NMEA 0183 SENTENCE FILTERING
The 4 input serial ports, the serial output port and the 2 input
wifi ports can be set to pass or to block certain NMEA0183
sentences. This is governed by a filtering string of up to 43
characters that you can enter in the wide text boxes for each of
the referred to ports.
The first character is a number, between 0 and 9. When this
number is 0 no filtering takes place. When the number is 9 the
input is completed ignored (not even read by the multiplexer).
When the number is between 1 and 8, it specifies the number of
sequences that are periodically dropped independently of the rest
of the filtering string. For example, if that number is 3, it
means that on 4 successive sequences, one is taken and three are
dropped. The second character should be a separator. The
characters "-", ".", "~" and others could be used as separators
but we will use "-". The third character can only be "A" or "R".
"A" means that only the sentences that are referred to in the rest
of the filtering string are ACCEPTED by (pass through) the ports.
"R" means that only the sentences that are referred to in the rest
of the filtering string are REJECTED (blocked) by the ports. Each
NMEA0183 sequence is specified by 3 characters inserted between
separators. For example, the string "2-A-RMC-GGA-GLL-GRS-GSA"
means that only 1 in 3 successive sentences is considered and the
considered sequence only passes if it is 1 of the 5 that are
specified. It is interesting to note that the shown sequence has a
length of 23 characters and specifies 5 sentences. For each
additional sentence you need 4 more characters (the separator and
the 3 characters). Therefore, in order to specify 10 sentences we
need 43 characters which is maximum length of the filtering
string. An important note is to understand that the filtering
strings for the UDP and TCP ports refer to data input. No
filtering takes place when data is transmitted by UDP or TCP.
You can use the RIGHT and LEFT arrows to add or to remove the
sentences that you select to appear in the Filtering String. You
can press CLEAR to remove all the sentences from the selected box.
You can use the 2 radio buttons to select "Accept" or "Reject" and
you can set the initial number of the Filtering String referred to
as Skip.