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WIRING: RS485
On the controller’s top side, two RS485 ports operate as COM1 and COM2. Each port is capable of up to 115,200 baud, and uses a
3-position, screw terminal connector.
1. RS485 port A (COM1) is often used to support a LMJA-IO-R-34 module. NOTE: Do not mix LMJA-IO-R-34 modules with other
types of RS485 devices on the same RS485 trunk.
2. RS485 port B (COM2) supporting a network of other field devices using RS485 communications.
3. RS485 devices on the same network should use the same protocol and baud rate. Up to 32 or more devices may be supported,
depending on device specifications.
Use shielded, twisted-pair, 18/22 AWG cabling to wire in a continuous multi-drop fashion to other RS485 devices: “minus to minus,”
“plus to plus,” and “shield to shield.” Connect the shield wire to earth ground at one end only, for example at the controller.
RS485 Bias Switches
Each RS485 port has an adjacent 3-position biasing switch with these settings:
BIA
- (Default, middle) RS485 biasing and termination: 2.7K Ohm bias resistors with no termination resistor
END
- RS485 biasing and a termination: 562 Ohm bias resistors and 150 Ohm termination resistor
MID
- RS485 biasing or termination: 47.5K bias resistors with no termination resistor
Often, adding RS-485 biasing can improve communications by eliminating indeterminate idle states.
BIA
- (Default, middle) Often best if the RS485 trunk needs biasing, but when the controller is not installed at the end of the trunk.
END
- Often best if the controller is installed at the end of an RS485 trunk of devices that is not already biased
MID
-Often best if the controller is put in the middle of an already-biased RS485 trunk
If desired, you can change the position of an RS485 port’s bias switch while the controller is running. Each RS485 port has two LEDs.
NOTE:
The power for the LMJA-IO-R-34 could be sourced from the
LMJA-8xxx if adequate current is available
-OR-
a dedicated Class 2, 24V
transformer, or to a 24VDC power source. The LMJA-IO-R-34 can share
transformer power with an LMJA-8xxx. If powering from a 24V transformer,
do not also power equipment (other than an LMJR-8xxx) with the same
transformer. Otherwise, conducted noise problems may result. Also,
DO
NOT
ground either side of the transformer’s 24V secondary.
OUT TO SEGMENT NETWORK
LMJA-8xxx
RS485-A
LMJA-IO-R-34
LMJA-IO-R-34
LMJA-NPB-2X-485
LMJA-IO-R-34
SH
RS485-A
SH
MID BIA
END
RS485-B
A-RS485-B
TX
RX
TX
RX
PWR
1
RS485-B
2
