A method for transmitting messages between a communications controller and devices in a factory process control system, wherein each device has an associated identification number which is unique to that device. A message to be transmitted is compiled, and then based on the criticality of the message, one of a standard, secure, or critical message protocols is chosen for the message; each protocol being defined to generate a message checksum for the message in a manner different from the other protocols. The generated checksum is then included in the message. Using the secure and critical protocols, a message checksum is generated from the message to be transmitted and the identification number of the intended recipient device. In the secure protocol, the message checksum is included in the message, but the identification number is not included in the message; in the critical protocol, both the message checksum and the identification number are included in the message. A crosstalk query message may be sent from the controller to determine if a link of a factory process control system is susceptible to crosstalk. The controller sends to a link on which a particular device is known to reside a diagnostic message indicating the identification number of the particular device and requesting any other devices that unintentionally receive the message to transmit a response message to the communications controller.