Automatic back-up switching
Automatic backup switching is a feature of the Brookhouse NMEA multiplexer that can be activated in the setup menu. One of the standard NMEA input ports is declared the "primary" input channel, another input port is the "secondary" or backup channel. The multiplexer monitors the primary input channel and as long as data traffic is detected, data coming in via the backup channel is ignored. If for a period of 10 seconds no data is detected on the primary channel, the backup channel is opened and input data is accepted. As soon as the primary channel becomes active again, the backup channel is closed.
For the standard multiplexer one of three combinations of primary/backup channels can be defined:
This feature is primarily intended for GPS backup, but it can be used for any other instrument or NMEA data source. As only channel activity is monitored, it is not necessary to know which NMEA data sentences are received. However, if the primary input channel receives combined data from several sources, the above method cannot be used to determine if one of the sources has stopped outputting data. For example, if the primary input channel receives data from the NMEA output of an instrument network, it is not possible to determine if the GPS has stopped, as other instruments in the network are still generating traffic. This also applies to Seatalk data traffic (if the Seatalk option has been installed).
Using a script
A refinement of backup switching has been implemented to enable detection of specific data in the combined input data stream. This method involves uploading a small script to the Brookhouse multiplexer. Detailed information about scripts can be found under NMEA filtering and conversion on this website.
If backup switching is selected (from the setup menu) for channel combination 1 & 2 (primary =1, backup =2) it is assumed that instead of overall data traffic monitoring, the selective script method is used. The reason is that channel 1 is the Seatalk input channel, if the Seatalk option has been installed and has been activated. The multiplexer converts Seatalk to NMEA and the script can test for the presence of a specific NMEA sentence to determine if the Seatalk GPS or a Seatalk instrument is present.
Seatalk instruments and a Raystar Seatalk GPS are interconnected via Seatalk. The Seatalk bus is connected to a Brookhouse multiplexer. An NMEA backup GPS is on input port 2 of the multiplexer.
If the Seatalk GPS stops functioning, multiplexer input port 2 has to open to allow NMEA GPS data from this backup source to be entered in the multiplexer output data stream to replace the (converted) Seatalk GPS data.
The above is achieved by:
The script looks for presence of the GPS NMEA sentence RMC channel on ch 1, converted by the multiplexer from Seatalk GPS. If not found for 10 seconds, the Raystar GPS must be off and channel 2 is opened for the backup GPS.
This method also works for systems without Seatalk, if port 1 is used
for combined NMEA input.
Backup using a conditional wipe script
This method does not require selection of backup switching in the setup menu, but is fully script-driven. It was implemented as a convenient way to use GPS data in the output of a Class B AIS transponder as backup GPS data, but it has found many other uses.
The script checks for presence of a certain NMEA sentence in the input stream of one of the multiplexer input ports. If the sentence is found, the "found sentence" condition is set. The script also contains one or more instructions to "wipe" (ignore) other NMEA sentences from a different input port conditionally, if the "found sentence" condition is set. If the condition has not been set for 10 seconds, the conditional wipe instructions are no longer executed.
GPS data received from input 4 (AIS class B transponder) is deleted from the input stream as long as the $IIRMC GPS sentence is being received on channel 1, i.e. the AIS GPS is used as backup for the Seatalk GPS if the Seatalk GPS stops working.
GPS data received from input 3 (GPS mouse) is deleted from the input stream as long as the $GPRMC GPS sentence is being received on channel 4. If the AIS GPS stops, the GPS mouse data on ch3 is no longer suppressed.
The importance of having a backup GPS permanently connected in the integrated instrument/navigation system hardly requires explanation. For example, chart plotters often rely on a single GPS receiver unit, in many cases mounted in a vulnerable position, such as on the cockpit railings. It does not take much in heavy weather to knock the unit from its mounting or to damage the cable. The entire navigation system becomes useless if the GPS receiver no longer works and the vesselís position is no longer plotted in the electronic chart. Such a mishap typically occurs under difficult circumstances and even if there is a spare GPS on board, it takes time to install and connect to computer or chartplotter.
It is not desirable to have two GPS units sending position data to the chart plotter or computer at the same time. Apart from placing a high data-load on the system, small differences in lat/lon data can confuse the chartplotter or computer software and lead to various problems. Brookhouse backup switching prevents double GPS input.
Addition of a backup GPS via the multiplexer will substantially improve reliability of the integrated system at relatively low cost. There are many GPS receivers with NMEA output available that are suitable for for this purpose. For example a low-cost GPS mouse makes a great backup GPS and is sensitive enough to be installed inside the cabin, protected from the elements.