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FAQ: What to do if M241/251 ethernet communication gets interrupted after several hours?

Published date: 10 June 2019

This can come from READ_VAR / WRITE_VAR / READ_WRITE_VAR function blocks called with unreachable server IP address


For example:

  • if controller IP address settings are:


Subnet mask:



  • and if READ_VAR tries to communicate with server at address


then ethernet port may fail after several hours.



Workaround is :

  • Stop trying to reach servers which are not in the same subnet as the controller
  • Check ethernet cable status: ETH_R.i_wPortAIpStatus system variable value must be "DATA_EXCHANGE"
  • Use a gateway address (use controller address itself if there is no relevant gateway on the network): requests to unreachable server will be redirected to the gateway

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