Using NMEA and SeaTalk

Introduction

Alba has electronic equipment that predominantly interfaces using SeaTalk 1 and NMEA 0183.  The chart plotter and radar were bought in 2015 and are connected together using SeaTalk-HS.  This article describes the interfacing.  

What is NMEA 0183?

NMEA 0183 is an electrical and data specification for communication between marine electronics such as echo sounder, autopilot, GPS receivers and many other types of instruments.  (It is slowly being phased out in favour of the newer NMEA 2000 standard, but this is not used on Alba.)

The electrical standard is RS422, although most hardware with NMEA-0183 outputs are able to drive a single RS232 port.  Interfacing to a computer is usually done via an RS232 to USB interface.  Physically, the wiring is four wires made up of two twisted pairs - one pair for data output and one pair for data input.  One of the wires in each twisted pair carries a positive voltage signal and the other carries a negative voltage signal.  This is done to minimise data corruption from stray electrical noise.

The NMEA 0183 standard uses a simple ASCII, serial communications protocol that defines how data is transmitted in a "sentence" from one "talker" to multiple "listeners" at a time.  Through the use of "multiplexers", data from multiple devices can be combined into one data stream and fed into other devices.  The speed that data is transmitted is known as the Baud Rate and is the number of pulses per second.  Common values are 4,800 baud and 38,400 baud.

The NMEA 0183 standard defines the contents of each data sentence type, so that all listeners can read sentences accurately.  For example, the GPRMC sentence contains Latitude, Longitude, Speed Over the Ground and Track direction. 

To put this into simple English, each device on the NMEA network can transmit and/or receive data along twisted pairs of wires.  There are four wires - data going out of a device is transmitted on one pair and data is received along another pair, there is also a ground (earth) wire. 

Note that without using a "multiplexer" there should only be one transmitter going into a NMEA input.  i.e. you could not connect the outputs from Device 1 and Device 2 directly to Device 3.  However, if Device 2 is acting as a "multiplexer" then the output of Device 1 will be combined with the output from Device 2 and sent to Device 3.   (Device 1 should ignore it's own messages, but some devices will get confused when receiving their own messages, so the connection into Device 1 may have to be removed.)

What is SeaTalk 1?

SeaTalk 1 is a proprietary network designed by Raymarine, which is a serial data protocol similar to NMEA, but there is only one "Data" wire and the information flows in both directions along the wire.  There have been later versions of the protocol with different wiring, the latest of which is SeaTalk-HS.  SeaTalk-NG is a proprietary extension of NMEA 2000.

In SeaTalk 1, the devices are daisy-chained, so that data flows from one device to the next in a chain.  Each device listens on the data line and is allowed to transmit its data when there is a long enough pause in the communication from other devices.

In addition to the data wire, there is also a +12V wire and a ground wire.  The three wires are contained in proprietary cables with D-type connectors making the connection much simpler than NMEA.  

The data messages and the electrical wiring is different to NMEA, so the two systems are not directly compatible.  Raymarine produces an interface to pass data between the two systems and also many Raymarine chart plotters input and output NMEA 0183 data

Raymarine do not publish the technical details of SeaTalk, but there is an excellent technical description of SeaTalk on the following website:

http://www.thomasknauf.de/seatalk.htm

Devices

On Alba, there are six devices with data interfaces:

Vesper XB-8000 AIS - This outputs GPS data (Location, Speed Over the Ground, etc.) and AIS data (Positions of other vessels).

The output is either via USB, NMEA 0183 (38,400 Baud) or NMEA 2000. The device will accept NMEA 0183 (4,800 Baud) or NMEA 2000 and will multiplex (re-transmit) this data into its USB output. 

Raymarine ST60 Boat Instruments - These are the Boat Speed, Wind and Depth Instruments.  They communicate with each other and are connected to the Autopilot via a SeaTalk network. 

The ST60 Multi Instrument has a NMEA input and a NMEA output.  The NMEA Input will receive GPS data and re-transmit onto the SeaTalk network.  The NMEA Output will transmit SeaTalk data such as Depth, Boat Speed and Wind Speed.  

Raymarine  C97 Chart Plotter -  This has a SeaTalk-HS port, a SeaTalk-NG port and a two NMEA 0183 interfaces. The digital radar is connected via the SeaTalk-HS port.  Unfortunately, the Chart Plotter doesn't support SeaTalk 1 without buying an expensive SeaTalk to SeaTalk-NG interface, so the chart plotter is only connected to other devices via a NMEA interface.  This causes a small loss of functionality.  The chart plotter also accepts data from other devices - in our installation it accepts data from the AIS.

Standard Horizon Quest+ VHF Radio - Accepts GPS data from other devices via a NMEA 0183 interface.

Raymarine  Type 300 Autopilot - Accepts SeaTalk information from the Boat Instruments.  It also has a NMEA 0183 output, which will transmit some of the SeaTalk data e.g. Boat Speed, Wind Speed and Autopilot data.

Laptop - Accepts NMEA data from the AIS device via a USB Port.

Garmin GPS128 -  Can output GPS Data via NMEA 0183, but is not connected at the moment.

The diagram shows the various interfaces implented on Alba.  Note the following:

* The primary navigation device is the Laptop running OpenCPN, so the interfacing is set up to maximise the data sent to OpenCPN.  (The Raymarine chart plotter is used as a backup device and a radar display.)

* The AIS is the primary source of GPS data and obviously creates AIS data.  It also receives Wind, Depth and Boat Speed from the Multi instrument and multiplexes it out to the Laptop via USB.  The instrument data is not multiplexed out to the Chart Plotter, but the GPS and AIS data is sent via the NMEA 0183 link.

* The VHF Radio receives GPS data, but only when the Raymarine Chart Plotter is switched on.  It's only a nice-to-have feature, if you want to use the DSC distress functionality.

*  There's a NMEA 0183 output from the Raymarine Chart Plotter to the autopilot, which will transmit Route instructions to the Autopilot allowing it to automatically follow a route, but it seems to be a bit flaky. I guess that if the link was changed to be SeaTalk-NG (by buying an expensive interface) then it would work properly, but we wouldn't ever use that functionality.

Wiring on Alba

The wiring for the NMEA interfacing is contained in the panel behind the instruments above the chart table.  The connections are: