Originally Posted by jwalker
Presuming you're using port 1; what you should actually have is the data out from the 505 connected to the white of the 5012 and the black data ground of your 5012 connected to the screen of the 505. This is your data path. Then the dual colour wire for that port (orange/white) is connected to ground to switch the 5012 into the mode you require. (Unbalanced connection.) If the orange/white wire is not connected to ground, the 5012 port will function in the balanced mode.
Just in case anyone drags this up in the future, I have just spoken to Icom and they have advised me that their NMEA ports are 'balanced'. As such, when conencting to modern Garmin with balanced ports, you should not 'ground' any of the wires. Simply connect TX+(A) on VHF to RX+(A) on GPS and TX-(B) to RX-(B) respectively and all should work. Older Garmins are unbalanced and there is no TX- or RX- as such, but instead the ground wire acts as the 'minus' connection. Garmin refer to this older version as 'single ended' devices.
If Icom are correct in what they tell me then JW's wiring method should not be followed.
Having had to understand all this, which took some time, here is a brief explanation.
Balanced (NMEA v2 and up) means that neither wire is technically a ground wire, but the units sense a voltage difference between the pair of wires. If properly manufactured then the ports are opto-isolated from the rest of the electronics, so actually connecting one to ground by accident wouldn't cause any damage. This type is also known as differential NMEA.
Unbalanced (older method mainly NMEA v1) uses one lead as the signal wire TX+ or RX+ and the other is a ground, though confusingly sometimes referred to as TX- or RX-. In this case it is fine to connect the minus wire to ground. This is known as single ended sometimes.
Important thing to remember is that a TX-(B) should never be conencted to ground on a listener. If you see A's and B's in brackets it means you are dealing with differential or balanced NEMA.
All clear? Yeah thought so!
And finally, any AIS NMEA is balanced as it uses 38k baud rate which is only available in recent NEMA versions.