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Wire Multiple Modbus Sensors
Want to wire three sensors (4 wire, RS485) and use Modbus RTU communication.

Hi!

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.

By W.L. Mostia on 18 March, 2018 - 9:16 pm

Modbus.org (http://www.modbus.org/) is a good resource website. Have a look at the below link and search for RS232 and RS485 :

http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf

These sites can also be of help:

http://eidetec.com/modbus-rs485-wiring-diagram

http://www.obvius.com/sites/uat.obvius.com/files/TN27-ModbusRS485QandA.pdf

http://www.plcdev.com/modbus_faq


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:
https://s9.postimg.org/j3a8i55en/rs385_4w_2w.jpg

This is a 2 wire RS-485 multidrop:
https://s9.postimg.org/kv37d36rz/2_wire_multi-drop_with_terminating_resistors.jpg

Thank you!

What can I use to do the connections? Breadboard and jumper wires?
Can I email you?

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.