Backbone query

[wonkywinch]

Think you've had a senior moment... With the correct number of terminators (two) in place the resistance can be measured as 60 ohm between the blue and white data wires.

Do this measurement with the power to the bus/backbone turned off.

If your measurement is about 120 ohm across blue and white, then you've got just one terminator connected and will need to add a second.

Isn't that what I said? 2x 120R (ohms) in parallel makes 60 ohms?

 

[dgadee]

I was at boat today and after sorting out the wiring and fitting the new S/H VHF (with NMEA2000) I looked at the backbone but didn't measure any ohms. The Triton was displaying wind and depth but not getting anything from the VHF, so I presume that part of the system isn't connected up properly.

Thanks all.

 

[PaulRainbow]

I was at boat today and after sorting out the wiring and fitting the new S/H VHF (with NMEA2000) I looked at the backbone but didn't measure any ohms. The Triton was displaying wind and depth but not getting anything from the VHF, so I presume that part of the system isn't connected up properly.

Thanks all.

Might be a good example of my comment "terminator must be fitted for a reliable system.

Data systems on boats can be a bit weird. You can put a system together wrong and they still work OK. You add something and it all goes wrong, or you get peculiar issues. Best to start of correctly and keep it that way.

 

[Martin_J]

Isn't that what I said? 2x 120R (ohms) in parallel makes 60 ohms?

You did... Correct... Unfortunately '12v supply pair' was in the same sentence..

 

[wonkywinch]

You did... Correct... Unfortunately '12v supply pair' was in the same sentence..

View attachment 179841

Ah, I am getting old. The loading resistors are on the data pair, not supply rail. My mind was on fault finding, check presence of 12v on a plug, then power off and check 60R on data pair.

Thanks for correction.