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RS422 (NMEA 0183) reading issues
#1
I am using the RS422 adapter suggested on the website but after trying to connect it to my AIS receiver for most of the day, I have been unable to get any valid responses in the software.

I'm just trying to figure out if my device could be faulty or not.

My receiver is an MXA-5000 (http://www.icomamerica.com/en/products/marine/ais/mxa5000/default.aspx), which has NMEA 0183 output. I connected the RS422 output to the adaptor R+ and R-, then connected that directly to the Pi.

The Pi recognizes the USB interface and the green light flashes with incoming data but the inspector shows nothing. I have made both a hex dump and a raw data dump of the incoming data (also tried without ground to see if it made any diff) and attached it here.

Do the packets look correct in any way? Or is the data just garbage?

I've actually had a look myself and can't see they match a normal NMEA sentence but maybe this is due to protocol packing or something. Unfortunately I don't know enough about it all to say one way or another.

(EDIT) Just a little more info:
Data output is RS422 balanced @ 38,400 baud (http://www.icomamerica.com/en/products/m...tions.aspx)

I tried this, and 4,800 baud inside the software but received nothing. The dump is just a 'cat' from the device in Linux.

The AIS data WAS being received into my plotter before I switched interfaces.


Attached Files
.zip   rs422-dump.zip (Size: 77.43 KB / Downloads: 160)
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#2
I used to be a diagnostic engineer. One of the common issues with rs485 and rs422 is a missing groung/common/neg/ zero volts connection.
Rs 422 is a 3 wire system. Do you have three wires connected or two?
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#3
I tried with ground and without when I was testing originally but good news is I solved it today by switching the positive and negative terminals as suggested in another thread on these forums. Seems maybe that the device is labelled incorrectly. Works fine now, even without ground - would you recommend I ground it still?
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#4
(2016-06-14, 08:30 PM)Kraavok Wrote: I tried with ground and without when I was testing originally but good news is I solved it today by switching the positive and negative terminals as suggested in another thread on these forums. Seems maybe that the device is labelled incorrectly. Works fine now, even without ground - would you recommend I ground it still?

If you are using the same power supply for all equipment, then you dont need a ground.
A common problem on boats and indutrial sites is not having a common voltage reference.
For example, 5 volts is only five volts with reference to ground or 0volts. If you have two power supplies isolated by transformers then connect the two communications cables together, you can get what is called a ground loop The potential difference can damage the device at chip level. An RS485 and RS422 driver chip, has a maximum voltage input of 12volts. RS422 standard is 5 volts. D+ sits at 2.5 volts going to five lolts, and D- sits at 2.5 volts going to zero volts. If you have these ground loops, then voltages can get up to 80 volts.
The NMEA standard calls for optically isolated inputs to stop damage from these loops. As i dont have isolated drivers, i just make sure, all my equipment uses one power connection point on the boat. Each item of equipment has its own fuse.
As a side note, the most common cause of damage to AIS devices is the use of the USB cable to a laptop, where the ground of the laptopis not the same as the AIS unit. This where i first found OpenPlotter, as i can use the RS422 output from my AIS transponder and keep all the ground planes at the same voltage. I no longer need my laptop..
Whilst on a boat, we use our house batteries as common power, so long as there isnt much resistance in joints, most of us wont have problems.
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#5
Great! Thanks for the detailed reply.

Yeah I'm using the same house batteries to power both the Pi and the AIS receiver. The wiring was done pretty nicely on the boat by the last owner or manufacturers. There's even a common ground between the starter and house batteries, with each then being isolated on the positive terminal, so the reference should be good.
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#6
Great explanation Tacmed. Thanks!
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