MODBUS RTU Hardware Connection


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I am posting this message to ask some question about Modbus RTU implementation. I want to use ST485EC transceiver. I am not sure that its A (non-inverting receiver input and non-inverting driver output) and B (inverting receiver input and inverting driver output) match Modbus A (D0) and B (D1), according to 2W-Modbus implementation explained in Modbus over serial line specification V1.02?

If you could clarify me that transceiver's A must be connected to Modbus' A, and transceiver's B must be connected to Modbus' B.

Thank you!
I don't know, but the way most of us deal with it is to hook 'em up and see if it works. If it doesn't, swap the wires.

Mr B&B Electronics wrote a white paper on the Polarity of RS-485 driver lines including the history of naming or misnaming the lines.

It's a 2 pager, and the last paragraph has a voltmeter test so that those who are not experiment minded can test to see which is A and which is B, according to Hoyle.

link to B&B's white paper:
Really helpful your answer David_2! Thanks a lot!

About swaping the wires, that is posible when you have screw terminals but not with RJ45 connector. So, it's really important to know the right polarity of RS-485 communication before you implement it.
> it's really important to know the right polarity of RS-485 communication

Yes, because it doesn't work when it's backwards.

> before you implement it.

Not necessarily, connecting it backwards does not damage the drivers, it just won't work.

> but not with RJ45 connector.

Google: RJ45 to screw terminal adapter

You might need two adapters wired screw terminal to screw terminal and an RJ45 coupler to mate the RJ45 plug to RJ45 plug, but it can be done.
Hi rfan,

yes, we generally connect transceiver's A to Modbus A and transceiver's B to Modbus B terminal for modus-rtu communication. I have used similar device from Texas Instrument's viz. ISO3082 for the same. I have used standard 2 terminal device. However you can use RJ45 plug also maintaining same terminology. Hope you have referred the Table :<b>2W-MODBUS RJ45 and 9-pin D-shell Pinouts </b> on page 29.