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ContinuousWave: Small Boat Electrical
|Author||Topic: NMEA-0183 Interface|
posted 10-30-2011 12:08 PM ET (US)
Details of the NMEA-0183 interface between two device are given below. The information is intended to be a comprehensive example of the steps used to successfully interface two NMEA-0183 devices, a Lowrance HDS-8 and a Standard-Horizon GX1500S, and also as an aide mémoire for my own use. Others may find it useful to improve their understanding of the process. There are several layers of connection and configuration to be accomplished.
The first layer to configure is the physical electrical connection between devices. We are able to connect the devices electrically because the manufacturers have provided serial data ports which are compatible, that is, they are rated as NMEA-0183 ports. Unfortunately the NMEA-0183 standard does not specify details of the physical components for these connections, so each manufacturer tends to implement them with their own selection of connectors, wires, and color codes. In the case of the two devices under consideration here, both provide their physical interface on short cables with wire pigtails, and wire color insulation is used to identify the signals carried on the wires. The names given for the signals on the wires is not uniform, adding additional complexity. The first step in untangling this confusion is to identify the signals and the wire insulation color used to carry them. The signals are divided into two categories: output, also called transmit (TX) or talker, and input, also called receiver (RX) or listener. We make three charts, two for the Lowrance and one for the Standard-Horizon.
Lowrance HDS-8 NMEA-0183 Signals--Single-Ended
Lowrance HDS-8 NMEA-0183 Signals--Differential
Standard-Horizon GX1500S NMEA-0183 Signals--Differential
Next, we sketch out the interconnections, using the output of one device to connect to the input of the other device. The electrical form of the inputs and outputs takes two general approaches: differential signals or single-ended signals. Differential signals are electrically neutral to ground. Single ended signals are referenced to ground. Again, to make the interface more difficult for us, we find that devices often use a combination of differential and single ended ports. In the case of the Lowrance HDS-8 device, its ports can be switched to either method. The Standard Horizon GX1500S uses differential ports. Accordingly we need two interconnection lists, depending on the configuration to be used with the Lowrance; one when it is using differential inputs and outputs, and one when it is using single-ended inputs and outputs. The connection arrangements are shown below.
Please note that the Lowrance HDS-8 can be configured for either one differential NMEA-0183 input-output pair, or as two single-ended NMEA-0183 input-output pairs. This change is accomplished in a configuration menu (explained below).
I have tested both of these wiring configurations and can verify that they work.
The above listing describes the physical wiring and interconnections between devices. In addition, each device must be configured so that the serial data characteristics of the ports which have been electrically connected are matched. The standard NMEA-0183 configuration is for 4,800-bps, 1-stop-bit, no parity bit.
HDS-8 Port Configuration
--select the PROTOCOL to be RS-422 (for differential) or RS-232 (for single ended)
--select the serial port speed to be 4800 (for standard NMEA communication). Note that when in the single-ended mode the speed of the serial ports can be set independently. This is very useful if a second device is to transmit data to the chart plotter at a different speed, for example, an AIS receiver that sends at 38,400-bps.
--port speed is the only configuration; there are no options for stop bits or parity bits; these are assumed to be fixed as 1-stop bit and no parity bit.
GX1500S Port Configuration
We have now made the physical connection between our devices, and we have properly configured the data characteristics of the two serial ports to they can communicate with each other. Now we must configure the data that will be sent on these communication links.
The general assumption of the data to be exchanged is as follows:
--the GPS receiver will send data to the radio containing the vessel position, the vessel course, the vessel speed, and the current time; and,
--the radio will send data to the chart plotter containing information received from other vessels about their current position.
The purpose of the data exchange is to permit the following to occur:
--the radio will upon request transmit the vessel position, course, speed and time, either as part of a distress message broadcast to all stations, or as a message sent to a particular individual vessel or group of vessels; and
--the chart plotter will upon reception of another vessel's position, display that position on its chart display.
To provide for this information exchange, we have to configure the devices to send the correct data.
HDS-8 Data Configuration
The path to the configuration screen is via the following menu choices
--enable output to occur by toggling the OUTPUT button to the ON state;
--select the NMEA-0183 sentences to be sent with the OUTPUT SENTENCES... selection;
--choose output sentences from the many available options.
Note: for the interface to the radio, the only sentence necessary for the GPS receiver to send to the radio is the RMC sentence, available under the NAVIGATION sub-menu in the selection screen.
GX1500S Data Configuration
There are no configuration options for the radio. The radio specifications say the radio is anticipating receiving the following data sentences: GLL, GGA, RMC, GNS. The radio specification say the radio will be sending the following data sentences: DCS, DSE.
We have now finished three layers of interfacing between the NMEA-0183 devices:
--physical wiring and electrical connections
--serial data port configuration
--data parameters to be exchanged
Fortunately, from this point onward in the data communication the NMEA-0183 standards take over, and there is no further configuration necessary. The devices will provide each other the particular data they can exchange in a format and form which is mutually compatible and understandable. We can take a peek under the hood of this process to see in more detail what is going on. Let's look at the structure of a NMEA-0183 sentence.
The NMEA-0183 sentence RMC is defined and structured as follows:
RMC--Recommended Minimum GNSS
RMC - Recommended Minimum Navigation Information from Global Navigation Satellite System
Verifying that the interface of two devices using NMEA-0183 protocols has been successful can be a bit more difficult unless the devices themselves provide for some means of monitor or diagnostic data interface.
In the case of the Standard-Horizon GX1500S radio there is a front panel display which will show the navigation data received by the radio from an attached device. The displays shows:
--time or epoch of the data;
It is important to note that the display shows the last values received, and the values persist for quite some time after a connection to the device that supplied them has been lost.
The display of the data can be selected from the front panel controls on the radio. Hold down the [H/L] button for a second or two; the display changes between showing navigation data and radio channel information. In this way you can observe the data from your connected device and determine if the radio is properly receiving it from the GPS receiver.
In the case of the Lowrance HDS-8 chart plotter, there will not be any data sent from the radio until it has received a digital selective calling (DSC) transmission from another vessel. The best method for testing is to arrange a test transmission from another vessel with a DSC radio. For example, you can request a position poll from a remote vessel, and the other vessel should send you their position. The HDS-8 chart plotter should display the other vessel's position on the navigation chart display.
posted 10-30-2011 03:09 PM ET (US)
ASIDE: One might wonder why the radio would retain out-of-date information about vessel position, speed, and heading. The basis for this is the notion that in an emergency it is desirable to know the epoch or age of the information transmitted. If a vessel were in distress and its GPS receiver became disabled but its VHF Marine Band radio were still operational, the radio could transmit a distress message with the last position information it received from the GPS receiver. The message transmitted would note the age of the information, and rescue searchers could use that information to make an informed estimate of the current position of the vessel in distress. In this way, it is better to have old information than to have no information.
It is also possible to enter a position into the radio manually, using the front panel controls. In this way a vessel whose GPS receiver were not operational could have an operator enter the position in the radio manually. The radio would then transmit that position in the distress call.
posted 11-07-2011 08:54 AM ET (US)
[At this point I introduced my concept of the Universal NMEA-0183 Interface. I have moved those articles to a separate REFERENCE section article and combined them into one longer and better illustrated article. Please see the REFERENCE article Universal NMEA-0183 Interface.
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