Low Cost Modbus Boards

[Visitor]

I've been seeing a lot of low cost Modbus boards on Banggood and ebay, like an 8 input / 8 relay output board for around $10. One or several of these, coupled with an ESP32 could be a powerful piece of gear.

Has anybody here used any of boards? Documentation appears to be lacking, so it may take some effort to determine registers and so on.

 

[rjenkinsgb]

Interesting devices!

Just been looking at some of the ones on ebay and found that a UK company is one of the resellers. They have a bit more info on their site, & note that the default modbus address is 255 but can be changed.
**broken link removed**

The modbus protocol is pretty well documented on Wikipedia, it looks straightforward enough.

>google< - And there is a PIC modbus library source for both master & slave units on github.

GitHub - worrajak/PICmodbus: PIC Microcontroller Master and Slave Modbus RTU Protocol

PIC Microcontroller Master and Slave Modbus RTU Protocol - GitHub - worrajak/PICmodbus: PIC Microcontroller Master and Slave Modbus RTU Protocol

github.com

Definitely worth some investigation, thanks for publishing the info.

 

[Visitor]

Thanks for the UK link. It does present things more clearly.

I was looking through some of the boards available at Banggood last night. There are a lot of different functions available and the gears are turning in my head.

Looks like Cat5 cable can be used for an RS485 network over short/moderate distances.

 

[rjenkinsgb]

Cat5 cable can be used for an RS485 network over short/moderate distances

Yes, it should work fine & be OK for hundreds of metres, at moderate speeds with proper termination.
Just remember you also need a shared ground (or an earth at both local and remote devices) to keep the data line signals in common mode range.

I often use audio twisted pair like mic cable / starquad for short links or a proper data-rated screened twisted pair for longer links.
Van Damme 278 401 000 is nice stuff for fixed data wiring, I use a lot of that.