Technical info NMEA Multiplexer

First of all, you don't need to understand the following. All that is necessary to get the NMEA Multiplexer working is to hook it up to the NMEA talkers and the Computer. Apply power and it works. One advantageous feature is that LEDs show if NMEA data is being received from the NMEA talkers. This provides a quick way of checking if the instruments are working. With each unit comes a detailed installation description. Our NMEA Multiplexer with repeater LCD does'nt have indicator LED's, because the LCD made these redundant.

For those who are interested in the internal workings, here is some additional information:

The Brookhouse NMEA combiner contains an advanced Micro Controller with on-board RAM memory, flash-memory for the software and Eprom.

The Multiplexer software does not decipher NMEA sentences. It looks for the start of NMEA sentences and for the LF (Line Feed) ASCII character, which terminates each NMEA sentence. Each whole sentence is transmitted via the RS232 port, no matter what the contents are. The Brookhouse Multiplexer with Repeater LCD does of course interpret the NMEA data to be able to extract the useful data from the sentences, but it does not interfere with the multiplexer function in any way. It is passive, does not modify any data but just picks out the useful parts for display if it recognizes the format. NMEA sentences cannot be broken up, so once transmission of a sentence via the output-port has started, it has to continue until the LF character is encountered. Marine instruments, in particular GPS's often send  series of NMEA sentences in one burst and repeat this every 2 seconds or so.

When the Multiplexer detects data on any of the input ports, it checks if the output port is currently transmitting an NMEA sentence. If not, the output port is reserved and received data is immediately transferred to the output port. If  the output port is occupied, the received data is buffered in RAM. For each input port, there is one buffer of  sufficient length to hold a number of NMEA sentences. After a complete NMEA sentence has been transmitted, the output port is freed up to give other instruments a chance to have their data transmitted. As soon as the output port becomes free, the software checks if any data has been placed in any of the input port buffers and if so, the first one in line is emptied and its data is transmitted, after the output port is reserved for that buffer. Transmission from a buffer will start even if there has not been a complete NMEA sentence received yet. The circular buffer mechanism allows simultaneous filling and emptying of a buffer. This ensures that maximum efficiency is achieved. It will only seldom happen that more than a few NMEA sentences per port have to be buffered.

The transmission speed of the NMEA Input ports is always 4800BPS (Bits Per Second). This is the NMEA standard data-rate. The speed of the output port can be selected (9600 or 4800). In case of high volume/high frequency data-input, it is advisable to set the RS232 output port to 9600 BPS.

If you have any questions, please feel free to e-mail Brookhouse.

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