NMEA numpty!

[mbird]

As an NMEA novice, I wondered if you experts could clarify something for me.....

All I have ever done with regard NMEA in the past is to connect my DSC VHF to my plotter for position polling. With the new boat, I am intending to revamp the nav equipment, as there is only a plotter/radar up on the flybridge and a 20 year old Autohelm Navcentre at the lower helm. There are depth/log repeaters up on the flybridge, and also autopilot control via a ST7001 controller upstairs and and ST5000 controller downstairs.

I know that NMEA0183 is the way the older equipment communicates, and I think I understand that this is done by the various instruments transmitting "sentences". If I've got this right, the "sentence" is actually a three character identification followed by a value. So, for example, a depth display would transmit the sentence for depth followed by the actual depth value. I assume therefore that the receiving equipment must listen out for the sentences that it can receive and ignore the rest? Does this therefore mean that the sentences are being sent continuously from the originating instrument, whether anything is listening to is or not?

I know this may all sound a bit basic, but I'm just trying to understand how the protocol works before I start ripping out wires

[Hockham Admiral]

Mark, you're a braver man than I if you start "ripping out wires"... and my other hobby is Amateur Radio!

[Antares_9]

Fairly correct Mark, remember a device can only take in one set of info per channel (cable pair) though a device can talk to up to three listeners at once otherwise it gets very confused, it becomes more complex as there are different NMEA levels and some stuff only listens for specific sentences, none of it is mission critical. You may find it easier in a number of instances to eithe use Seatalk on the Raymarine stuff and even use bridging through some devices or even using multiplexers.

I'm a certified advanced NMEA installer authorised by the NMEA to 0400 standard and even I find it a bit of a head scratch job sometimes, thank goodness for N2K I say. If you are going to connect then try to use (or replace any existing cabling) with twisted pair tinned shielded cable of between 22 / 28 AWG and ground the shield at the talker end only to give the job the best chance of working in a crowded loom.

[mbird]

Thanks David. I knew my understanding was fairly simplistic. I had assumed that a device could listen to several different types of information through just one in port, and the sentences were used to seperate out the various types of info.

The new kit I am thinking of installing does have the latest Raymarine Seatalk ports, and NMEA2000, as well as 2 NMEA0183 ports, but only one of these is an "out" port. I am guessing the autopilot would use the 0183 network for track data, but also the DSC VHF would need to as well for position data. Am I correct in thinking that both the listeners could be connected to this single talking port?

My head hurts already!

[Guest]

Hi Mark

Will photocopy some block diagrams for you and data stream information and bring up on Sunday.

James

[mbird]

Thanks James, any info is helpful!

[mbird]

Just found quite a good explanation here http://www.actisense.com/Downloads/TechTalk/NMEA%200183/The%20NMEA%200183%20Information%20Sheet.pdf

[Antares_9]

Yes Mark, you can connect to two listeners, even three, but the autopilot may well have a lot more than simple track info being fed currently, fluxgate heading, and the like, there may be a fast heading sensor connected, in fact there must be if you want to overlay the radar data to your plotter in a meaningful way. Block sentences and the like need not concern you as you have no control over them, save to switch them on and off and that is obvious in device menus, all you can affect is the physical connections between devices and that is very straight forward matter of physicalities.

[mbird]

Hmmm, that's then brought up another point with the autopilot. It is quite and old Autohelm ST5000 system, which doesn't have a course computer, just a fluxgate, a rudder position transducer and a head unit connected to a hydraulic pump. I think all it can do is receive GPS location and route data from the plotter. It's a very basic system, but should it still interface with new kit do you think?

[Antares_9]

Would you want it to? it will be connected up to seatalk I imagine which will work very well, if you wish to replace the current plotter with new and get it to work with that then yes it will via the control head, I am sure I have and old scanned copy of the install manual in my archives if you do not have one. The only issue I see with interfacing to modern stuff is the 0183 version but most stuff is backward compatible.

[mbird]

Thanks David, the voice of reason as always! I'm probably overthinking things at this stage anyway, so I'll just have to see what transpires once I've established which route we are taking with new plotters & radar.