Python Modbus

Modbus is a communication standard to transfer values between computers. It is commonly used with Programmable Logic Controllers (PLCs), Human Machine Interfaces (HMIs), and other networking applications. It has several limitations that motivated the creation of newer standards such as OPC UA. Although it is an old standard, several legacy systems still support this protocol. There are several serial or network connection possibilities including RS232, RS422, RS485 (serial) or TCP/IP (network).

A Modbus server has coils, discrete inputs, input registers, and holding registers. The coils and discrete inputs are a 1 or 0 (on or off) while the registers are 16-bit values (0-65,535 for unsigned integer). A floating point 32-bit number uses two registers, and a double precision 64-bit number uses four registers. The coil numbers start with 0 and span from 00001 to 09999, discrete input numbers start with 1 and span from 10001 to 19999, and holding register numbers start with 4 and span from 40001 to 49999.

The coils and discrete inputs hold 1 bit each as a 0 or 1 (off/on). The input registers and holding registers have 16 bits each. A 16-bit unsigned integer can store integer values between 0 and 65,535 while a signed integer uses an extra bit to store the positive or negative sign and can store values between -32,768 to 32,767. A 32-bit (7 digits of accuracy) or 64-bit (14 digits of accuracy) number can also be sent, and the values are sent across multiple 16-bit registers with 2 registers for 32-bit numbers and 4 registers for 64-bit numbers.

The pymodbus Python package is a full-featured set of methods that includes a Modbus server and client. Install the pymodbus package with pip from the PyPI repository. The code in these examples use version 3.0.2.

[prashkd]

Would love to see videos on Python application for other Industrial comms like Profinet, EGD 👍

[paulramasco]

This was very informative, thank you. I'm about to start working with some motor drivers that use modbus so it's timely as an overview.

[PLCAutomations]

Are legacy restrictions on the address space still used in real life? It seemed to me that in the modbas specification this was indicated as outdated back in 1996.

[embetronix]

I want to control a VFD from the computer. Can you give me some ideas? Like how I should proceed, what all would I be needing, etc. Any guidance will be really helpful. Thanks.

[matthewmckenzie]

I work with Eurotherm 6000 as a legacy data historian. My goal is to import the data from the historian to python and plot it in an HMI sort of fashion. Any advice on how to tackle the problem?

[javierarredondo]

The modbus server script you have had create is for a serial modbus server right? what changes i can do to create a ethernet modbus server?

[edwintoapanta]

hello, it is possible to read/write negatives values from the input and holding register with pymodbus?

[abhishektiwari]

Hi @APM,
Your tutorials are amazing! 🙌

I've set up a Modbus TCP server in Python following your guidance. Now, I'm trying to make the register data dynamic, updating them based on real-time variables in Python.

please provide solution achieve this?

Thanks a bunch!

[michaczarnocki]

Thanks for tutorial, i have a ubiquotus problem with the line 'from pymodbus.client import modbustcpclient'. Have you ever come up with this problem?

[LinoJr.Asmolo]

Hi. Is it possible for pymodbus package to retrieve the data from Modbus TCP/IP driver from Kepware KepServerEX? If yes, how do I get the actual values of a tag? Like in your previous videos on OPC UA Client, you were using the nodeid? Thanks.

[travelcompass]

Could we just scrap MODBUS and work with sensors conected via LOAR, 5G, WIfi, etc directly from a Linux such as Ubuntu core 22?

[agustinguzman]

How do you setup the Client if I'm getting modbus messages via the serial port? (i.e. USB port COM1)

[fburton]

What are coils? Does the name come from a very specific initial use for modbus?

[fburton]

I am planning to trigger activity (image acquisition) on a slave computer using a signal from another master computer. (Not sure if master/slave are acceptable terms these days - apologies if not.) I will also be transferring strings. A preliminary trial shows that using IP (Internet Protocol) and an ethernet cable connecting the two computers works reliably. Seeing this video, I am now wondering if I should be considering Modbus as an alternative. If I want the lowest latency, should I stick with 'simple' IP? I suspect the answer is try both and measure, but if one is known to be significantly faster than the other maybe I don't need to.