Why the Modbus communication over Ethernet between a Saia PCD and a Siemens S7-314C-2 does not work correctly?

FAQ #101973

The PCD is configured as Modbus client with the Saia-Modbus client F-Boxes and you have to communicate over Ethernet with a Siemens S7-314C-2 with the Modbus protocol.
Randomly, some read or write F-Boxes are working fine and others not.

The problem is that the Siemens device sends a ‘reset’ telegram after sending the response to the PCD.
Since the PCD has received the response from the Siemens, the PCD does exit from the Modbus communication state machine and end the task correctly.

If the PCD now receives this unexpected ‘reset’ telegram from Siemens device, then the PCD thinks, that this is an error message from the Siemens device and does close the communication and the F-Box goes in error state.

The transmission/reception F-Box which is in the queue to proceed the next Modbus request is now no more able to communicate with the Siemens device.


Use the Modbus Initialization Client Extended F-Box.
In this F-Box you have the possibility to set a minimum request delay.
Set the minimum request delay to 100ms

This problem occurred for example with a Simatic S7-314C-2 Siemens device.



PG5 2.0 / Modbus

PG5 2.0 / Fupla

Last update: 19.11.2015 15:39

First release: 19.11.2015 15:37

Views: 2

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