Advantech MOS-1120Y Series, User Manual, Page: 32 / 34

MOS-1120Y/1121Y Series User Manual 24
5.3 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
Request-To-Send signals) are normally used to control the direction of the data flow
and to switch transmissions accordingly. In RS-485 mode, MOS-1121Y cards auto-
matically sense the direction of the data flow and switch the transmission direction,
thereby eliminating the need for handshaking. This means users 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 functions without requiring any
modification.
Termination Resistor Setup
If necessary, termination resistors can be installed for impedance matching. MOS-
1121Y cards have mounting spaces for termination resistors, but no resistors are
installed at the factory. Depending on the application, users may need to solder on 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).