The Modbus serial port of a Unity Momentum PLC will not automatically change back to a slave port after an XMIT/XXMIT function is done

Issue
This Resolution explains the behavior differences between a legacy Momentum Modbus serial port and a Unity Momentum Modbus serial port when using an XMIT/XXMIT function block.

When enabling/activating an XMIT/XXMIT function block on a legacy Momentum PLC (171CCC980x0 and 171CCC960x0), the Modbus Master serial port will automatically change back to a Modbus Slave port when the function has completed.

When enabling/activating an XMIT/XXMIT function block on a Unity Momentum PLC (171CBU98090 and 171CBU98091), the Modbus Master serial port will not automatically change back to a Modbus Slave port when the function has completed.  Instead, it remains as a Master port as long as the PLC remains powered.  If power is cycled, the serial port will revert back to a Slave port until the XMIT/XXMIT is activated.

This difference in behavior required a solution to allow the user to change from a Master Modbus port to a Slave port on a Unity Momentum PLC.


Product Line
171CBU78090
171CBU98090
171CBU98091

Resolution
The solution to change the port from a Modbus Master to a Modbus Slave is to set bit 15 (0x8000) in the Command word which triggers the reconfiguration of the port using the internal buffer. The “done bit” is then set to notify that serial port went back to values before EFB was active.

• This issue exists in Unity 8, 8.1, and 10.
• HotFix UnityPro_V110_HF0258202R.exe is available for Unity 11.
  (HotFix UnityPro_V110_HF1 must also be installed in Unity 11)
• The solution is available in Unity 11.1.