When using serial output data in control modules, remember that the output values are only sent to the Modbus devices when the value changes. This might cause problems if there is a program running in the Modbus device (for example, a PLC) that could overwrite a register being used for output data by a DeltaV control module. The DeltaV control module might think that the output register has one value in it when it actually contains a different value written by a third party. The control module does not send the output value to the Modbus device until the output value changes. To prevent this situation, enable the Output Readback option for any output datasets that could be changed by a third party. When the Output Readback option is enabled, the Serial Card reads the ouput values from the Modbus device during its normal input scan and updates the corresponding values in the DeltaV system if they change. This ensures that the output register values in the DeltaV system always match what is actually in the Modbus device.