SJ23 Tech Tip E13, (Updated 2018-12-08 Under Construction) Bob Schimmel

Index

Connect a GPS to an Autopilot or VHF.
(Send NMEA data to an autopilot to head to a waypoint or follow a route)

VHF-DSC Radio, GPS, Waypoints, Auto Pilot, Data Wiring, Panache Installation, About NMEA, Plotter

NOTE: In 2016 I installed a Standard Horizon Explorer GX1700 VHF-DSC radio.  It is equipped with an internal GPS receiver that necessitated a change of the NMEA wiring I had with a Uniden VHF that died after only 2 years of use.  The replacement Standard Horizon is much easier to operate.

The amount of navigational data on some boats can be downright phenomenal, what with multiple programmable displays.  Most manufactures of marine electronics sell a family of interconnected instruments that are designed to "talk" to each other for enhanced operation.  This integrated operation generally requires a "data hub" with proprietary protocol to direct the data to/from each instrument, all with perfect signal quality for error free data transmission.  The expense of this class of instrumentation is hardly feasible on the average SJ23 used for weekend sailing, but if you have deep pockets and want this degree of sophistication then by all means fill your boots.  I still want some of the enhanced operation so opted for a simpler configuration, with the lowest possible power consumption.  The correct data wiring is still required though, so don't go cheap by using the wrong cable.

On Panache I have three navigational instruments; a Magellan GPS320 navigator, a Standard Horizon Explorer GX1700W VHF-DSC radio (Digital Selective Calling) and a Raymarine ST2000+ Tiller Pilot.  I also have a Signet analogue knot meter, a Ritchie compass and a Hawkeye depth sounder but they don't count in my wiring scenario.  I wired the three together for the enhanced features and convenience of operation.  Given compatibility, these basic instruments can be swapped for other manufactures.

"An autopilot equipped with NMEA tracking capability is a good choice for an SJ23.  Being able to automatically follow a route programmed in a GPS navigator, not just a heading selected in the autopilot, will make life much easier for a solo sailor.  This is especially true if you traverse a long stretch of cross current or side wind that will set you sideways to a dangerous area."  Hal Mueller.

THE CONCEPT -  My idea was to hardwire the NMEA leads from my GPS navigator directly to my Tiller Pilot so it could follow a track as Hal suggests.  The GPS receiver will source the following NMEA data sentences; Cross Track Error, Bearing to Waypoint, Distance to Waypoint, and Waypoint Number at 4800 baud async.  I can view the GPS navigator screens to confirm I'm on track while cruising to a waypoint or along a route.  Add an iPad with Navionics software and you have the overall picture.

"A sailing buddy demonstrated a perfect use of this technology when he decided to motor back to his mooring after the midnight fireworks.  This was during one of those rare pitch black, clear nights up here when, if you stare long enough into the dark, you can actually see.  It is the perfect clarity for fireworks by the way.  By selecting a route on his navigator he knew the boat would stay on a safe course to his mooring.  As he passed by each waypoint, and the very important cardinal buoy, he heard the expected warning beeps, keeping him oriented.  It should be noted that there is no floating debris and only two well lit Cardinal buoys to mark hazards on this lake.  He knows the lake very well and we both keep a chart on board for back up use.  The route took his boat right to his mooring ball 8KMs away.  He updated his progress to me with text messages, keeping me in the loop!  Pretty impressive when you know where you are but can't see more than 75' ahead."

When my analogue VHF died I replaced it with a VHF-DSC radio equipped with an internal GPS receiver.  While a VHF-DSC radio has lots of useful capabilities, it wasn't my first choice because the nearest Coast Guard station for is 1,200 KMs away!  Duh, I would never be in RF range to send a distress call to them so why buy one!  "But I have a certain old sailing friend who convinced me to move up to current technology in the hopes that my influence would entice other "fossils" on the lake to start using their DSC functions; familiarity leading to proficiency of use.  It's actually working, albeit slowly.  We're also working out the "subtle details" of duplicating the Coast Guard function here for handling a distress call, which is no minor task this far inland."  After studying the VHF manual my simple configuration had a possible alternate if it was wired correctly.  The goal being a configuration that draws minimal power, to navigate with a bullet proof operation, so any crew can transmit a distress message with the push of the DISTRESS switch.  You want simplicity & speed during an emergency. 

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VHF-DSC RADIO (Standard Horizon Explorer Series GX1700W) - This VHF radio is equipped with an internal GPS receiver and waypoint management software.  If the radio has poor GPS reception then it must be relocated or wired to an external GPS navigator from which it receives continuous position updates in the form of NMEA data sentences.  When this radio has a position fix it displays a "satellite icon" in the top right corner of the screen and shows the coordinates.  It takes about 45 seconds to acquire a position fix after power up.  The primary purpose of the coordinates is to transmit your position to the Coast Guard with a push & 3 sec hold of the DISTRESS button.  Your coordinates will show on their chart plotter and they can acknowledge your call with an "ACK" message displayed on your radio screen, giving you confirmation they received your distress call.  Hope just when you need it!  By sending your coordinates to the Coast Guard it greatly reduces their time (and cost) to rescue you.  Now, get yourself ready to be rescued!  The secondary reason for displaying the coordinates is to plot them on a printed chart to discover where you are!  Keep a chart handy!  This radio has never failed me.

NOTES

  • Enter your MMSI number in your VHF-DSC so you can transmit your current position/coordinates.  Without it the DSC functions cannot work.

  • DISTRESS CALL (BEARING, DISTANCE & COORDINATES) - If this radio receives a distress call, it immediately goes to the compass screen and shows the bearing and distance to the waypoint, you can then store it as a waypoint and use it as a GO TO for the Tiller Pilot (to be confirmed).  You should plot the waypoint on a chart to confirm an obstacle free path to the vessel in distress.  A "Remote Position Request" works the same way. 

  • Distress call NMEA data is decoded from the signal received via the antenna.  If your chart plotter is wired to the NMEA OUT connections it can show the location of the distress call on its display.  The advantage of this feature is to instantly show the location of the vessel in distress to quickly render assistance.  You have only to select those coordinates as a heading for the Tiller Pilot and you are underway to rescue the sailor.  You might even arrive before the Coast Guard or assist them if they beat you, which is the concept of this feature.  The Coast Guard uses similar technology on their rescue vessels or at their shore station where they have access to the MMSI data base. 

  • From a recreational boater point of view the following VHF-DSC options should be enabled to assist you;

TYPICAL VHF-DSC RADIO OPTIONS
Enter and save your personal MMSI number mandatory
automatic position request enabled
automatic test call reply enabled
automatic channel change enabled
weather alert enabled
Store frequently used MMSI numbers 100 max

It is important to review and understand the configuration of your VHF-DSC radio.  The manufacturer defaults may not match the requirements of your country, which can create confusion.  In addition, other manufacturers enable these options different, naturally!  With one manufacturer you have to actually enable the DSC function, despite the fact that it is sold as a VHF-DSC radio.  Thank God for protocol rules so they can connect with each other.  The control panel of many VHF-DSC radios still requires maturing to reduce the number of key strokes to handle a DSC call. While it should be as simple as operating a cell phone, a marine radio doesn't have the benefit of a telephone switch to route calls and has to monitor an open channel, which a cell phone can't.

EXTERNAL GPS ANTENNA - If your VHF-DSC is not equipped with an internal GPS receiver then an external navigator or active GPS antenna is your next best choice.  An external GPS antenna works best if mounted on the transom since this spot has unobstructed visibility to the sky.  But it should be installed as far away from the secondary VHF antenna as the received GPS signal is extremely weak, being 10dB below noise threshold.  Your body does not affect signal strength as long as you don't cover the top of the antenna.  If the sails are above the GPS they can also affect the signal strength.   TOP

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PORTABLE GPS NAVIGATOR (Magellan GPS320) - This 1999 vintage navigator can send NMEA data sentences to a NMEA compatible device; VHF-DSC radio, Tiller Pilot, chart plotter, data hub, radar, etc, when the receiver is locked to the satellite network.  Without a position fix a GPS navigator cannot send NMEA data.  This makes sense but I discovered it through testing as it is not in the manual.  The navigator sends NMEA 0183 data via a Magellan serial data cable equipped with a DB9 connector (see photo).  This cable is also equipped with a 12V plug to power the receiver from boat battery (50MA + 10MA for light) thereby preserving the two internal AA batteries for an emergency.  If you don't have a serial data cable for your older GPS I suggest you scour the Internet till you find one.  They are becoming rare.

Match the following configuration:

  1. Marine mode (waypoints, NM/KNOTS).

  2. Set the GPS receiver to magnetic North to match the flux gate compass of the Tiller Pilot that can operate only on a magnetic heading.
    If the GPS is set to geographic North the two will differ by the magnetic variation in your area.

  3. Calibrate the Tiller Pilot flux gate compass to operate in your area so it agrees with the magnetic heading in your GPS. 

  4. A Tiller Pilot can only recognize the first 4 characters of a waypoint name.  A name longer than 4 characters will stop the waypoint advance feature from working.  So use 4 characters to name your waypoints.

  5. NMEA enabled 4800-8-N-1 (4800 baud, 8 data bits, no parity & 1 stop bit).  Data signal (TTL 0/+5 VDC). 
    The Magellan GPS can send one of three suites of NMEA sentences.
    - V1.5 APA - BWC, APA, GLL, VTG Remote display autopilots.
    - V1.5 XTE - XTE, BWC, GLL, VTG Marine autopilots.
    - V2.1 GSA - GSA, GSV, GLL, GGA, RMB, RMC, APB NMEA is recommended for navigation data, satellite data and autopilots.
        * I will use V2.1 GSA since it includes all the sentences required to acquire a new track automatically;
        Cross Track Error (APB & RMB),
        Bearing to Waypoint (APB & RMB),
        Distance to Waypoint (RMB), and
        Waypoint Number (APB).

MANAGING GPS WAYPOINTS & ROUTES the EASY WAY- A PC equipped with a GPS management application like EasyGPS 6.11 (Win 7 or 10) is the only "civilized" way to manage the waypoints and routes for a GPS navigator.  It is so much easier and quicker to use the PC keyboard than to trip your fingers on that tiny GPS keyboard.  (For Linux or Apple operating systems try GPS Drive). 

  1. Most PCs as of about 2010 don't have a COM port.  If your PC doesn't, then buy a USB/SERIAL adapter and plug it into a USB port.  Let the PC install the required driver while you have Internet connectivity.  A serial adapter is useless without the driver installed.
    - If you want to see the WIN7 configuration of the SERIAL adapter, click on: Start, Control Panel, System, Device Manager, Ports.  Expand PORTS and you should see the name of your USB/SERIAL port (COMx) adapter.  Right click on it to view the properties.  Click on the "Port Settings" tab to see the speed (9600), data bits (8), parity (N), stop bits (1) & flow control setting (none).  Not sure why mine is at 9600 but it works and I'm not touching it.  There is speed translation within the adapter since the port speed of 9600 is communicating with the GPS speed of 4800.
    -     I don't take a laptop onboard as I think it is too risky.  Its much easier to manage the waypoints at home.

  2. Plug the GPS cord (previously connected to the GPS) into the SERIAL adapter.

  3. Launch EasyGPS and go through the connection configuration.  Click on: Edit, Preferences, My GPS Receiver, Settings, Select your USB/serial adapter (by name) and set the options to 4800-8-N-1 to match the GPS.

  4. Switch the GPS navigator on and wait till it is booted up.  Set the speed to 4800-8-N-1.

  5. In EasyGPS click on the RECEIVE icon to download the waypoints from your GPS.  If all settings are correct the waypoints will populate in a table at the bottom of the screen.  
    - The GPS does not have to be locked to the satellites to download or upload waypoints. 

  6. Edit the waypoints to suit your needs.  Determine which table columns show on the GPS and edit those for an understandable readout on the navigator.  When you are done SEND them to the GPS.  Remember to save a backup file of your waypoints. 
    - It is entirely possible to save different suites of waypoints for different operating areas or needs.  Then you simply upload the appropriate suite to the GPS so you have fewer waypoints to scroll through when out in the field.  May as well simplify your life.
    -     How you name your waypoints to organize them, given the cryptic rules of a GPS navigator, is your job.  Each navigator has its own limits but is one of the reasons why I like to do this job with a PC.  I numbered mine counter clockwise around our lake, going with the outbound traffic flow.  I'll take a copy of the table on board the boat and stick waypoint numbers and names on my chart.

  7. You can also create and save a route using your waypoints, which is infinitely easier than doing it from the GPS keyboard. 

As a side note it is interesting to view my tracks over the course of a sailing season, now that I switch my GPS on every time I sail.  Think you aren't a creature of habit?  Its surprising how close my tracks are to each other. TOP

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AUTO PILOT (RAYMARINE ST2000+ Tiller Pilot) - A Tiller Pilot is capable of being directed by a NMEA 0183 compatible device (GPS navigator, chart plotter, etc) to head to a waypoint or follow a programmed route.  This is far superior than using the AUTO switch on the control arm to "hold this heading".  "Admittedly, just pushing the AUTO switch is quick and likely good enough for most of the time."  To control the Tiller Pilot from my GPS I ran the power and data wiring (NMEA) from the Tiller Pilot connector (port side of cockpit), in a cable tray along the top inside of the port locker, through the port cabin bulkhead (sealed), and across the cabin under the companionway to a terminal strip (nickel plated) on the starboard bulkhead below the VHF-DSC radio.  The power leads continue to the breaker panel on the starboard side.   TOP

NMEA DATA WIRING - The default NMEA data speed of most marine device is 4800 baud for which you can use category 3 (Cat3) cable (voice grade, stranded, twisted pair).  Use the shortest length for minimum loss. 
- Its best to use shielded cable to protect the signal from radiated radio frequency interference (RFI).  Ground the shield at one end only, preferably to the boat's battery return terminal. 
- If you use Cat5 unshielded twisted pair (UTP) cable equipped with Teflon insulation (most cables are) it is virtually immune to cross talk or interference from radiated RF.  If you have shielded Cat5 (for a really noisy electrical area) connect the shield at one end only, preferably to the boat battery return.  Secure the UTP well enough to withstand vibration because the conductors are usually solid. 
- I wired Panache as per the connections in the table below.  Hopefully the logic is clear enough to understand. 
- NOTE - Seatalk takes precedence over NMEA.
- UPDATE (Unfortunately I have not had the time to test the functionality of this wiring.  Perhaps in the near future as I have many other pressing projects). 
 

POWER, AUDIO & NMEA WIRING from GPS to VHF-DSC RADIO to TILLER PILOT (2018-12-08).
PORTABLE GPS
(Magellan GPS320
e/w Power/Data Cable)		 DB9-M
(pin)
 		 Data
Flow
 		 VHF-DSC RADIO
(Standard Horizon 1700
e/w GPS)		 Data
Flow
 		 Term Strip
(below VHF)
 		 TILLER PILOT
(Raymarine ST2000+)
Designation DB9-F
(pin)		 Lead Designation Cockpit
Connector (pin)		 Designation
        Ext SPKR (+) WH   1 - WH    
        Ext SPKR (-) BK   2 - BK    
        NMEA In (+) BL  > 3 - BL    
        NMEA In (-) GN  > 4 - GN    
        * NMEA Out (+) GA > 5 - GA    
        * NMEA Out (-) BN > 6 - BN    
NMEA Out (+) 2 2 > RD > 7 - WH 6 - WH NMEA In (+)
NMEA Out (-) 5 5 > BK > 8 - GN 5 - GN NMEA In (-)
              3 (not used)
Data In (+)
(from a PC)		 3 - YL           4 - YL Raymarine Seatalk Data
(not used)

BAT +12V

 RD  

BAT +12V

 RD   1 - BN BAT +12V
   BAT Return BK     BAT Return

BK

   2 - BL BAT Return

  NOTES:
 - The BLUE CELLS indicate VHF audio out wires to the cockpit speaker. (3W AUDIO works well).
 - The TAN CELLS indicate NMEA out data wires from the GPS (TTL, 0/+5VDC) connected directly to Tiller Pilot. 
       Required for the Tiller Pilot tracking feature.
 - The VHF NMEA In data wires. Unterminated.
 - The VHF * NMEA Out data wires.  Unterminated.
       Distress, position request, etc. data is decoded from a received RF signal to show on a chart plotter.
 - The GPS Data In wires connect to a PC.  Upload waypoints & routes to GPS.
 - The red cells indicate a battery connection from breaker panel, (+12VDC), to the Tiller Pilot.
 - The black cells indicate a battery return connection from breaker panel, (-12VDC), to the Tiller Pilot.
 - Battery return is also a common signal for NMEA data reference for all devices.

  RADIO & DATA WIRING GUIDELINES:

  • CAUTION - Always connect a radio to battery with the correct polarity, red to + & black to -.  Opposite polarity will fry the radio. 

  • CAUTION - Never let a data (NMEA) data lead touch battery voltage.  It will fry the radio.

  • Always connect all navigation devices to the same battery and battery return for equal potential between them and so NMEA data can pass between devices.

  • Always connect a NMEA positive to a positive (+ to +) and a NMEA negative to a negative (- to -).

  • Always connect a NMEA out to a NMEA in, (send data to receive data).

  • All wires must measure 0 Ohms continuity. If in doubt, use an ohm meter to confirm. 

The block wiring diagram below looks amazingly simple but can be a tad difficult to trouble shoot once installed.  For this reason I connected and configured all the devices on a work bench to confirm overall functionality, then installed it on the boat with confidence.  "I am not going to be responsible for your grey hair!"  
NOTE - This radio is powered via a circuit breaker and the battery return is equipped with a water proof quick blow fuse to protect it from ground surge induced power.  The holder is made of rubber with internal ribs to keep the fuse dry.  Twisting and pulling the two halves apart will reveal the 6A, 1" bus fuse.  Make sure it is pushed together well for a secure connection.  Grease the ends to prevent corrosion.

VHF TERMINAL STRIP - At right is a close up view of the audio, NMEA data and power wiring on the terminal strip installed below the VHF-DSC radio.  The DE9F connectors to the left are for the external GPS NMEA data leads that go directly to pins 7&8.  There is a M/M gender bender between the two.  The DIN connector, barely visible behind the heat sink, is for the VHF radio NMEA data leads 3&4 5&6.  For clarity the co-ax cable to the antenna is not connected.  These connectors, along with the white power connector, make it quick and easy to remove the electronics for winter storage in the warmth of my humble abode.  My original aluminum mounting bracket around the VHF continues to house the radio and protect the terminal strip wiring. 

NOTES -  Refer to power, audio and NMEA cabling table above. 

  1. Pins 1-2 - Terminated to the external 4 Ohm cockpit speaker. 
    - The volume is good without an amplifier.

  2. Pins 3-4 - (VHF-DSC NMEA input wires). 
    Future - May be connected to the GPS NMEA Out (DE9) for an external input.

  3. Pins 5-6 - (VHF-DSC NMEA Out wires).
    Future - May be connected to the Tiller Pilot NMEA In.

  4. Pins 7-8 - (Tiller Pilot NMEA In wires)
    - Connected directly to GPS NMEA Out (DE9). 
    Future - Once this configuration is functional I may install a switch here to select the NMEA source as GPS or VHF-DSC. 

    VHF TERMINAL STRIP PIN DESIGNATIONS.
    VHF AUDIO VHF NMEA DATA TILLER PILOT DC POWER
    SPKR + SPKR - NMEA in + NMEA in - NMEA
    out +    		 NMEA
    out -    		 NMEA in + NMEA in - -12 VDC +12 VDC +12 VDC
    VHF
    WH    		 VHF
    BK    		 VHF
    BL    		 VHF
    GN    		 VHF
    GY    		 VHF
    BN    		 Tiller Pilot
    WH    		 Tiller Pilot
    GN    		 Battery Rtn
    BK    		 Instrument Lights
    BKR3  RD    		 Depth Sounder
    BKR6  RD
    pin 1 pin 2 pin 3 pin 4 pin 5  pin 6 pin 7 pin 8 pin 9 pin 10 pin 11
    RD
    (SPKR)    		 BK
    (SPKR)    		 future future future future GPS RD
    (NMEA out +)    		 GPS BK
    (NMEA out -)    		 2 BK wires
     Windex, Compass, Knotmeter, depth sounder    		 2 RD wires
    Windex, Compass & Knot Meter    		 RD
    Depth Sounder

     

  5. Pins 9 - Battery Return for instrument lights and depth sounder.

  6. Pin 10 - Instrument lights

  7. Pin 11 - Depth Sounder.  TOP

 

 

 

 

NMEA BENCH TESTING - This is the electrical connection to the Tiller Pilot for my bench testing of NMEA control.  NMEA wires (BK-neg, WH-pos) on the left and DC power (BK-neg, RD-pos) on the right.  The connector key is at the top.  This allowed me to confirm the wiring configuration and various GPS options at home during the winter.

My bench testing is now complete with the Tiller Pilot displaying the correct NMEA commands on the screen.  While I'd love to say it works, I'll reserve my conclusion till Spring of 2019 when I will run it through a sea trial to confirm operation.  Can't wait for that.   TOP

 

 

 

 

 

 

 

 

 

PANACHE INSTALLATION - Shown here is the completed Standard Horizon GX1700 installation.  The coiled GPS cable on top of the radio allows me to extend the GPS to the cockpit.  It's always nice to see a screen, tiny as it is, to navigate to a waypoint.  The required DSC & channel 16 warning labels are strategically placed above the radio.  The terminal strip (nickel plated) is visible just below the radio.  Each connection on the terminal strip is labeled to facilitate repair.  I hate having to reverse engineer one of my installations especially when labeling is so easy at installation time.  My installer background!

Using this hardware makes the job of navigating easier when traversing tricky shallow water, going around an island or motoring to a location over the horizon.  It frees me up from the tedious task of steering so I can complete chores on board while maintaining a lookout.

Later I will add an iPad Mini 4G equipped with internal GPS and mapping software as a substitute to a chart plotter.  While a chart plotter doesn't give you visibility in fog or the dark it will help to keep you oriented, making it easier to deal with an issue around you.  The screen resolution of an iPad is very good and it is so easy to zoom in/out with the touch screen.  To make an iPad waterproof, tuck it inside an acrylic topped Pelican case.

Alternatively if you use a laptop as a chart plotter, then wire the GPS NMEA data output +/- to the PC RXD data +/- input.  Connect the laptop to boat battery for long term operation.

Notice the tiny toggle switch on the aluminum mounting plate at the top left of the radio bracket.  It operates the LED light to illuminate the power panel.  Up is permanent on and down is momentary on. 

NOTE on GPS COORDINATES - Just because your GPS navigator shows "GPS Data OK" does not mean you should take the coordinates as gospel.  It is sometimes possible (although rare) for a GPS navigator to have a fix that is off by some degrees.  In our case both navigators were off by 1500 meters with absolutely no indication of a problem.  I've experienced this only once as my buddy and I floated near a railroad trestle which is also under a high tension line.  Both navigators were off by the same amount and changed at the same time as confirmed by a second trip into the affected area.  The fix was to leave the area and power cycle both navigators.  If you don't power cycle the navigator, the coordinates will remain off till it is power cycled.  This is the only place I have experienced this problem but other people tell me they have experienced it in several places.  I now regularly compare the GPS coordinates against a chart (another good reason to carry a chart).  By the way, the aircraft version of a GPS navigator is protected against this by checking itself.  This is the basic difference between a commercial and a personal unit.  Problem is, you can't afford the aircraft version!  TOP
 
CHART PLOTTER (PC or iPad) - Most lake sailors don't need a chart plotter to see where they are.  The shore close by makes it somewhat self evident!  But every once in a while somebody goes off the beaten path and then a chart plotter comes in real handy to get you unlost!  Hopefully you can figure out how to get back!

(Laptop equipped with mapping software) - While a laptop equipped with mapping software is hardly practical in the cockpit as a navigation instrument, in a pinch it can be used as a chart plotter at the navigation station.  Good luck tying it down for sailing.  Its also more difficult to operate from the boat battery.

(iPad) - I prefer an iPad equipped with Navionics in the cockpit, especially if the pad is in a water tight case.  However, it can shut down with high temperature so be careful exposing it to the sun.  The details that Navionics can display are excellent.  The really nice feature is that users are encouraged to submit corrections to update or correct the map.  Two of us are doing this at our lake and the accuracy has really improved over this past summer.  Everybody benefits.  The application is free, its the map you have to pay for.  It is real efficient to charge or operate an iPad from the boat battery using a 12V USB adapter.  The 2A version will get it running that much quicker and is more dependable than the el-cheapo crap purchased at a convenience store.  (Just because the iPad battery charge indicator shows 100% it does not mean the battery is full.  When the charging icon disappears, then the battery is fully charged). TOP
 

ABOUT NMEA DATA - NMEA (National Marine Electronic Association) data is a communications protocol for marine navigation devices.  The data consists of sentences that are transmitted sequentially to provide essential machine commands and position fix data necessary for electronic navigation.  A subsequent sentence may supply new data but many of them are repeated and therefore result in no change to the receiving device.  Each device watches for a sentence that applies to itself and ignores the rest.  Effective and simple.

Data transmission is uni-direction, outbound from the GPS to other devices.  In the NMEA standard there are no commands to indicate a GPS navigator should send something specific.  Instead a GPS navigator just sends all the data, fully expecting much of it to be ignored.  The GPS navigator cannot detect if data was read correctly or re-send data that wasn't received.  Instead the navigator just checks the checksum and ignores the data if the checksum is bad, figuring the data will be resent later.  There are many sentences in the NMEA standard for all kinds of devices used in a marine data network. 

It is possible to view the NMEA (ASCII data) transmitted from a GPS navigator by monitoring it with HyperTerminal, a software communications application that comes with Windows XP & earlier.  This is a handy testing technique to determine if the GPS navigator is sending NMEA data.  If HyperTerminal can log the data then you can also build a history of the entire session by saving it in a file.  I used to monitor async data with a "dumb" async terminal as a test technique in the late 1970s.  Today you have to monitor async data using HyperTerminal. 

There is little specific information about NMEA wiring.  The physical interface is actually EIA422 (resembles RS232) with isolated inputs and outputs.  The NMEA standard uses a simple ASCII data format; 4800 baud asynchronous, 8 data bits, 1 stop bit, no parity, serial communications protocol.  A talker (sending device) can have a unidirectional conversation with a nearly unlimited number of listener (receiving devices).  The majority of the data payload is in the form of "sentences" that contain machine commands.  The only readable sentences are coordinates.  If the PC is equipped with USB ports then it will require a USB to serial adapter.  While I have not tested the interface connection to the boat network, it will require a serial (COM) port with a null modem adapter to satisfy the EIA handshake protocol.  If a bulkhead DE9 connector is incorporated in the boat network it would make it convenient to connect the PC to.  You might learn some interesting things about NMEA data protocol while a GPS navigator talks to a Tiller Pilot and plots your track on the PC screen. 

NOTE About Garmin GPS Navigator - A Garmin GPS navigator converts lat/long coordinates using the datum selected by the user when sending NMEA data.  This is indicated in the proprietary PGRM sentence that a program uses for map datum that is not a NMEA standard.  Be sure to set your Garmin navigator datum to WGS84 when communicating to a NMEA capable device.  There might be an equivalent setting for Magellan and other GPS navigators.
 
IMPORTANT - All the "toys" available on a boat aren't necessary on a lake with visibility to shore, but there is something inherently comfortable in telling it to take you to a destination and unfailingly getting beeped as you pass each waypoint.  The use of this technology does not excuse a vessel operator from maintaining a vigilant lookout or using a printed chart. 

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