Multiple Modbus RS485 device On same RS485 to USB convertor

M

Thread Starter

Manthan

Hi All,

I am new to Modbus area. So, please guide me even if my question is stupid.

Background:
I have multiple RS485 modbus slave device. Each having different slave id values. Right now I am using separate RS485 to USB converter to communicate with each device. There is only one master which is communicating with all slaves.

Problem:
Now my question is that, is it possible to use only single RS485 to USB converter in such setup?

Or if above is not possible then any other solution so i can connect multiple RS485 device(may be 4/5) to single USB port?

Thanks in advance.
 
Assuming they all talk Modbus RTU (or all talk Modbus ASCII), yes.

Usually a single USB-to-RS485 converter is used on a master, like a PC.

Multiple slaves on one network is called multidrop.

It is usually wired (+) to (+) to (+) , (-) to (-) to (-), or A to A to A, B to B to B.

Some manufacturers label the lines differently so it becomes A to B, but try it the way it should be first.

The master has to be smart enough to poll the first, wait for the response, poll the 2nd, wait for the response, etc.
 
> Some manufacturers label the lines differently so it becomes
> A to B, but try it the way it should be first.

Yup. Better to look for D+/D- labeling that relying on "A", "B" and "C" (common, ground, pick your favorite).

I've mostly used FTDI USB-to-RS485 converters and haven't had any issues with connecting to multiple devices. My only beef with that product is the wire gauge - my recollection is that it is 22 or 24AWG, which is a pain to work with.

> The master has to be smart enough to poll the first, wait
> for the response, poll the 2nd, wait for the response, etc.

Modbus/RTU over 3-wire RS-485 is always half-duplex, so any software written for Modbus/RTU should support response delays. I've never worked with 5-wire devices, so I don't know if they support full-duplex.

For Modbus/TCP, as long as you aren't pounding on a single slave device, you should be able to send requests without waiting for the first device, assuming your software can handle asynchronous responses.
 
Hi Manthan,

If the amount of slaves is less than 32, you should be able to communicate with all slaves.

In many cases you will need to put termination resistors on first and last node of the RS485 network. The resistance should match the impedance of the cable, but usually 12Ohms will do.

You wrote that there is a master present. If you need to communicate with a different master (PC), you will need to disconnect the existing master, else you will have collisions of data traffic.

My 2 cents
 
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