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jimh

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Posts: 5,747
Reply with quote  #1 
I have received several questions on this topic in the form of private messages. This is an interesting topic and should be discussed in public.

The actual specifications of NMEA for the maximum length of a NMEA-2000 drop cable are not available to me (that I can find on line and at no charge), but based on published information from a respected manufacturer who is a NMEA member, I infer that they allow for the maximum length of any drop cable to be 6-meters or 20-feet:

http://www.maretron.com/products/pdf/Network%20Installation%20Guide.pdf

One can also infer the maximum length of a drop cable to be 6-meters from an illustration found in this NMEA publication:

http://www.nmea.org/Assets/20090423%20rtcm%20white%20paper%20nmea%202000.pdf

and also from slides shown in this one:

http://www.nmea.org/Assets/2012%20ibex%20full%20%20nmea%20installation.pdf

The E-TEC engine-to-network cable is about 5-meters or 16.3-feet long. This length was possibly chosen because it is just about the maximum length that a drop cable ought to be. This suggests that it is not a recommended practice to extend the length of an E-TEC engine-to-network cable by more than about 1-meter (3.28-feet).

The typical installation in a small boat for an E-TEC engine is to carry the network backbone wiring to the stern of the boat, employ a Network-T, a terminator, and connect the E-TEC to the Network-T. 

If you have a very small boat, it may be possible to run the E-TEC engine-to-network cable from the stern to your helm area for connection to the network backbone. But I suspect that it will not be possible in many boats because the E-TEC engine-to-network cable will not be long enough to reach. The better approach is to extend the backbone to the transom area, and to connect the E-TEC engine-to-network cable to the backbone without extending the drop cable length.

It is also likely that if you insist on not bringing the network backbone to the transom, you may be able to extend the E-TEC engine-to-network cable a few feet to reach the backbone, without having any effect on the network. This is not a recommended practice, but it may work. I do not see any compelling reason for taking this approach, but if there is some overriding reason why one would want to try it, it may turn out to work in some instances.

It should be noted that the maximum length of the network backbone using the Micro-category cabling is 100-meters or 328-feet. There is no concern in any practical installation on a small boat that this length could be exceeded. This, again, suggests that extension of the backbone is much preferred over extension of the drop cable when the length of a drop cable is already close to or at its specified maximum length.

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jimh

Registered:
Posts: 5,747
Reply with quote  #2 
Again, based on continuing email regarding this topic:

Nothing in the above should be interpreted to mean that there is any change in the basic configuration of the network, its terminators, its power node, or other nodes connected to the network. All networks have to follow the basic rules of the network configuration. 

For a refresher on how to construct a NMEA-2000 network, please see 

http://www.maretron.com/products/pdf/Network%20Installation%20Guide.pdf

or also see

http://continuouswave.com/whaler/reference/NMEA2000.html

__________________
Make your own EV-Diagnostics cable.

I am currently sold out of EV-Diagnostic cables. 
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