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wettek

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Reply with quote  #1 
Gidday guys
Does anyone know if running a backbone with the engine interface cable hooked up to the bus port of a tee instead of the device port and obviously no terminator on that tee (its on the end of my backbone) will have any effects?
I found that my backbone was hooked up that way (not sure how I never noticed it before).
One of the other learned members on this forum advises that there will more than likely be no adverse effects, but I am just curious if anyone has had any issues due to connection/setup issues.
I have been having issues with my trim display (coincedentally started after a technician had been under my dash playing with my network) and while I doubt I have found the magic bullet, it may be a contributor.

Regards to all

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wettek
PropNut

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Reply with quote  #2 
NMEA networks are pretty tough, they can put up with a lot before they actually go down.  The robustness of the network is why it a lot of times works when something is not right.

The network needs 2 terminators, 1 at either end of the buss and devices connected at the drop of a T.

When not assembled right they may work for a while then all of a sudden start giving you issues after days/months/even years for no good reason other than it wasnt installed correctly in the first place.
jimh

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Reply with quote  #3 
I am always surprised at how often installers neglect the most basic rules of a NMEA-2000 network. The network requirements are incredibly simple, very inexpensive, and can be easily assembled from pre-configured components. Yet there is always someone who manages to think of a new way (and incorrect way) to wire things together. Deviation from the network layout recommendations cannot be justified; it seems to be just sloppy work that causes this.

I recommend following the recommended configuration. And why wouldn't you?

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wettek

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Reply with quote  #4 
Yeah, I rectified the issue pretty much straight away, pretty cranky at myself for not noticing it earlier. It may not fix my issue, but at least it's done correctly now.
What do the terminators actually do on the network?

Regards to all.

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wettek
jimh

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Reply with quote  #5 
The purpose of the terminators is to terminate the twisted-pair data circuit in a specific resistive load. The data cable is a transmission line, a special form of electrical signal carrier, and unterminated stubs on the transmission line can cause reflection of signals. Termination of the far ends of the network backbone with 120-Ohm resistor terminators helps to prevent reflected signals. Reflected signals can seriously degrade the network communication.

For more about transmission lines see

https://en.wikipedia.org/wiki/Transmission_line

NMEA-2000 is based on the J1939 standard. The physical layer is based on CAN bus. See

https://en.wikipedia.org/wiki/CAN_bus#Standards

The Evinrude brand name for their older NMEA-2000 instruments is I-Command.

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wettek

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Reply with quote  #6 
Thanks Jimh.

I learn something every time I come to the forum.

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wettek
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