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.

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