INTERCONNECTING NMEA-0183 DEVICES
FOURTH STEP: CONFIGURING RADIO TO TALK TO CHART PLOTTER
I am jumping ahead because I think NADOL has actually got the chart plotter talking to the radio. Now we want the radio to send to the chart plotter. The wiring necessary is given above in my first step instructions.
I think this is where NADOL is struck, because he has only a one-way communication wired between the chart plotter (as TALKER) and the radio (as LISTENER). I show that wiring above. When completely wired, we have two-way communication on two separate circuits, one in each direction. Therefore we need to make a second path, with the radio as TALKER and the chart plotter as listener. See my wiring advice to implement this second path.
Since the baud rate adjustment on the chart plotter is properly set, there should not be much to do. We now turn to see what data the radio can send to the chart plotter. This is an important second phase of connecting the radio and chart plotter.
A DSC radio can send data to a chart plotter
based on radio transmissions it receives via digital selective calling methods. We turn to the LINK-5 radio documentation to see what it has to say about NMEA-0183 output. Again page 52 we see:
NMEA output sentences: DSC (for DSC call), DSE (for enhanced position).
In the case of this radio, I do not believe there is any sort of configuration necessary. It only knows two sentences to send, and it will only send them if it receives a DSC message containing that data. So the radio is ready to send these two sentence, and no configuration necessary. There is one option for the COMM PORT, shown on page 22 of the manual: NMEA CHECKSUM. This options refers to how the radio will send data. Usually data is sent with a checksum character at the end of the packet. The default in the LINK-5 radio for NMEA CHECKSUM is to be set to ON. I recommend leaving it at that setting.
An article about NMEA-0183 at
Wikipedia says the checksum is optional on most NMEA-0183 sentences; it is only compulsory on a few, including RMC. But this is contradicted by other sources, such as Eric Raymond's excellent website,
NMEA Revealed, which indicates the checksum is mandatory. I suspect that in many implementations of NMEA-0183 the LISTENER may ignore the checksum, if one is sent. Because the data is being sent only a short distance on a wired connection, it is unlikely that the data transmission itself would create errors in the data, and ignoring the checksum is probably unlikely to result in a problem. However, the option to send or not send the checksum creates one more variable. I do not recall seeing this option in most NMEA devices, and therefore I would leave it set to NMEA CHECKSUM ON.
Note also that the LINK-5 appears have the COMM PORT speed fixed at 4800 baud. No adjustment needed there, that is the standard rate.
Now we turn to the chart plotter to see if NMEA sentences DSC and DSE are in its vocabulary. Using the documentation (linked above) we again look on page 61 to see if these sentences are supported by the HDS-7. Under the table heading AIS/DSC, we find the chart plotter can receive DSC and DSE. That is good news. We now know that the radio and chart both understand DSC and DSE sentences. When they are properly wired together, the chart plotter should respond to data from the radio.
Exactly what the chart plotter will do when a DSC radio sends it data from a digital selective call received on the attached radio is not exactly set in stone. Typically the chart plotter will respond to input from the radio by providing some sort of presentation on the electronic chart. I have actually explored this behavior is some detail. Rather than repeat my findings here, please see my lengthy article about digital selective calling at
Chart Plotter Interface in Digital Selective Callinghttp://continuouswave.com/whaler/reference/DSC_ChartPlotter.htmlWhen interconnecting a chart plotter to a radio for this function, there is no simple way to test if the connection is working. The only way to test is to send a digital selective call to the radio
from another DSC radio. You need a second station to help you in testing, or you need to have to radio at your test location. The simplest test is to send a DSC request from your radio to the second radio asking for the position of the other vessel. That vessel should respond with a reply DSC call--if working properly--and send your radio their position. Your radio--if all is connected properly--will send that data to your chart plotter. And your chart plotter--if working correctly--should plot the position of the other vessel. The only way to determine if this works is by testing the entire system. Make a DSC call to the other radio, get a response, and see what the chart plotter will do. There are several chances for this test to break down, other than in your wired connections. For that reason, test with a second radio that is known to work properly with DSC calls. That will take some of the other places for failure out of the test system.