RS485 and power on the same cable

Projects General Guidance
1

I'm building multiple PCBs which have ATMEGA328P and distance sensors, connecting in a line for watering system.
To provide power to the PCBs, i plan to use RS485 for both communication and power (as shown here: https://www.ti.com/lit/ug/tidu993/tidu993.pdf?ts=1738579704640&ref_url=https%253A%252F%252Fwww.google.com%252F)

The first PCB in the chain will initially receive power through its connected FTDI adapter, primarily for programming and initial setup. Subsequent PCBs in the chain will be powered directly from the RS-485 bus. This bus will receive its power from an external power supply.

Do you think this is possible, and what should i consider?
The RS485 section of the first PCB would look something like this

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2

The longer the distance the more the dimension of the powering cables should be considered.

3

After a quick read, is the TI app note - and your design - supplying power to the remote nodes over the RS485 A & B lines?

If you kept the power separate and ran it down a pair of chunky cores would that make your design easier?

You could look at the Arduino MKR485 board design as that also takes an external supply voltage (but on separate cores to the data).

4

Maybe with only two boards and Manchester encoded data.
It would be an experiment to say the least.
Let us know if it works.

P.S. The schematic looks like it was drawn by a novice or someone who just did not care.

5

Following the documentation you provided, I see no chance this will work other than between TWO devices, one at each end of the bus.

6

Makes sense if you have existent wiring with only two conductors, otherwise I think you are looking for challenges on purpose.

7

I use a 4 Wire cable with a pair for CAN and a pair for 24V to power the remote nodes. Works great.

8

because i want to reduce the number of wires for the subsequent PCBs, i wanted to use only two wires for communication and power.
But it's becoming unnecessarily complicated and not really reliable, so i will get rid of this option :smiley:

9

You can do what you want, but will have to have terminating resistors only on the end units. The low pass filters will have to be designed for the complete system, as they will all be in parallel. Mind the notice that the data speed must be as HIGH AS POSSIBLE.
I use a bias-T system in my ham radio antenna system to power a remote tuner through the coaxial cable and it works just fine for frequencies above 1mHz. And only two units on the coax.

10

Instead of putting power on the data lines, it might be better to put data on the power lines (if you really can't afford the extra 2 wires).

11

:thinking:

12

Ok check the SIG60 IC, I came across it a few years back. Itis by Yamar and here is a link: https://www.yamar.com/datasheet/DS-SIG60.pdf To the best of my knowledge it is out of production but several distributors will still have some stock. It is also possible a newer version is available. With this in mind get a few extras.

13

The X10 stuff could be made to operate pretty reliably communicating over the power line.

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14

I may be wrong but I assumed the OP was looking at a DC bus. X10 works on AC and sends its code at the zero cross point in the signal.

15

Yes, that's right. I was just drawing an analogy.

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16

RS485 also need a ground wire

17

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