Testing the communications link

Applicable receivers:  All receivers

To determine if the receiver can accept RS-232 commands, the protocol request ENQ (05h) is used. The response is either ACK (06h) or NAK (15h).

"ENQ", "ACK", "NAK" correspond to “Are you ready?”, “I am ready”, and “I am not ready”. This quick 1-byte test can be sent by the remote device before any other command to make sure that the RS-232 line is clear and operational. A NAK response can be expected due to an error, the requested command is not supported or else the command being sent has a syntax error.