WR21 Serial Port 485 Modbus

Hi all,

I’m trying to read modbus data from a RS485 device using a Digi Transport WR21 but I’m completely lost on this issue. I’ve been reading the examples available on the Digi ESP libraries, but have no luck so far.

Wondering if there’s a working example for opening and send data to the serial port of a Transport that is configured as RS485.

I’ve seen so much information on the forums but most of it it’s related to Xbee 485 adapters.

I think I’m missing some clues about:

  1. Connecting the serial port of the Transport to the 485 device. I think it should be:

TDA-RDA (together)-------->> RS485 +
TDB-RDB (together)-------->> RS485 -

  1. Setting up serial port on the Transport properly (where is the option TCP packets, do I need it?

  2. Managing the port propertly from python. So far I’m using the serial libraries without any luck so far.

There’s so much that I don’t know about python and serial since I’m new to it, so I was looking for kind of working example to start from.

Sorry for writing so much and thanks a lot in advance for your help.


I found myself the answer to question 1 in this excellent wiki, wich I assume that applies to Transport products too:


I’ll continue working on the other points, so your help is still very much appreciatted. Thank you.

You are right about the connections; usually always positive connected to positive and negative to negative when dealing with RS485.

Oh sorry… I didn’t see that you already found the answer for this yourself.