Raymarine SPX-5 seatalk connections and earthing

[Ric]

I'm currently fitting a Raymarine SPX-5 course computer. I'm curious to know the reasons for some of the rather odd electrical connection instructions.

Firstly, although the unit has to be connected to 12v supply, the instructions also insist on having a separate earth, to either a grounding plate or back to the batteries. Why is this? Surely if the boat has a negative earthed 12v supply then this is unnecessary?

Similarly, the instructions require that the red-wire (carrying 12v+) of Seatalk should not be connected. Why is this? I always try to have my Seatalk network connected to 12v at as many points as possible to avoid voltage drops around the network.

 

[Simondjuk]

I'm currently fitting a Raymarine SPX-5 course computer. I'm curious to know the reasons for some of the rather odd electrical connection instructions.

Firstly, although the unit has to be connected to 12v supply, the instructions also insist on having a separate earth, to either a grounding plate or back to the batteries. Why is this? Surely if the boat has a negative earthed 12v supply then this is unnecessary?

Similarly, the instructions require that the red-wire (carrying 12v+) of Seatalk should not be connected. Why is this? I always try to have my Seatalk network connected to 12v at as many points as possible to avoid voltage drops around the network.

On the Seatalk red wire issue, it shouldn't be connected if you intend to have your instruments and autopilot individually switched. It should be connected of you intend to have your Seatalk network powered from the course computer. This is confirmed on page 24 and 25 of the SPX-5 manual (Wheel Pilot manual in this case, as that's just what I found, but the course computer Seatalk connections are almost certainly the same for a linear drive). Of course, if you're not switching independently, an additional 12V connection just gives some redundancy.

 

[Ric]

Aha that makes sense for the red-wire. Still mystified why I need a separate earth when the SPX-5 is already wired to negative earth 12v

 

[David2452]

Aha that makes sense for the red-wire. Still mystified why I need a separate earth when the SPX-5 is already wired to negative earth 12v

It is not a DC return, it is an RF ground and should be connected along with all other RF grounds e.g. NMEA 0183 shields to an RF ground, I say should because they rarely are in DIY installs and people often get away with it, but it is there to reduce the possibility of RFI in your electronics and is worth the effort if you want to do the job right.

 

[robmcg]

Just pray that the autopilot will successfully go through the calibration sequence. Mine still won't and Raymarine are shall we say less than interested! :disgust: