hacor
Posts: 8
Joined: Mon Sep 11, 2023 7:00 pm

CM4 Breakout board RS485 issue

Mon Feb 26, 2024 6:56 am

Hello

I wanted to use this channel to ask a question about my Raspberry Pi CM4 breakout board design.
- It uses a SP485 chip from MaxLinear to create a RS485 bus
- This SP485 chip is driven by a CH340E chip (USB -> serial)

With most devices this design works perfectly! But some other devices completely destroy the CM4 and the breakout board completely. Some devices causing this problem:
- A Solax Inverter
- A Sungrow EV charger (Sungrow inverters work perfectly)

When one of those devices (is connected to the RS485 port of my breakout board it destroys the PoE 5V power supply and the CM4 itself. The CM4 is also unusable in a working breakout board (it's completely fried)

Currently working perfectly (all connected to the same RS485 bus):
- Huawei inverters
- Goodwe inverters
- Solis inverters
- Heidelberg Chargers
- Phoenix Contacts Chargers
- Eastron energy meters

I place a picture of the Schematic of the RS485 design. I hope someone could help me find the mistake in my design.

Thank you!
Kind regards

Hacor
Attachments
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dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 1293
Joined: Thu Dec 29, 2011 5:46 pm

Re: CM4 Breakout board RS485 issue

Mon Feb 26, 2024 7:07 am

Do you have a ground connection with A and B if not add one.

hacor
Posts: 8
Joined: Mon Sep 11, 2023 7:00 pm

Re: CM4 Breakout board RS485 issue

Mon Feb 26, 2024 7:51 am

Hi dp11!

Thanks for the quick reply!
You mean connecting the ground shield of the cable between both devices? Or do you mean something else?

To be honest my board doesn't have a ground screw terminal, and some of the devices (Solax) doesn't have such a wire either.
Strange thing is that everything blows up but the Zener Diodes seem in tact.

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 1293
Joined: Thu Dec 29, 2011 5:46 pm

Re: CM4 Breakout board RS485 issue

Mon Feb 26, 2024 8:48 am

Unless you have fully isolated system you need a ground between the two systems. The shield would be fine.

hacor
Posts: 8
Joined: Mon Sep 11, 2023 7:00 pm

Re: CM4 Breakout board RS485 issue

Mon Feb 26, 2024 2:47 pm

Thanks! I'll give this a try!

6by9
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Location: ZZ9 Plural Z Alpha, aka just outside Cambridge.

Re: CM4 Breakout board RS485 issue

Mon Feb 26, 2024 6:32 pm

I'd (very carefully) look at the DC voltages on the RS485. Yes you have the Zener diodes there, but something nasty is obviously getting through.

If you're killing that many devices I'd also look at opto-isolated RS485 transceivers. At least that way you can get it working, and then look at what the voltages are and try to determine why it kills parts of your system.
Software Engineer at Raspberry Pi Ltd. Views expressed are still personal views.
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hacor
Posts: 8
Joined: Mon Sep 11, 2023 7:00 pm

Re: CM4 Breakout board RS485 issue

Wed Feb 28, 2024 3:07 pm

Thanks for all the replies! I'll do some more testing and post the results.

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