Connection diagrams

 

Please note that the text and diagrams below apply to the standard Brookhouse NMEA multiplexer. The diagrams are a number of years old and many of the options available now are not mentioned. However, the information has proven to be valuable for a general understanding of NMEA multiplexers, so we have kept it on the website.

 

The following diagrams show integrated instrument/computer installations, whereby the NMEA Multiplexer plays a crucial role. In all cases the objective is the same: to combine all instrument data in one data stream which is then sent to a computer, a chart plotter, to a repeater instrument, auto pilot etc.

 

There are many applications for the multiplexer, in the text below we will concentrate on combining data to be used by a (laptop) computer. 

 

For the navigator of a cruising yacht, the GPS connection to the computer is the most important because positional data is required for the navigation/chart plotting software, but the various software packages offer an ever increasing number of features, which also require other instrument data. For example, it is very useful  for the navigator to be able to read out the actual water-depth and wind direction at the current position of the vesssel as plotted in the in the electronic chart. Many navigation programs can display a window with all instrument data in analog and/or digital format, even with graphs. Wind strength graphs can play an important role in decision making. Also, automated log systems can make log-entries at certain time-intervals,  which include magnetic heading, wind speed and direction. To make optimal use of what the on- board computer has to offer, all the available NMEA data has to be fed into the computer via a NMEA multiplexer.

 

For the race-tactition other programs help determine the best course/tack. Again, all instrument data has to be available as input to this software to enable it to perform its calculations.  Therefore, after the decision has been taken to build an integrated computer/instruments system, it is important to hook up the instruments and computer in such a way, that the set goals can be achieved. If the basis of the system is not properly designed, it will not work, not even with the best software.

 

An important point when planning how to interconnect instruments and computer, is whether an autopilot will be part of the integrated system and which features of the auto-pilot will be used. Most auto-pilots can be used in three ways:

 

  1. To steer a set course. The navigator  works out a course, the boat is manually steered on that course and the helmsman presses the “auto” button of the auto-pilot. The auto pilot will maintain that course from this point onwards. It is the simplest use of the AP and no other instrument data is required.
  2. In “track mode”. The boat is steered along a track to a waypoint. The advantage is that the boat is not only steered in the right direction, but it is also steered exactly along the line (track) that the navigator has plotted to the waypoint in the (electronic) chart, i.e. by following the track, fixed obstacles such as shallows are avoided. This is ideal for navigation in bad visability or at night and as the track is “over the ground”  the autopilot automatically compensates for drift and current. In most cases the autopilot does a better job than a helmsman.
  3. In “wind vane mode”. The boat is steered at a set angle with the wind direction. This mode is useful to maintain the sailtrim, but if the wind direction changes, the course changes. Most auto-pilots will give an audible warning if the change is more than a set maximum.

 

 

To be able to work in track mode (2), the auto-pilot needs special NMEA sentences with data such as the bearing and range between origin and destination, the bearing and range from present position to destination and cross track error (NMEA APB). A GPS can produce this data, but it is necessary to load the waypoint(s) in the GPS and to set the GPS to navigation mode. In an integrated system whereby the navigation is done on the computer, waypoints are set with the mouse, directly in the electronic chart. This much user-friendlier, faster and less error-prone method is one of the advantages of using a computer. The GPS is only used for providing the computer with latitude and longitude of the position. Therefore, instead of connecting the auto-pilot to the GPS NMEA output, it is hooked up directly to the computer and the navigation software generates the NMEA sentences for auto-pilot control.  When selecting the navigation software, it is important to check if auto-pilot control is supported if this is required.

 

If wind-vane mode is required (3), the autopilot needs NMEA data from the wind-instrument. However, if it is connected to the computer for track-mode, no direct NMEA connection with the wind-instrument is possible at the same time. Therefore, the navigation software has to be able to pass the NMEA sentences it receives from the windinstrument on to the output port, where the autopilot is connected to. Naturally, the wind-instrument data has to be available in the computer in the first place. In the setup section for most navigation software, the user can specify which NMEA sentences should be sent to the output port. Sometimes this is referred to as “NMEA pass-through”.

 

In most cases the output port for the auto-pilot will be the same as the port where the NMEA multiplexer is connected to. The auto-pilot will most likely require a transmission speed of 4800BPS (NMEA standard). As transmission and reception speed for PC ports have to be the same, this dictates that the computer input port and therefore also the output of the NMEA Multiplexer has to be set to 4800bps as well.

 

 

Diagram  1

 

 

This shows a relatively simple system with individual NMEA outputs of  the instruments. The instruments may be of different manufacture. The speed instrument is directly hooked up to the wind instrument for true wind direction and speed calculation. All instruments and a GPS are connected to opto-isolated input ports of the NMEA multiplexer. The combined NMEA data is sent to the computer via RS232 or USB and/or to a chart plotter or other NMEA listeners via RS422.

 

Diagram 2

 

 

This diagram shows a basic instrument network. Instrument manufacturers often have their own protocols for connecting instruments to eachother, but usually an NMEA output is provided for connection to the “outside world”. This NMEA talker port outputs sentences for data from all instruments in the network. The GPS is not part of the instrument network and therefore an NMEA multiplexer is necessary to combine the data from the separate NMEA data sources.

 

 

Diagram 3

   

The instruments in this example are Raymarine Seatalk instruments. Seatalk is the name of the proprietry Raymarine protocol that links the instruments. No NMEA output is provided. However, the Brookhouse NMEA multiplexer with Seatalk option can accept the Seatalk bus signal on input channel 1.  The multiplexer converts the Seatalk data of all connected Seatalk instruments and Seatalk GPS to standard NMEA sentences and outputs these sentences combined with the NMEA data from the GPS and other “NMEA talkers” via the RS232 port or USB to the computer and via RS422 for other NMEA listeners. This is an efficient and cost-effective solution because the multiplexer provides both the combiner function and the Seatalk-NMEA conversion.

 

 

Diagram  4

 

Here, an autopilot is added to the configuration of diagram 2. As discussed in the intruduction of this chapter, it is advantageous to control the autopilot directly by the navigation software.  Computer output is sent to the multiplexer via RS232 or USB and enters the multiplexer via a 5th input port. Therefore, the standard Brookhouse multiplexer is in fact a 5-channel multiplexer. The auto pilot is connected to the multiplexer’s NMEA OUT (RS422) port.  Please note that in this case the multiplexer’s output baudrate has to be set to 4800 bps, because most auto pilots will only support this standard NMEA baudrate. There are now also Brookhouse multiplexer models available with baudrate conversion, that allow connection of a standard 4800 bps “listeners”, whilst the output baudrate to the computer or chartplotter is higher.

 

The combined data stream sent to the auto pilot contains GPS data plus computer output. Therefore, both the GPS and the computer (or chartplotter) can control the auto pilot, without physically switching.

 

Many autopilots also have a NMEA-out (talker) port. NMEA data such as magnetic heading (from the fluxgate compass) is output via this port. In this diagram the autopilot NMEA output is fed back into the NMEA multiplexer, so that the magnetic heading is available to the navigation or other software running in the computer.

 

 

Diagram 5

 

In this diagram both the instruments and the autopilot are Raymarine Seatalk. The Raymarine autopilots that support track-mode also have an NMEA IN (listener) port, so that the NMEA sentences for autopilot control from non-Seatalk devices can be accepted. As in diagram 4, the navigation software running on the computer or the GPS controls the autopilot. The autopilot is also connected to the other instruments via Seatalk and therefore the data from the wind instrument is already available to the autopilot for wind steering mode. The autopilot also has a NMEA output. The primary function of this output is to send out heading data from the fluxgate compass. In the diagram, this is linked to NMEA multiplexer Ch3 with the dotted line. The reason is, that this NMEA connection is not always necessary, as the NMEA multiplexer also translates the heading data it detects on the Seatalk bus to NMEA and the computer does not need the same data twice.

 

Note: Although the autopilot is connected to the Seatalk bus and also has a NMEA output port, this does not mean that the other instrument data is available as NMEA sentences via this port. Only heading data is sent and Seatalk-NMEA conversion in the multiplexer is still required.

 

 

Diagram 6

In diagram 6 the connection of a radar is shown, in combination with an autopilot. It is assumed that this radar unit is capable of displaying navigation data such as L/L, bearing and range to waypoint (BWC sentence), heading and speed. Provided the required data is available, some radars are capable of plotting the current active waypoint on the screen. This is an vey useful feature for navigation in bad visability. Either the computer or the GPS can be used for auto pilot control without physically switching. The full combined data stream is sent to both the autopilot and the radar NMEA IN ports from the multiplexer’s NMEA OUT port (RS422). The radar can therefore display waypoint data originating from either the computer or the GPS. A maximum of 5 NMEA listeners can be connected in parallel to the RS422 output port.

 

 

 

 

 

 

Note 1:

If the instrument system includes a universal NMEA repeater instrument, that is capable of displaying bearing & distance to waypoint and cross-track plus other NMEA data, this instrument can be connected in parallel with auto pilot and/or radar to the mux RS422 port.

 

 

Note 3:

In the diagrams, the Brookhouse NMEA Multiplexer model  without the LCD display is depicted. The model with compact LCD provides exactly the same functionality, with the additional advantage that the data that passes through the multiplexer can also be displayed, independently from the computer.

 

Note 4:

VHF or SSB radio connections are not shown in any of the diagrams. If the VHF or SSB radio supports DSC, a GPS can be connected to automatically transmit the position of the vessel in distress. In any of the diagrams shown above, the GPS signal can be split to go to both the NMEA multiplexer and the radio.

 

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