Using Comms Link Operator Panel

The operator panel is used to convey link specific data about the internal state of the comm link driver and to provide counters and other status to show link health and assist in diagnosing communication issues.

To view and utilize a Comms Link operator panel:

  1. From EPM Device User Interface screen, select Options - Device I/O - Comms Link:

  2. The Comms Links screen displays:

    Note - Diagnostic counters are not retained when power to the EPM device is lost. After the device powers up, all counters restart at zero.

  3. The operator panel displays the following for Modbus Server:

    • Internal Error - Indicates an internal problem in the EPM Modbus server. This flag is used for non-communication issues and is false during normal operation. When true, an alarm exists; contact your supplier.

    • Request - Number of requests received by the EPM Modbus server from all connected Modbus clients.

      • The counter increases by one for each request received.

      • If it does not increase, check whether:

        • Any Modbus clients are connected.

        • The client configuration matches the EPM Modbus Server configuration.

        • An installation issue, such as a disconnected cable, is preventing communication between the client and server.

    • Good Response - Number of requests received by the EPM Modbus server that were valid and generated a successful response to the Modbus client. Under normal operation, this increases with the Request count. If the Request count and Good Response count do not increase together, the following counters may help indicate the issue.

      Note - A difference in the count may not always indicate a current issue, the difference may be historic and the cause may already have been resolved.

    • Illegal Function - Number of requests where the function code is not supported by the EPM Modbus server . This causes the EPM Modbus server to respond with a 0x01 exception code. See Modbus Server supported function codes for details.

    • Illegal Data Address - Counts requests in which the client specifies an address or address range that the server does not support for the selected function code. For each invalid request, the EPM Modbus server returns exception code 0x02.

      Check the client configuration and ensure that it uses only the function-code and address combinations listed in the exported EPM address map.

      For example, if the EPM Modbus server supports holding registers 1000–1010 and the client attempts to read register 2000, the request is invalid because that register does not exist. Similarly, a request to read three registers starting at 1009 is invalid because it includes register 1011, which is outside the supported range.

    • Illegal Data Value - The number of requests where the Modbus client sent data that is not supported by the EPM Modbus server. For example, when a client requests to read more items than the maximum quantity supported by the EPM Modbus server, the EPM Modbus server responds with a 0x03 exception code. Check the Modbus client configuration and update it to use only valid quantities and addresses for the EPM Modbus server. See Exporting a Modbus Address Map Modbus Server Protocol Absolute Limits for details.

    • Reset - Select to reset all Modbus diagnostic counts to zero. A reset can be useful if there are historic counts in the diagnostics and you need to diagnose a current issue.

Select the following to expand the details:

The EPM Modbus server supports the function codes listed below. Your application may only use a sub-set of these function codes. The exported address map displays the function codes in use. See Exporting a Modbus Address Map for details:

Function Code (decimal) Function Code (hexadecimal) Function
1 0x01 Read Coils
2 0x02 Read Discrete Inputs
3 0x03 Read Holding Registers
4 0x04 Read Input Registers
5 0x05 Write Single Coil
6 0x06 Write Single Holding Register
15 0x0F Write Multiple Coils
16 0x10 Write Multiple Holding Registers

The following are the EPM Modbus server protocol limits supported by each function code. The valid range could be much smaller than these absolute limits, depending on the application configured. If a Modbus client sends a request that exceeds these limits, the EPM Modbus server responds with Exception code 0x03 irrespective of application configuration. The exported address map shows which function codes and address ranges are used by the application. See Exporting a Modbus Address Map for details:

Function Code (decimal) Function Code (hexadecimal) Function Absolute Limits and checks
1 0x01 Read Coils Quantity 1 to 2000 (0x0001 to 0x07D0)
2 0x02 Read Discrete Inputs Quantity 1 to 2000 (0x0001 to 0x07D0)
3 0x03 Read Holding Registers Quantity 1 to 125 (0x0001 to 0x007D)
4 0x04 Read Input Registers Quantity 1 to 125 (0x0001 to 0x007D)
5 0x05 Write Single Coil Data value must be 0x0000 for False and 0x00FF for True
6 0x06 Write Single Holding Register No limit/check
15 0x0F Write Multiple Coils Quantity 1 to 1968 (0x0001 to 0x07B0)
16 0x10 Write Multiple Holding Registers Quantity 1 to 123 (0x0001 to 0x007B). Data Byte Count must be 2 × Quantity
Related topics - Configuring a Modbus Server, Deleting a Comms Link, Exporting a Modbus Address Map, Deleting a Modbus Address Map

Last update - October 2026