How important is galvanic isolation?
I'm considering using a Pi with a CAN hat that also provides power. However, that hat doesn't provide galvanic isolation. But that may not be important because it does provide power, so the only ground that the Pi will see is from the NMEA network.
https://copperhilltech.com/pican-m-nmea-0183-nmea-2000-hat-for-raspberry-pi-with-smps/
Basically, the Pi will only be attached to the NMEA 2k network for power and data, and will not be connected to any other power source.
Or is this a bad idea? If so, I'll use NMEA 2k-to-ethernet bridge, and connect the Pi and the bridge to my ship's wifi hotspot.
Copperhill
PICAN-M - NMEA 0183 & NMEA 2000 HAT For Raspberry Pi With SMPS
The PICAN-M (M = Marine) is a Raspberry Pi HAT with NMEA 0183 and NMEA 2000 connection.
14 Replies
If the Pi is not connected anywhere else, lack of isolation does not matter, just like you suggested. Note, though, that NMEA 2000 limits individual device current draw to 1 A which a Pi can easily exceed.
If you want to have a board that is compliant in both regards, you might want to have a look at the SH-RPi (disclaimer - I designed that board).
I'm not an EE so I can't speak to the implications of isolation, but I have the PICAN-M HAT. My first one failed within a couple of months and it took around 6 months for the distributor to evaluate and send me a new one. I'm using this HAT on my RPI now https://www.waveshare.com/rs485-can-hat.htm
And a converter for power: https://www.amazon.com/gp/product/B07MYPCM73/
It's hard to beat $21 in parts that you can get from Amazon in 2 days, compared to $90 for the PICAN-M.
I also highly recommend SH-RPI if it's going to be an integral part of your boat's systems instead of something you're just playing with. (No disclaimer--he didn't pay me to say this 🙂
Piggy backing on this. I have a PiCAN-M hat, which I don't want powered off the n2k bus, for a variety of reasons. Is there a way to not have that happen? It's too power hungry and it doesn't want to play nice with my display when running like that
They are available without the power supply but I guess you have the version with a power supply. You can either unsolder the power supply board, or Garmin has an adapter plug that doesn’t pass through the power. Or replace it with a waveshare board and a SH-RPi.
Well Drats. I am allergic to replacing components that should be fine, so I'll take it into the shop today and desolder the power connections.
Thanks for the tip!
It's not fine. Since the device is not isolated, by powering it from a different source, you're introducing a ground loop in the N2K network.
Well. Drats. That's no good either. I had the vague idea it was isolated, but I'll look up the documentation first, or otherwise I guess I'm going shopping. Thanks!
To be fair, lack of isolation might not be the end of the world. CAN bus uses differential signalling which is by design quite insensitive to ground loops and other noise. There are other devices like Victron Cerbo GX that aren't isolated and people are using them without issues. But it better be a conscious decision! N2K is often a safety-critical system and a single misbehaving device can bring the network down. I don't think we could disengage our autopilot if N2K failed, for example.
I wouldn't have chosen to power it this way. Had been running on an old 0183 network with external power for about a year, and fully forgot that it could power over n2k. Was a surprise when I plugged it in. Honestly, I'm probably moving past the form factor of the pi, and going to a nice running Linux , but that needs time to set up, which I don't have much of right now. Thanks!
Thank you all! I think I'm going to get a NMEA-to-ethernet gateway, like the Yacht Devices, and use cabled ethernet to my ship's wifi, and then a small computer and power it from 12V with a buck converter, maybe like this: https://www.amazon.com/CyberGeek-A1-Desktop-Computer-3-4GHz/dp/B0CNZJ6C2P/ref=sr_1_2_sspa
I'd hate to bring down my autopilot.
I did have a problem with my n2k network last year. I’m running mostly Raymarine kit and I have a ITC5 analog converter which takes in wind , speed, and depth. It has built in backbone sockets, so I run the backbone through it. One of the sockets had a problem if there was pressure on the cable, the network would cut out. My plotter at the helm could see everything behind the fault, and my RPi could see everything forward of the fault. The problem was that the AP compass is in the front cabin so the AP would randomly cut out. Easy fix, by fitting a tee instead and using a drop cable to connect the ITC5.
My two cents: in a complex multi vendor system (N2K), and some times even multi-battery, having isolation is just the simplest choice to pre-empt hard to diagnose issues. Our next generation Cerbos (in stock and shipping) have their VE.Can 1 port isolated for that reason; just like the Ekrano GX already had from first unit shipped.
Great, apologies for the misinformation!
Welcome, and np - its quite the detail. See Cerbo Mk2 here:
https://www.victronenergy.com/live/venus-os:start