Seatalk Cable Voltages

[Tim Good]

What voltages should I be seeing on my seatalk cable from my Type 300 course computer.

I get 12v between Red and Screen (Bare)
I get 5v with minor fluctuations between Yellow and screen
I get 7.8v with minor fluctuations between Red and Yellow.

Should I be getting any voltage between red and yellow?


This thread goes hand in hand with another about trying to diagnose voltage issues up to an MHU wind direction/speed instrument here:

Autohelm ST60 Wind issues

 

[prv]

The red core is 12v power, the screen is 0v. The yellow core is the Seatalk data, which idles at 12v but is pulled down to 0v to signal each logic 1. A multimeter won’t follow those pulses, so it’ll report an average somewhere between 0v and 12v, but 5v seems lower than I’d expect. When I’ve put a scope on a Seatalk line while developing my depth-alarm-shutter-upper, it spend most of its time at 12v. I suppose a busier bus than my single test instrument would spend more time signalling hence average lower, but 5v still seems surprising.

Pete

 

[cpedw]

I think you are confusing yourself by measuring voltages between too many places. If you confine your measurements on Seatalk and on the Wind instrument to voltages compared to the Screen i.e. Measure Red to Screen, Yellow to Screen, other colours to Screen but not one colour to another, then things should be clearer.

I agree with Pete about what to expect on the Seatalk - probably OK.

I don't know details about the ST60 wind but I would expect the voltages on the inputs to the Display Unit if it's connected to power will be about 0V measured against the Screen.