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Low cost AIS

2 replies [Last post]
Mon, 2010-03-08 11:29
nozomi
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Joined: 04/15/2009

At the recent Pacific Cup prep seminary no.2, in the course of discussing AIS the electronics presenter mentioned that the low cost AIS receivers offered by MilTech Marine (SR 161, SR 162) are not compatible with chartplotters due to the fact that they are designed with an RS-232 serial interface instead of the RS-422 interface standard on most chartplotters.

While this is true, it apparently only impacts the newest generation of chartplotters.  For example, our 3 year old Garmin 3206 has an RS-232 interface and works fine with the Miltech Marine SR 161/162, however Garmin informs us that new models (4000 and 5000 series) have RS-422 interfaces and are therefore not compatible with Miltech SR 161/162 units.

Check with the manufacturer of your chartplotter to determine which interface standard is built into your chartplotter and whether of not you can use the Miltech Marine or other AIS receivers with an RS-232 serial interface.

FYI, we have had a Miltech SR161 installed for more than 2 1/2 years and have experienced outstanding performance both with our chartplotter and our laptop navigation computer.

 

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Tue, 2010-03-09 07:52
#1
nozomi
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Joined: 04/15/2009
RS-232 vs. RS-422 (AIS)

Very interesting explanation, I did not realize that NMEA-0183 was RS-422. I have always used RS-232 and NMEA-0183 interchangeably when interfacing onboard electronic equipment and have never experienced any problems whatsoever.

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Mon, 2010-03-08 15:23
#2
Valis
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Joined: 04/24/2007
I, too, have had good results

I, too, have had good results with the SR-161 (and SR-162).  While I'm not saying the problems described at the recent seminar didn't happen, I wonder if there wasn't something else going on.


[edit]


I just got off the phone with Doug Miller at Milltech Marine, and he does confirm the problem with connecting the SR-161 to a true NMEA-0183 input.  He says this only happens occasionally, and only with some chartplotters.  The cause is not understood, but the SR-162 has a different output design and there have been no problems noted with this.  Milltech has a webpage that discusses the AIS / Chartplotter connection issues: http://www.milltechmarine.com/Chartplotter-Tech-Notes_ep_55.html


I suspect that there are grounding issues at play here.  One other factor is that the SR-161 RS232 output only swings from 0 to about 6 Volts, it doesn't swing negative as a true RS232 output should.  In most cases this isn't an issue, but it could cause data to not be transferred in some cases.  I still don't understand how the damage is occuring, but obviously something is going on.


[end edit, resuming my original posting:]


In general, an RS232 (single-ended) output can drive an RS422 (NMEA balanced) input with no problems.  There may be noise issues if the grounding is poor or the distances are long, but in the typical installation it's going to work fine.  Just connect the RS232 output from the AIS reciever to the NMEA "+" input, and ground the NMEA "-" input.


I'm going to talk to Doug at Milltech Marine and see what he says about the SR-161 / RS422 interface problem.  I will report back here.

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by Dr. Radut.