In the proAIS2 Diagnostics page, you will often see DSC and TX Attempt failed warnings displayed. These warnings are referring to normal AIS transponder behaviour and here is some more info on these messages…

The RX 1 / RX 2 DSC start / Stop message is a completely normal message, indicating when one of the AIS receivers has started/stopped a DSC channel management listening period. Class B equipment is required to listen to channel 70 periodically (at specific times defined in the standards) for DSC channel management commands. As Class B lacks the third receiver necessary to do this full time it is achieved by occasionally switching an AIS receiver to this channel.

TX attempt failed (Msg 18) and CP Busy messages are often seen, as Class B AIS 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 – we have never witnessed it outside of test conditions).

In summary these messages are normal and can be ignored if the “Power” LED indicator is green.

NOTE – these conditions are not shown as an error or alarm in proAIS2 , just in the ‘messages’ box. Anything in the ‘messages’ box is just for info and not a fault or alarm; those are shown in the ‘Alarms’ box.