Modbus RTU Communication is getting STOPPED Randomly

saqib47 · Feb 3, 2016

Hello Everyone,

I am reading parameters of Energy Meters, Schneider PM 710, using 'M340 BMX P34 2020' PLC and Modbus RTU Communication Protocol. The devices communicate very well for roughly around 1 hour and then the communication STOPS randomly. The PLC keeps on working properly/correctly. Then the only way to restart the communication is to Manually Power OFF and then ON the PLC.
There are 37 Meters (+ PLC), so 38 Nodes and not using Repeater.

Thanks.

tragically1969 · Feb 3, 2016

saqib47 said:
Hello Everyone,

I am reading parameters of Energy Meters, Schneider PM 710, using 'M340 BMX P34 2020' PLC and Modbus RTU Communication Protocol. The devices communicate very well for roughly around 1 hour and then the communication STOPS randomly. The PLC keeps on working properly/correctly. Then the only way to restart the communication is to Manually Power OFF and then ON the PLC.
There are 37 Meters (+ PLC), so 38 Nodes and not using Repeater.

Thanks.

Well you already know you are over the physical node limit of 32 for Modbus so that won't help, got terminating resistors in the network i hope ?

Are you using the Ethernet port to a Modbus Gateway or are you using the in built RS485 port ?

Either way i would recommend updating to the latest firmware for the in built Ethernet ports and processor, there was an issue with this sort of thing a while back

saqib47 · Feb 3, 2016

Yeah, I know I am bit over the Limit

and also the PLC is sending request for only 17 READ_VAR blocks in Unity Pro Program, for rest of the blocks the error is '16#0B No processor system resources'.
@ tragically1969
Thanks, I am using RS485 (RJ 45 connector) for Modbus RTU and Ethernet port for Modbus TCP/IP, communicating with Vijeo Citect.

tragically1969 · Feb 3, 2016

saqib47 said:
Yeah, I know I am bit over the Limit and also the PLC is sending request for only 17 READ_VAR blocks in Unity Pro Program, for rest of the blocks the error is '16#0B No processor system resources'.
@ tragically1969
Thanks, I am using RS485 (RJ 45 connector) for Modbus RTU and Ethernet port for Modbus TCP/IP, communicating with Vijeo Citect.

How often are you polling the meters or are you using the finished bit in the previous block management word to trigger the next read ?

redbarron55 · Feb 3, 2016

Make certain that you have timeout values for the modbus comms. Otherwise if there is an error it will timeout and not restart.

saqib47 · Feb 3, 2016

tragically1969 said:
How often are you polling the meters or are you using the finished bit in the previous block management word to trigger the next read ?

Since, wasnt able to send request to more than 17 Nodes (process 17 READ_VAR Blocks) so I made 4 segments with abt 10 READ_VAR blocks in each. Then Activating One Block for 3 seconds, then all segments Deactivated for 3 seconds and after that Activating Next Segment for 3 seconds and this GOES ON.

tragically1969 · Feb 4, 2016

So you mean you activate 10 read_var blocks at once and requesting data simultaneously from 10 slaves at the same time, i think that's pretty much your issue right there.

Its better to index a word after each read_var using the read complete bit, use this word to trigger the next read, just cycle them round like that.

Have a search, there was some examples of comms routines using read_var a while back

Lare · Feb 4, 2016

32 limit comes from RS485 network, not from modbus protocol.

But you can have only one read_var/write_var block active/ serial port.

Unity manual says that 2000 CPU can have simultaneus communication blocks 16 at same time. This is valid for TCP or if you have several communication cards.

saqib47 · Feb 9, 2016

@ tragically1969
Yes, exactly this was the Problem.
Thank you very much guys for helping me out. This was my first time with Modbus and Modicon M340.

Modbus RTU Communication is getting STOPPED Randomly

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