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LINK-8 NMEA-0183 Interface; DSC and AIS Datagrams
|Author||Topic: LINK-8 NMEA-0183 Interface; DSC and AIS Datagrams|
posted 05-12-2014 12:47 PM ET (US)
LINK-8 VHF Marine Band Radio
The Lowrance LINK-8 VHF Marine Band radio provides many attractive features at a comparatively modest price point of about $300. The radio is a Class-D DSC qualified radio, as it must be to be sold in the USA, and it has an internal automatic identification system or AIS receiver. The marine radio and AIS receiver share a common antenna. The radio also has data interfaces for NMEA-0183, which is quite common, and for NMEA-2000, which is not so common. Radios from other manufacturers with all these features sell for about twice as much as the Lowrance LINK-8.
I have a LINK-8 on my bench for some testing, and I needed to work out the NMEA-0183 wiring interface. The radio provides all of its serial data interfaces on an accessory socket, and a mating plug connector with a short cable to a wire fan out is provided. The wiring of this interface is a bit complicated, and it took me a few minutes to work out the signals and wire colors. I found there were three sets of interfaces on this connector or wire fan out.
The first set of signals from this interface are apparently fixed at 4800-BPS, the standard NMEA-0183 baud rate, and are intended to interface with an external GNSS receiver (for input of position data to the radio from the receiver) or to interface with a chart plotter (for output of digital selective calling data to the chart plotter from the radio).
Lowrance has attached two tags to the Yellow and Green wires as aids in identifying their purpose. The tags say GPS(+) on yellow and GPS(-) on green. This just confuses me, but maybe it helps someone else.
The LINK-8 also provides additional data lines, which are operating at the NMEA-1083-HS (high speed) baud rate of 38400-BPS. This interface is apparently intended to send AIS data to a chart plotter or AIS plotter. The data rate is eight times faster because an AIS receiver can produce a lot of data output per second, and all the data might not fit into a lower speed serial data link. Using a menu selection on the radio interface for set up, the port speed on this "AIS" port can be set back to 4800 if desired. Lowrance does warn about possible loss of data at this speed.
The above signals are identified as being RS422. This is a differential signal output signal. There is no input port provided.
Curiously, the LINK-8 has a third serial data output port, which also operates at the 38400-BPS rate, but is configured as an RS-232 port, an unbalanced interface. The wiring for that is as follows:
This is also an output port, and there is no input port provided.
To interface the LINK-8 to a chart plotter or terminal in order to observe the chart plotter response to its data output or to capture the data on a terminal, and using my Universal NMEA-0183 wiring and connector plan, I interfaced the radio data wires as follows:
posted 05-12-2014 03:38 PM ET (US)
With the NMEA-0183 interface correctly configured and wired, I made a little test of the LINK-8 radio.
The LINK-8 sends a position request to another DSC station. The other DSC station replies with its position. The LINK-8 outputs some data about the other boat's reply call to an attached chart plotter. The chart plotter is also a Lowrance product, an HDS-8.
If the LINK-8 is connected to the chart plotter via NMEA-0183, the response of the chart plotter to the data from the radio is "UNDESIGNATED DISTRESS" received and an on-screee alert pops up. A waypoint is plotted at the other vessel position.
If the LINK-8 is connected to the chart plotter via NMEA-2000, no on screen alert occurs. A waypoint is plotted. In the "Get Info" screen on the waypoint, the description says: "Grounding Buddy". Neither seems very appropriate.
More testing to follow.
posted 05-14-2014 04:22 PM ET (US)
Here is a full write up of the LINK-8 radio testing with the HDS-8 chart plotter:
posted 06-27-2014 09:52 AM ET (US)
Using NMEA-0183 for connection between a LINK-8 and a chart plotter that has only one port will likely result in a compromise. The LINK-8 appears to sends its DSC data on one port, and its AIS data on a second port. I do not believe there is a way to combine both the DSC data and the AIS data onto one port.
An example of this problem is seen in interfacing the LINK-8 to the HDS-7. The HDS-7 has only one NMEA-0183 port. The speed of that port will be the same for its TALKER and LISTENER, so the port speed is limited to one setting of the baud rate.
The HDS-7 TALKER will send GPS information to the LINK-8 LISTENER at 4800-BPS. This connection is wired as follows:
HDS-7 sends GNSS to LINK-8
This give the DSC radio the vessel position data it needs to properly send DSC calls with the vessel position automatically provided.
To get data to the chart plotter from the radio, it appears a choice has to be made. One must decide if the chart plotter will get AIS information or DSC information.
To send AIS information to the chart plotter from the radio, the AIS port has to be set to 4800-BPS from its normal 38400-BPS setting. A menu option in the radio configuration provides for this. The LINK-8 TALKER will send AIS information to the HDS-7 LISTENER at 4800-BPS. This connection is wired as follows:
LINK-8 sends AIS to HDS-7
With the HDS-7 and LINK-8 wired in this manner, the HDS-7 chart plotter should display the position of AIS target vessels. The chart plotter will not plot the position of other vessels that have sent their position data to the LINK-8 using a DSC call. For example, if another vessel sends a DSC distress alert message with its position, the chart plotter will not plot that position. Or if a buddy-boat replies to a position poll request, that position will not be plotted.
In this configuration the AIS data is being sent at a slower baud rate than normal. Usually AIS data is sent at the NMEA-0183-HS (high speed) rate because it anticipated that in an area with a dense population of AIS transmitting vessels the amount of AIS data will be so large it could not be sent at the standard baud rate. With the AIS baud rate turned down to 4800 it is possible that some AIS data might be lost, but I don't expect this unless there are a lot of AIS vessels in range of the receiver.
If it is desired to have DSC data plotted, there are two options:
--disconnect the AIS data and re-wire for DSC data
--purchase a NMEA-0183 multiplexer, combine the AIS and DSC outputs from the LINK-8, and connect the multiplexer output to the chart plotter
The first option removes display of AIS vessels from the chart plotter. The second option requires purchase and wiring of an expensive multiplexer. Neither option is a very good solution.
The overall best solution for interconnecting a LINK-8 and HDS-7 is to move to a NMEA-2000 connection. Using NMEA-0183 will permit both the AIS data and the DSC data to be sent to the chart plotter from the radio, and the high speed of NMEA-2000 will eliminate any bottle neck in the data rate for AIS information.
posted 06-27-2014 02:33 PM ET (US)
For completeness, if it is desired to pass the DSC data to the chart plotter from the radio, then the HDS-7 chart plotter's LISTENER is connected to the non-AIS TALKER of the LINK-8 radio, as follows:
LINK-8 sends DSC to HDS-7
posted 12-12-2014 08:10 AM ET (US)
The question of interfacing a Lowrance LINK-8 VHF Marine Band radio with digital selective calling (DSC) and automatic identification system (AIS) receiver to chart plotter devices using NMEA-0183 interfaces comes up from time to time. This discussion has excellent information on accomplishing those interconnections, and details are given above.
To summarize, the LINK-8 has quite an array of NMEA-0183 inputs and outputs. I briefly review them again.
The most conventional interface is a set of standard NMEA-0183 TALKER and LISTENER interfaces operating at 4800-BPS. The LISTENER function is provided on a differential pair:
LISTENER A = Yellow
The TALKER function is provided on a single-ended circuit:
TALKER A = Orange
These signals can be used to interface the LINK-8 to a chart plotter. The chart plotter will send GNSS data to the radio so the radio can know the ship's position. The radio will send DSC data it receives to the chart plotter so that the position of other vessels can be plotted.
The LINK-8's AIS receiver output is available on two interfaces. The most useful of these is a NMEA-0183-HS TALKER which operates at 38,400-BPS. This can be connected to a dedicated AIS display or a regular chart plotter in order to show the position of AIS target vessels. The outputs are
HS TALKER A = Black
In the radio's settings option, the speed of this interface can be reduced to 4,800-BPS if desired.
The LINK-8's third interface can also operate at 38,400-BPS, but it is designated as an RS-232 output, apparently intended to feed a serial port input on a computer. This interface is provided by
RS-232 TXD = Blue
Despite having three sets of interfaces, the LINK-8 radio will actually become a bit of a problem to interconnect to a chart plotter via NMEA-0183 if the chart plotter has only one LISTENER port. The problem is created because the LINK-8 does not combine its DSC data and AIS data onto a single TALKER port. If the radio is to be interfaced to a chart plotter that has only one NMEA-0183 port, a choice will have to be made between sending DSC data or sending AIS data to the chart plotter.
This same situation existed in early models of the Standard-Horizon VHF Marine Band radios with AIS receivers, but they corrected this in later receivers by providing the ability to multiplex both AIS and DSC data onto one TALKER output. This greatly simplified interfacing the radio to a chart plotter with only a single LISTENER input port.
Of course, the inclusion of a NMEA-2000 interface on the LINK-8 allows one to make a much simpler interconnection of the radio to a chart plotter via a NMEA-2000 network backbone. Many boaters will prefer that method.
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