Rs-485 Signal Wiring; Figure 5.2:Rs-485 Wiring Topology - Advantech PCI-COMM Series User Manual

5.2.3 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. Hand-
shaking 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 PCI-1601/1602/1611/1612/1622 cards
automatically sense the direction of the data flow and switch the trans-
mission 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.
Figure 5.2: RS-485 Wiring Topology
Termination Resistor Setup
You can install termination resistors if necessary for impedance match-
ing. The card has mounting spaces for termination resistors, but no resis-
tors 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 sig-
nal wires (approximately 120 Ohms or 300 Ohms).
95 Chapter 5