The MODBUS code reads the AMS ID and Version Level first to determine if the provided Configuration Register Address is correct. Assming the AMS ID is correct, it uses the Version Level to determine the size of the full configuration data structure and then reads it. The second read, reads the AMS ID and Version Level again.

If the first read was successful, the code assumes that the second read would also be successful and that the same AMS ID and Version level would be read the second time.

We have found that Allen Bradley PLC's that don't support MODBUS in a native way, may not respond properly on the second read. To detect this, we are now testing for communication failures on the second read. In addition, the AMS ID and Version Level between the first and second read are compared for equivelence. If they are not equivelent or there is a communication error, a descriptive error message is displayed.

The AMS ID and the Version Level of the first read are displayed int the PLC Diagnostic Screen.