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SeaTrac Serial Command Interface Reference
Page 110
8.2. ECHO Protocol Messages
The ECHO protocol is primarily intended for testing and validating remote beacons functionality
and diagnosing problems with acoustic communications.
The CID_ECHO_SEND command is used to start an ECHO transmission, where a user specified
payload of up to 31 bytes is sent to the remote beacon. Upon successful reception, the remote
beacon will then return a response message containing the same payload.
While the acoustic transceiver should receive the response message correctly, and not have to
validate the contents of the response data (guaranteed by the CRC checksum algorithms
used), the longer structures used by the ECHO protocol over the PING protocol allow acoustic
channel characteristics to be evaluated or the effect of Doppler shift on remote beacons.
8.2.1. CID_ECHO_SEND
The CID_ECHO_SEND command is issued to send a packet of data to another remote beacon,
and have this beacon return (or echo) the data back, also allowing its range and position to be
determined.
If successful, the CID_ECHO_SEND command will start an acoustic message
transmission, and at some time later an acoustic response will be heard.
The ECHO protocol handler will generate subsequent serial message to signal when
these event occur.
Additionally acoustic transceiver diagnostic messages are available to monitor activity
and obtain further information on the received signal levels – see section 7.6 on page
92 for further details.
When sending ECHO data, be aware that larger quantities of data will take longer to
send (by 80ms per byte), and during this time the receiver may become more
susceptible to errors caused by Doppler shift or other interference, and the statistical
probability of the CRC16 checksum algorithm (used by the acoustic transceiver)
uniquely validating the message content becomes reduced.
When testing the acoustic link, it is recommended to use data lengths of 16 bytes or
less if problems with communications are observer.
Command Message Parameters
Parameter Type Description
MSG_ID CID_E Command identification code (CID_ECHO_SEND)
DEST_ID BID_E The ID code of the beacon to send the Echo request
to, and receive position and ranging information
for.
Valid values are form 1 to 15.