FA50 NMEA 0183 problem

[amarin]

I have identified a negative ground leak on my boat, pinpointing the issue somewhere between the FA50 NMEA output and the NMEA 0183 to 2000 converter.

Upon conducting measurements, I discovered that the white (TD-A) and grey (SILENT-C) wires of the FA50 power/data cable are in contact with the FA50's chassis (ground) and, consequently, with the boat's bonding.

Subsequently, I disconnected all cables (antennas, ground, power, etc.) from the FA50 and inspected the contacts directly on the FA50's circuit board's connector. The outcome remained the same. So, the contact is inside the FA50.

1) Is it normal for these two wires to be in contact with the chassis?
2) Is it OK/safe for the FA50 to operate without being connected to the boat's bonding?

Thanks!

 

[Sparky Wire]

I will ask someone to dig out a FA50 and check the wiring. The Silent-C (common) wire shouldn't be connected to anything other than the matching Silent wire normally via a toggle switch to enable or disable silent mode. The wiring should never touch anything external to the unit. (Other than a switch and going right back to the unit). If that is used in some other manner, then it is wrong. The positive going wire for NMEA 0183 (TD-A = Transmit positive) would be impossible to go directly to ground as it goes to a 0183 transceiver chip that might have a ground for ground level (zero) and then is positive going as it puts out the positive side of the RS422 differential signal. The only thing that should be connected to that wire is the receiving 0183 positive transceiver on the other piece of equipment. Are you running the FA50 off 12v or 24v? What converter are you using? Is the NMEA 2000 power tee running from an isolated power source as required by NMEA 0400 installation standards?

 

[amarin]

Thanks a lot!

The power supply is 24V. The converter is Quarc QK-A032. I am unsure about the NMEA 0400 compliance, but considering a reputable shipyard, I believe it's done as it should be (there is a 24V to 12V converter as a source for the NMEA backbone).

I think the key indicator could be that there is contact between the wires (TD-A and SILENT-C) and the aluminium chassis (ground), even on a completely disconnected FA50.

Could lightning or perhaps corrosion be the cause of the problem? The electronic circuit looks in pristine condition.

 

[Sparky Wire]

Always check that the 24v to 12v converter powering the backbone says "ISOLATED" on top of it. Many builders go cheap and if not isolated it can lead to grounding loops on a mix voltage boat system.

 

[Sparky Wire]

Checking an FA50 we found no short between unit ground and either of those wires you asked about on the unit. Between those wires also no short is found. If you disconnect the unit completely from the boat and ohm them and still getting a short, the unit is clearly damaged.

 

[amarin]

Yes, I am afraid it is damaged.
Thanks a lot!