Hi!

I am using MODBUS RTU. I have several slave devices and one master device. I don't know how they should be connected together. Are they need a switch (like ethernet switch) or can we connect them together with any other way.

Thanks in advance.

 

Modbus RTU with multiple slaves works over RS-485.

RS-485 is designed to be parallel daisy chained, from A to A to A, B to B to B, sig gnd to sig gnd to sig gnd, device 1 to device 2 to device 3, etc.

There are RS-485 hubs that are 'active' devices which allow multiple devices to be wired to one common hub, but the hub replicates the 1-to-2-to-3 sequence internally,but avoids the problems inherent when long 'stub' wiring is used to connect a hub and spoke topology.

http://www.cc-concepts.com/products/rs485/hub/

 

I need to know they should connect physically. What are the parts I have to buy? I heard that I need to buy passive taps and line termination units. Can you please help me on that.

 

Hi there,

I posted "some" information about RS485 on a thread here a while ago, have look if you are interested.

http://www.control.com/thread/1267803349#1268119057

Good luck
Patrick L

 

Kudos Dave!

You included the sig gnd connection in your description!

There are lots of installations that are "missing" this very important connection.

Jerry Miille

Miille Applied Research Co., Inc.
http://www.miille.com/

 

passive taps and line termination units

If this is for Profibus DP, there are Profibus components.

B&B's tech page on RS-485:
http://tinyurl.com/7woca

Note that there's a pdf button in the upper right if you prefer to download the whole thing.

troubleshooting and cheat sheet are here:
http://www.bb-elec.com/guide/guide_info.asp