[CA] [CA11] Ais transmit error

[Chuck - Raymarine - Moderator]

Welcome to the Raymarine Forum,

Occasional “TX attempt failed (msg18 CP busy)” message is normal to encounter in areas featuring large numbers of AIS transmitters. Another AIS manufacturer, PicoSense, provided some good information on this subject within the following FAQ posted on their website:
"Class B AIS (The system on which the B100 operates) is a ‘carrier sense’ system. It has to find a free slot to transmit in by listening just before transmission to check the slot is empty.

When a transmission is scheduled a block of 10 potential slots is selected. It will attempt to transmit in the first of these slots. If the slot is busy the ‘CP busy’ (meaning ‘candidate position busy’) message will be output. It will then try again in the next slot, and so on for the 10 slot block. It is quite normal to see ‘CP busy’ messages in an area where other AIS vessels are operating. It simply means the first slot selected was busy. 99% of the time the transmission will be successful in one of the other 9 slots.

So long as the indicator remains green transmissions are occurring on schedule. If transmissions can’t be made because the AIS environment is simply too busy then the indicator will turn amber (this is incredibly rare though and extremely unlikely to occur outside of lab conditions)

In summary these messages are normal and can be ignored if the indicator is green. Anything you might see in the ‘messages’ box is just for info and not a fault or alarm. If there is an active alarm or fault, then the ProAIS2 will list these in the “Alarms” window of ProAIS2."