Just thought I would relay a tale about my experiences with the NMEA 0183
This was invented about 20 yrs ago more or less to establish a language for different manufacturer's instruments to "talk" and "listen" with other instruments.
There are many bits of data, but they developed a bunch of sentences which contain the data and the listener can "extract" it and "use it" to display, plot or drive an autopilot
for example. You probably know all this.
Back in the 90s I put this to the test and managed to get three manufacturer's instruments to have successful "conversations" and there was kind of a "gee whiz.. this sh*t works!"
Fast forward to the new mellenium and I decided to upgrade instruments... principally a chart plotter MFD by Raymarine
(C80). The other devices remain in the system... KVH sailcomp 103AC which is also a "cockpit repeater" of XTE, BTW and DTW which sends heading info TO the C80 and a B&G cockpit
repeater for position fixers which can supposedly display up to 12 different bits of data.
The main ones on the B&G being COG, SOG and XTE, BTW and DTW and then you can program one button to display some other data .. I used to leave it on TTG (time to go to waypoint) I found this data up in the cockpit
very handy... especially once a waypoint was programmed.... because I did not have to go up and down to see if we were "on course" etc... The autopilot
is set from the cockpit so I can observe the data as I steer. Makes sense! Today people are installing "pods" with MFDs in the cockpit these days.... but I have a "dash" type instrument panel above my companionway
with a row of displays. It works and it is readable from anywhere in the cockit.
My first shock was when the B&G would not report COG or BTW accurately. It displayed 2 digits and a decimal place. So 105.6° read as 05.6° which made little sense as it could mean 5.6, 105.6. 205.6 or 305.6° The old display dumped the decimal and rounded off... so it would read 106°. Weird ha? The TTG did not display at all and the KVH would not show XTE data!
I was told that the equipment
was too old to chat nicely... so I turned to a Brookhouse MUX which is a programmable NMEA
multiplexer. This device allows you to do lots of stuff, most of which I do not understand, but you do it by connected it to a computer and "programming" the NMEA
data stream. Brookhouse programmed my decimal place problem away, but not the TTG or the XTE on the KVH. They said if I could "read" the sentences from the devices with a PC then it WAS possible to get that data back with some reporgramming.
When I decided to add AIS
to the mix... I lost
the required baud rate of 4800 for my NMEA network. Without some fix I had to choose between the AIS
and the NMEA repeaters. Raymarine
had a device which offered promised - a NEMA Seatalk
is the "daisy chain" propriatary network for Raymaine... but it is NOT NEMA.... The Bridge will pop out and allow you to pop in NEMA data essentially "through" the Seatalk network. So for about $150 I had a possible fix.
The AIS got the C80 NMEA port at 38400 baud and the KVH heading went to the NMEA on the Bridge and the NMEA output of the Bridge went to the MUX mulitplexer and on to the cockpit repeaters. Mind you this involves lots of tiny little wires connect to "eruostyle" busses/terminal blocks. What a mess!
So I do all the little wires and fire everything up... and lo and behold... the heading data DOES get into the C80 (as promised) and the 4800 baud data DOES go to the MUX... but the B&G now does NOT display BTW or DTW but it DOES display TTG. The KVH seems to be working fine as a repeater. WOW this never ends...
Brookhouse tells me I need to "find the BTW and DTW" data in one of the sentences of the Bridge... because it has decided to select one of many ways to communicate this sentence... which the KVH CAN read but the B&G cannot! Is this insane or what?
So now I have to get a laptop
out to the boat and the software
to "read" the sentences from the Bridge and compare that with the sentences from the C80 at 4800 baud without the Bridge and then reprogram the MUX to do the BTW and DTW sentence that the B&G likes. Is this possible? Dammed if I know.
I wonder how many other sailors find themselves sucked into the NMEA 0183
magical mystery tour? Oh... BTW the AIS is not working with the C80 yet... still have to figure that one out!
Break.... more to follow....