-test period, when the ADAM-
5511 has just booted.