Chapter 2
Hardware Installation and Configuration
2-10
ni.com
An RS-485 stub is created when you tap the RS-485 backbone to form a
T-junction. RS-485 stubs must be less than 3 in. long. However, you can
create longer stubs by using an RS-485 repeater to start the stub, ensuring
that the repeater is close to the junction.
The RS-485 connector is a 5-pin Combicon connector whose pinout is
shown in Figure 2-11.
Figure 2-11.
RS-485 Connector Pinout for the FP-1000 and FP-1001
RS-485 Network Termination and Biasing
An RS-485 network must be terminated at each end of the network, but not
anywhere else. At each end bank, termination resistors should be installed
between the RX pair and between the TX pair. These network terminating
resistors are nominally 120
Ω
and are provided in your kit. To install them,
twist the resistor leads with the RS-485 signal wires and then insert them
into the RS-485 Combicon adapter as shown in Figure 2-12. The 120
Ω
resistor in the figure is a terminating resistor.
Figure 2-12.
Terminating RS-485 Using the Combicon Adapter
An RS-485 network also needs biasing resistors to protect the devices
on the network against noise during intervals when no RS-485 drivers are
transmitting on the network. The host computer’s RS-485 interface
normally has provision for such biasing resistors. When you are using
FP-1001 network modules directly connected to a host computer’s RS-485
interface, use the biasing feature of the host computer’s RS-485 interface
for better reliability and noise immunity. The RS-485 repeater interface on
RX–
RX+
TX–
TX+
GND
1
2
3
4
5
120
Ω
120
Ω
+
–
+
–
RS-485
Signal Pair
RS-485
Signal Pair
RX+
RX-
GND
TX-
TX+