6.3 RS-485 Signal Wiring
The RS-485 standard supports half-duplex communication. This means that just two wires are needed to both
transmit and receive data. Handshaking signals (such as RTS, Request To Send) are normally used to control the
direction of the data flow and to switch the transmission accordingly. In RS-485 mode, the PCM-24D2R4/24D4R4
cards automatically sense the direction of the data flow and switch the transmission direction
— no handshaking is
necessary. This means a user can build an RS-485 network with just two wires. This RS-485 control is completely
transparent to the user. The software written for half duplex RS-232 works without any modification.
Termination Resistor Setup
You can install termination resistors if necessary for impedance matching. The card has mounting spaces for
termination resistors, but no resistors are installed at the factory. Depending on your application you may need to
solder in a single resistor to handle the DATA+/DATA- pair (and a corresponding resistor on the other end of the
connection). The value of the resistor should equal the characteristic impedance of the signal wires (approximately
120 Ohms or 300 Ohms).
MOS-1120/MOS-1121 User Manual 9
