Chapter 3. Hardware Description
3.2.4
Terminal Resistors
Terminal Resistors are installed in RS422 or RS485 communication to prevent noise and distortion of
long distance data transfer. They help by matching impedance in communication line, and normally
120 ohm resistors are used. The decision to use terminal resistor lies with the user, and one may
choose to either install or not install terminal resistors in RS422 or RS485 mode. Installation can be
done by connecting RX+ and RX- pin on J1 pin header to the resistor if in RS422 mode, or TRX+ and
TRX- pin in RS485 mode.
3.2.5
Terminal Resistor Installation Guide
◆
RS422
Install terminal resistors at each end (terminal)
RS422 Slave Device N
TXD-
TXD+
RS422 Slave Device B
When using RS422 Point to Point Mode
TXD-
RS422 Master Device
RXD+
TXD+
RXD-
Terminal REG
120R
TXD+
RXD-
Terminal REG
120R
RS422 Slave Device A
RXD+
RXD+
TXD+
RS422 Device B
TXD-
RXD-
TXD-
Terminal REG
120R
TXD-
RXD+
TXD+
RS422 Device A
TXD+
RXD-
When using RS422 Multi-Drop Mode
RXD+
RXD-
RXD-
Terminal REG
120R
TXD-
RXD+
◆
RS485
Install terminal resistors only at each end (not in the middle ones in Multi-Drop structure)
TR
X
D
-
TR
X
D
+
TR
X
D
-
TRXD+
TRXD+
TRXD-
RS485 Device A
RS485 Device G
TR
X
D
+
TRXD-
RS485 Device F
TR
X
D
+
Terminal REG
120R
TR
X
D
-
RS485 Device E
TR
X
D
+
RS485 Device D
TR
X
D
-
TR
X
D
-
RS485 Device C
TR
X
D
+
TR
X
D
+
RS485 Device B
TR
X
D
-
RS485 Device N
Terminal REG
120R
Figure 3-6 RS485 Terminal Registor
3-7
