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Rene

Unistream Modbus communication freezing problem

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I have configured a Unistream PLC as Modbus Master (periodic working mode) and connected with 11 Modbus Slave devices using RS485. Communication works five or six hours as it must, but suddenly Modbus RS485 communication hangs up. It stops to polling just one (random) slave device and does not poll other slaves at all. After hard reset of whole PLC (power off and on), it starts polling all slaves normally. I make tests with "Modbus Poll" software and PC, result was that the communication with all slaves working correctly, but Unistream PLC as master does not.

By the way, there is no possibility to adjust "retries count" and "time out" values for Modbus communication. This is very serious disadvantage. If a one of the slaves does not answer to the master requests, the communication with other slaves must still work!

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Hello Rene,

 

Please ensure you are using the most up to date OS (Firmware Version 1.12.7, associated with VisiLogic 1.20 Rev 20).  We have not experienced any issues with Modbus over RS485 with this OS version.

 

Please let us know if this helps, or if this does not resolve your issue.

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To follow up with this I have noticed a similiar issue with the Modbus RS485 Master. It will stop polling various points at various times. When the controller is first turned on, OR if you toggle the Modbus Active bit it will run through all the points, however after that some of the points drop out and no longer update (read or write). I have found by toggling the Modbus Active bit again you can get around this. I have noticed this in periodic and the on-demand version both. I alerted the Boston Office of this issue and they were looking into it. IN the mean time I've just set my program to toggle the Modbus active bit every couple of seconds and that has allowed me to limp through testing.

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Hi,

 

There was an old issue that when one of the slaves on the RS485 network was down, in specific cases the MODBUS polling would stop.

This issue was already fixed - please download the latest UniLogic version from our website, update the UniStream Bin_Files and this should solve the problem.

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it would be nice to have a user assignable field to set the MODBUS master retries and timeout parameters!!

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Try checking out my Modbus Read UDFB :) It includes Retries/Timeouts on a PER READ basis - not globally, so you can adjust for different slaves etc.

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