P3 Terminal Resister usage in RS485 Bus (Modbus)
Issue
The communication between P3 relay and SCADA system will sometimes fail when using Modbus protocol through RS485 port.
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Product Line
Easergy P3 relay
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Environment
All firmware version
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Root Cause
Besides the basic configuration mistake on Slave address/Modbus bit rate/Parity, there is one more reason which can cause communication failure to Easergy P3 relay using RS485 Modbus protocol. It is terminal resister.
For all RS485 ports in Easergy P3 relay, we have a resistor parallel with RS485 port in our relay and the resistor can be used(or disconnected) in the wire bus of RS485 by setting the DIP switch ON(OFF).
But actually this resistor only should be used at both ends of RS485 bus, all the other devices in the middle of RS485 bus should not have parallel resistor, otherwise the communication of this RS485 bus will not be stable, even sometimes all devices in this RS485 bus fail to communicate with SCADA.
_____________________________________________________________
Solution
Make sure only the relay at both ends of RS485 bus has resistor connection, which means DIP switch for terminal resistor is ON, others should be OFF.
The communication between P3 relay and SCADA system will sometimes fail when using Modbus protocol through RS485 port.
_____________________________________________________________
Product Line
Easergy P3 relay
_____________________________________________________________
Environment
All firmware version
_____________________________________________________________
Root Cause
Besides the basic configuration mistake on Slave address/Modbus bit rate/Parity, there is one more reason which can cause communication failure to Easergy P3 relay using RS485 Modbus protocol. It is terminal resister.
For all RS485 ports in Easergy P3 relay, we have a resistor parallel with RS485 port in our relay and the resistor can be used(or disconnected) in the wire bus of RS485 by setting the DIP switch ON(OFF).
But actually this resistor only should be used at both ends of RS485 bus, all the other devices in the middle of RS485 bus should not have parallel resistor, otherwise the communication of this RS485 bus will not be stable, even sometimes all devices in this RS485 bus fail to communicate with SCADA.
_____________________________________________________________
Solution
Make sure only the relay at both ends of RS485 bus has resistor connection, which means DIP switch for terminal resistor is ON, others should be OFF.
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