17 February 2011

Simnet Noise Filter

Recently I became aware of the fact that (a) Simrad has switched to a new specification for their Simnet cabling and (b) that there is a "Simrad Noise Filter" that they recommend for installation in sailing boats.

As you can see here in an report from Ben @ Panbo on N2K cable mixing, Simnet cables used to contain a yellow/blue pair for the data wires. Apparently this type of cable caused issues as the impedance of a long stretch was not according to NMEA specifications. This naturally ends up giving problems when you have a IS 20 wind instrument at the other end of a 20m cable. In my case it's even worse as I had to order a 30m cable as my mast is 25m tall.

I've been told that Simrad has switched to a different cable that has white/blue data wires which has better characteristics. To 'fix' existing installations Simrad has also produced a "Simnet Noise Filter", P/N 24006934. The Simrad NSE documentation states on PDF page 32:

For a large systems or sailboats with a mast head wind sensor and long mast cable, it is recommended to use a SimNet noise filter (24006934). On sail boats the filter should be inserted at the mast junction. On larger systems without a mast head unit, the filter should be inserted centrally along the backbone.

The original 2006 era SimNet installation manual contains no reference to this. Probably these issues have become more prevalent with larger networks being deployed.

I've ordered such a noise filter. I've been told it contains a 120 Ohm termination resistor as well as a 220 μF capacitor to damp out supply issues. Once it gets delivered I'll report on what it contains and the effects on my network.


  1. So did you get the noise filter and did it fix the problem? I can't seem to find a source for one...

  2. Hi Alexus,

    Yeah I got it and didn't really notice any difference. My problems were software related, see my post on Airmar software updates.

    I was able to obtain one from the distributor. Cost was about 40 euro's if I remember correctly.

    What is good about the noise filter is that since it contains a terminator it will also mean that the network is still within spec if you remove the mast and disconnect the wind transducer cable.