Need help with Modbus connection break micro820

cdogmillion

Member
Join Date
May 2016
Location
Darwin
Posts
6
Hi all,

Im working on a tank filling project where a pump is started remotely which fills a tank locally.

At the remote location I have a micro820 acting as a remote modbus relay (Slave) to switch on the remote pump.

Its connected via modbus RTU to my local master PLC which is also a micro820.

Master PLC tells the slave PLC to turn the pump on and off, everything is well.

However I have a problem if the following series of events occur:
- Master tells the slave to hold the pump on
- Slave holds the pump contactor on
- There is a connection break in the modbus cable
- Master tells the slave to turn the pump off due to tank being full
- Slave does not receive the message and keeps filling the tank (holds its last value from before the connection broke)

This is a major problem for me if the tanks continue to fill due to a modbus connection break.

So my question is, is there any bit/register/function in connected components workbench which I can use to monitor the status of the modbus link which I can implement to disable the pump output if the slave PLC detects a bad modbus connection?

Cheers
 
I was thinking the same thing as Tarik. A "Heartbeat" would be a good way to go.

One of the simplest Heartbeats is to have a timer in your slave PLC that is reset by the master via Modbus. If the timer is ever done, then you have lost communications and the slave can shut down.
 
Thanks Bullzi

So simple and worked like a charm..

I Put a timer on both ends which are always counting and reset by flashing one shot signals from the opposite ends. If the timers reach 20 Seconds: On the master it brings up a modbus comms error on the HMI and at the slave it blocks the pump from starting until the timers are reset again.
 
Glad it worked!!
I can't take credit for that idea, I got it from another forum member. Just trying to pay it forward.
 

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