Filtering is an advanced feature of the Nmea3Wifi and Nmea4Wifi multiplexers. In most cases you can ignore filtering by accepting its default value of 0.  

The 2 (Nmea3Wifi) or 4 (Nmea4Wifi) input serial ports, the serial output port and the 2 input wifi ports or the 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 ports referred to. The first character is a number, between 0 and 9. When this number is 0 and nothing follows, no filtering takes place. When this number is 9, the port is ignored (neither read nor written by the multiplexer). When this number is between 1 and 8, the multiplexer tests the checksum of the sequence. If it finds a wrong checksum, the sentence is ignored. If the starting number is between 2 and 8, some sequences are periodically dropped independently of the rest of the filtering string. For example, if that number is 4, it means that only the 4th sentence is taken from 4 successive sequences. The first 3 sentences are dropped. The second character is 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 "3-A-RMC-GGA-GLL-GRS-GSA" means 3 things. Firstly, a sentence only passes the filter if it has a correct checksum. Secondly, only 1 in 3 successive sentences is taken. And finally, the sentence only passes the filter if it is 1 of the 5 specified sentences. 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.

In the case of the Nmea3Wifi multiplexer, it is possible to filter the datagrams that are received or are ready to be sent out, on the SeaTalk1 bus. Datagrams are also specified by 3 characters. For example, datagram 53 (hexadecimal) is specified by S53. 

You can use the tool Nmea0183Tester available here, to construct Nmea0183 filtering sentences. This tool is a Windows programme that was used in the development of the multiplexers. It is also a convenient tool to test many aspects of data communication with the multiplexers. 

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