B&B Electronics Celeron M Universal Network Controller with PC/104 Extension UNO-2170 - Manual (Page 15)

2.3 RS-232/422/485 Interface (COM3~COM4)
The UNO-2170 offers two RS-232/422/485 serial communication
interface ports: COM3 and COM4. Please refer to Appendix A.4 for their
pin assignments. The default setting of COM3 and COM4 are RS-422/485.
2.3.1 16C550 UARTs with 16-byte standard
Advantech UNO-2170 comes with TI16C550 UARTs containing 16
bytes FIFOs.
2.3.2 RS-422/485 detection
In RS-422/485 mode, UNO-2170 automatically detects signals to match
RS-422 or RS-485 networks. (No jumper change required)
2.3.3 Automatic Data Flow Control Function for RS-485
In RS-485 mode, UNO-2170 automatically detects the direction of
incoming data and switches its transmission direction accordingly. So no
handshaking signal (e.g. RTS signal) is necessary. This lets you
conveniently build an RS-485 network with just two wires. More
importantly, application software previously written for half duplex
RS-232 environments can be maintained without modification.
2.3.4 Termination Resistor (JP6)
The onboard termination resistor (120 ohm) for COM3/COM4 can be
used for long distance transmission or device matching. (Default Open.)
9 Chapter 2