Modbus RTU enabled on controllers comm port, loaded with correct XDriver, and configured correctly but does not communicate.
There is a rare occurrence where older implementations of Modbus RTU upgrade cookies may not be properly installed even after seeming to uprev properly. Enabling both the Modbus RTU and Modbus IP bits in the upgrade file seems to resolve this. The upgrade procedure has now been changed and all future implementations of Modbus will have both the Modbus RTU and Modbus IP options enabled on the comm port being upgraded.
For existing controllers that have already had the Modbus RTU option added to the comm port but have not validated that it works and are now seeing this problem, obtain a new upgrade cookie from Repairs requesting that both the Modbus RTU and Modbus IP options be enabled.
All new orders for Modbus RTU or Modbus IP will have both options enabled on the comm port so this problem does not occur in the future.