The following settings work well for me:
Interfaces: Serial: SerialPort 0:
Enable, set Baud Rate, Data Bits and Parity to match the connected serial device. Set Flow Control to NONE.
Legacy Protocols: MODBUS Gateway: MODBUS 0:
Enable, Async Port = 0, Async Mode = RS232, Duplex Mode = Half, Operation Mode = Act as Master, Idle Gap = 20 ms, Fix Slave Addr = 0, Adjust Slave Addr = 0
In the IP Port Table: Port 502, 1 Socket, TCP Mode