How do I wire a Modbus Master to multiple Slaves via RS485?
Issue
User would like to know how to correctly wire a Modbus Master device to salve devices using the RS485 port.
Product Line
ION and Powerlogic Meters
Environment
Serial Communications
Cause
There are two different configurations for wiring Master devices to slave devices. The two configurations are depending upon if the RS485 port is 2 or 4 wire communications.
Resolution
The pinout for 4-wire RS485 is TX+ to RX+, TX- to RX-, RX+ to TX+, RX- to TX-. See chart below.
Terminating resisters are required across the Master receive signals and the receive signals in the last Slave. The value of the terminating resisters
depends on the impedance of the cable.
| Master | Slave | Slave | Slave |
| TX+ | RX+ | RX+ | RX+ |
| TX- | RX- | RX- | RX- |
| RX+ | TX+ | TX+ | TX+ |
| RX- | TX- | TX- | TX- If the device is 2 wire communications simply wire the master device "TX+ and TX-" pins to the same pins on the slave device. |
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