Emerson Exchange 365

Can MODBUS Function Code 8 be disabled on VIM2

We are trying to enhance the security of our firewalls on a network using VIM2s and using MODBUS. The is a read only connection, so we intend on only allowing MODBUS FC 16 to read holding registers.

We are getting bad status for the logical card in diagnose if we set our rules to only allow that FC 16.

We see FC 8 being transmitted.

 I cant block FC 8 without causing bad status for the cards.

I suspect that this is the mechanism being used to prove valid communications.

Can FC 8 disabled in any way?

MPHymel

• 14 Feb 2020 2:56 PM
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2 Replies

• MPHymel

  

  • 3 Jun 2021 8:21 PM

  from VIM NET explorer Help....
  Some external devices do not support Modbus Function 8. The VIM firmware allows
  users to select any input dataset (one that uses Device Data Type of 0, 1, 2 or 3), and
  use it as the ping. By adding a 10 to the Device Data Type of an input dataset
  indicates to the standby VIM that this dataset should be used as the ping. Instead of
  sending Function 8 command, the standby VIM will send the read command
  associated with the dataset. For example, if there is a dataset that reads Holding
  Registers, it will be configured with a Device Data Type of 3. By changing the Device
  Data Type to 13 allows the standby VIM to use this dataset as a ping. Note that the
  data read from the external device is discarded. The active VIM, of course, continues
  its normal read of Holding Registers.
  
  The ping command will be sent every 1500ms. However, user can configure a custom
  delay. The custom delay is configured in the datasets Special Data 5, most significant
  7 bits, in units of 100ms. For example, if the most significant byte of Special Data 5 is
  512, the ping is delayed 100ms. Refer to Section 5.7 for details.

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• Mark Millar

  

  • 4 Jun 2021 11:51 AM

  In reply to MPHymel:

  We use this extensively for Bently-Nevada equipment along with a 6 sec transmit delay as the older BNs dont like normsl speed subsequent requests. The Rockwell 20CommM card falls into the custom ping category too but without the delay.

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