Modbus wiring í'am lost

Hardware
1

RS485 mode and wiring. I’ve been searching for a while on google and youtube, but I haven’t found an answer yet. As stupid as it sounds.

Each unit has its own wire connection, these wires run to one collection point. I still understand that I can’t tie them together.

But then I lost it. According to the explanation you have to wire from unit to unit. That would mean I need to have 2 send wires and 2 return?

Then loop the send and return wires unit to unit through at the collection point to make a correct chain?

2

RS485 can be run in many ways, however it is a bus - shared or multidrop, so each signal pair is looped from device to device, then to a termination resistor (might be selectable in an end device).

Full-duplex RS-485 has 4x signal wires. The only connection between TX and RX is in software - i.e. ask a question on TX; get the reply on RX.

TX+ ---- A --- B --- C --¬
TX- ---- A --- B --- C -- R

RX+ ---- A --- B --- C --¬
RX- ---- A --- B --- C -- R

GND ---- A --- B --- C

Half-duplex has only one signal pair.

D+  ---- A --- B --- C --¬
D-  ---- A --- B --- C -- R

GND ---- A --- B --- C
1 Like
3

Then I indeed need a send and return wire to each sensor.

YouTube and Google only show cheerful pictures where everything is next to each other. Not how people have wired their homes. It is impossible to go directly from point to point and much more logical to have a collection point

4

Transmission line theory doesn’t like star bus networks as each stub causes a different reflection from the end of the wire and degrades the data signal (that’s what the terminator resistor reduces).

You might get away with star links over short distances, but watch for data errors. Modbus doesn’t use the data rates of thin-wire Ethernet, but the same physics apply: 10BASE2 - Wikipedia

Many bus networks have ‘hubs’ to connect multiple branches together, but I’m not an expert on Modbus to give product examples.