Lost Communication Situation
You may want to detect if there is something happening at the Modbus master side that stops
communication with the Modbus slaves. Or, the communication cable might have been
disconnected. In this situation, you may want the Modbus slaves to take an action. For
instance, you may want to shut down the outputs on the slave CLICK PLUS PLC when the
communication with the Modbus master is lost. We offer an easy method to accomplish this.
The CLICK keeps counting how long it has been since each com port received a message from
the Modbus master, and enters the time duration in the following system data registers.
Each register is reset to zero automatically when the com port receives a message from the
Modbus master. Then its value increments by 1 per second until the com port receives another
message from the Modbus master. If one of these registers has 60 as its value, it means the com
port has not received any message from the Modbus master for 60 seconds. You can use this
info to shut down the outputs on the slave CLICK PLUS PLC. Here is an example program.
System Data Registers
System Data Register Nickname Range
SD41 _Port1_No_Comm_Time 0 - 32767 (sec)
SD51 _Port2_No_Comm_Time 0 - 32767 (sec)
SD61 _Port3_No_Comm_Time 0 - 32767 (sec)
SD214 _WLAN_No_Comm_Time 0 - 32767 (sec)
This contact is on when com port 1
has not received any message from
the Modbus master for 60 seconds.
Reset the out
uts Y001 to Y006.
CLICK PLUS PLC Hardware User Manual, 1st Edition, Rev. B – C2-USER-M
4-28
Chapter 4: PLC Communications
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