35
Options
On the Options page several options and conversions can be chosen.
Figure 36
Priority
This option deletes duplicate NMEA sentences received on multiple inputs. When enabled, the multiplexer
assigns a priority to incoming NMEA data based on the input on which it is received. The host interfaces
have the highest priority, followed by NMEA
In1, In2, In3 and In4 in descending order. NMEA conversion
results have the same priority as the input that received the source of the conversion.
If for instance two GPS receivers are connected to In1 and In2 and both GPS receivers send GPRMC
sentences, only those from the GPS on In1 are passed. This feature can be useful to set up a second GPS
as a backup for the main GPS.
The multiplexer only uses the Sentence Formatter (the RMC part) for comparison - the Talker ID (the GP
part) is ignored.
Another useful application of Priority is when a GPS and an AIS transponder are connected to the
multiplexer. An AIS transponder often sends sentences from its internal GPS. A navigation program will
now receive GPS data from two sources that might differ in position due to GPS inaccuracy. This will lead
to navigation errors. When the AIS transponder is connected to In1 and the GPS to In2, all the NMEA
sentences from the AIS transponder is passed while duplicate NMEA sentences (e.g. GPRMC) from the
GPS are blocked. When the AIS transponder fails, GPRMC sentences from the GPS will be passed again
after an adjustable timeout.
Note that the priority system does not block an entire input, it only blocks duplicate sentences. Sentences
received from the GPS that are not received on the AIS input are passed.
The following example shows what is passed and what not: the left column in
Table 1 shows sentences from the AIS transponder on In1 and the right
column sentences from the GPS on In2. The greyed sentence in the table is
blocked by the priority system.
The priority feature can store up to 50 different sentence types to determine
their priority. A time out mechanism ensures that sentences received on
lower priority inputs are passed again after an adjustable time out when their
duplicates on higher priority inputs are no longer received.
Check GPS status
Normally, the priority system detects the
absence
of NMEA sentences. With “Check GPS status” enabled,
it detects
invalid
sentences, specifically from a GPS.
When a secondary GPS is connected as a backup to the primary GPS, the primary GPS must completely
fail (stop sending data) before sentences from the secondary GPS are passed. In a situation where the
primary GPS no longer receives any signal from satellites (broken antenna, cable etc.), it will still send
NMEA sentences to the multiplexer. Thus the priority system will not fall back to the secondary GPS.
When “Check GPS status” is enabled, the multiplexer checks the status field of GPS sentences. When the
status field indicates “Invalid data”, the multiplexer will block these sentences, allowing the priority
system to time out and pass sentences from the secondary GPS. Sentences of which the Status flag is
checked are APB, GGA, GLL, RMA, RMB, RMC, VTG and XTE.
AIS
(In1)
GPS
(In2)
GPRMC
GPRMC
AIVDM
GPGGA
Table 1
