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post #3 of Old 07-19-2012
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Re: Raymarine SeaTalk Troubleshooting

Originally Posted by szigi View Post
Seatalk uses a "one-wire" communication. All the seatalk instrument are connected to one wire, that is the yellow one (the others are +12V (red) and 0V (black or shield) for power).
All instruments on the line can talk and/or listen for data.
Normally, the seatlak wire is at 12V (because internally in every instrument it is connected to 12V via a resistor), and whenver one instrument wants to talk, it will pull it down to 0V with the digital pulses (it switches it to 0v via a transistor).
It also listens for its own data transmission, and if accidentally two instruments want to talk simultaneously, they will hear different data that they are transmitting, because any instrument can pull the line to 0V. In this case they will cease transmit and wait some random time to start transmission again. An instrument will also wait for the seatalk line to become idle (+12V) before starting transmission again. If the line never comes to idle, or it is always at 0V, or have invalid data because of noise etc. on it you get a seatalk error on the instrument.

So I would do the following:

First check to see that you have around +12 on the seatalk line, which can occasionally go to 0V. It is quite hard to see it with a normal voltmeter, but you might. You can see occasional drops in the voltage, or some average below 12V. If you can get an oscilloscope that can show it very nicely. Unfortunately seatalk uses 4800bps data like NMEA, but with binary coding for shorter messages and uses the parity bit for signalling, so it is not that easy to simply connect it to a serial port of a PC and see that data.

If the line is constantly at 0 or 12V, check for any shorts in the cable. Shorts to 0V is OK, it wil not damage anything, short directly to +12 might damage the instrument when it tries to pulse the line to 0V.

Disconnect all instruments and use an Ohm meter to see if there is any short to 0 or 12V.

My best advice would be to try disconnecting/connecting the instrument one-by-one from the seatalk line. A faulty instrument can disable the whole setup. Check that when not connected to seatalk, a device should have +12 on its seatalk line, with possible data.

Make sure that all instrument have good power connection. Bad connection on power can introduce noise to the seatalk line.

Check all connections. I have once seen that by accident the seatalk line was connected to the NMEA out of an instrument, which will obviously disrupt it.
Great post!!!

I have noticed that if one instrument has a bad connection, it screws them all up or makes them flaky.


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