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).

Modbus Source Code: https://apmonitor.com/dde/index.php/M...

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.