19 February 2011

NMEA 2000 connector pinout - Smart!

Today my attention fell to this diagram in a NMEA 2000 device manual from Maretron:

As you can see the pin-out is:

Pin 1Shield-Shield
Pin 2RedNET-S+12V
Pin 3BlackNET-C0V
Pin 4WhiteNET-HCAN-H (data high)
Pin 5BlueNET-LCAN-L (data low)

I noticed that the center pin is not carrying ground or even shield, which I naively thought would be the case. In fact the designers of this pin-out were a lot smarter! As this is a circular connector, a dumb user could theoretically try to push two connectors together whilst forcing a wrong orientation. That would force power down the wrong wires.

Let's see what happens if you rotate the connectors:
FemaleMale 90° Male 180° Male 270°
Shield+12V 0V CAN-H>
+12V 0V CAN-H> Shield
0V CAN-H Shield +12V
CAN-H Shield +12V 0V

The chosen layout ensures that there is no case where both +12V and 0V are connected at the same time to the two data connections, so if you are really obnoxious and try to ram a connector the wrong way into the T socket the worst that will happen is that the fuse will blow. Once you remedy the situation and replace or reset the fuse the entire system should come back up unharmed. That is Really Cool!

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