I am new to Modbus so I would appreciate some help. I have three digital sensors that I want to use as slaves over Modbus RTU communication. The sensors are RS485, 4-wire (A and B), supply 6-36V DC, output 4-20 mA.
I can connect one sensor to a USB tool, but how should I connect three of them? I have a breadboard, would this be ok? I am not sure how to do the wiring.
Modbus.org (http://www.modbus.org/) is a good resource website. Have a look at the below link and search for RS232 and RS485 :
These sites can also be of help:
William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Eng. (TUV Rheinland)
WLM Engineering Co.
RS-485 multidrop networks are daisy chains, as shown in the graphics in the links below.
This is a 4 wire RS-485 multidrop:
This is a 2 wire RS-485 multidrop:
I'm an industrial guy so I think in terms of protected wiring: trays, conduit, panels, ducts, etc; and long wiring runs between sensors: hundreds of meters.
And you're talking breadboarding.
If your sensors are all local and fit on a breadboard then have at it.
Industrial signal wiring is usually TSP: twisted, shielded pair. For short 485 runs on a breadboard you're not likely to need shielding, but you might want to twist the copper conductors that you use to connect A/B lines. Parallel lines pick up a lot of noise.