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Ethernet communication problem

Marko SramlPLC

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Hello everybody!


I have a problem and I don't know how to solve it. I use ethernet communication between 2 Unistream PLCs. I use 1 PLC as MASTER and other for SLAVE. If I SET the MASTER output , SLAVE's input becomes SET. If I physically disconnect ethernet connection between them, SLAVE still gets "1" on it's input, even if the ethernet "connected bit" is "0". I would like that if the connection between them is lost, all SLAVE inputs become "0". I tried to reset them, when the ethernet "connected bit" becomes "0" but it doesn't work. 

Is there any way to solve this problem?


Thank you for your help!



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  • MVP 2023

I don't use UniLogic often enough to remember enough detail to answer your question definitively, but one universal solution is to incorporate a "heartbeat" function within your communications. If the heartbeat stops changing for the appropriate period of time, then reset your bits (or registers).

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Once that register is set to a value it will continue to have that value until something else acts on it.

You have two choices; you can either clear the register on a timed basis, perhaps just before the other PLC sets it, or you can have a routine that 'pings' or keeps track of the communication between the two.   This can be in the form of a heart beat setup where one PLCs set a register to the a 1; the other sets sit a 0 and then if the heart beat stops you know the comms has gone down.   There are a ton of ways to go about it.  

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Under the section "Global", in the "EIP Scanner Node Struct" is a "Connected Bit" which represents the state of ethernet communication. I tried to use it as an inverted contact and reset numeric inputs, but it doesnt work. When the connection is lost, "Connected bit" becomes "0", but I can't reset the IO inputs. I think I sould try to copy IO inputs from ethernet protocol to a new struct and try to reset this new struct.


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