When must the COMM TEST button be used during the ETD installation process?

The COMM TEST button must be used after establishing communication during the ETD installation process because it is essential to verify that the communication between the ETD (Electronic Train Dispatch) unit and the locomotive's control system is functioning correctly. This check ensures that data transmission is reliable, enabling the train crew to receive accurate information about brake pressure and other critical systems. Successfully testing communication prior to proceeding with other steps, such as entering the ETD number or making pressure comparisons, helps prevent operational errors that could result from communication failures. Proper sequence in using the COMM TEST is critical for safe and effective train operations.