Controlling Inverters with Siemens PLC via MODBUS

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1. Overview of MODBUS Communication

The MODBUS protocol is divided into two types: MODBUS protocol over serial links and MODBUS protocol based on TCP/IP. The serial link MODBUS protocol includes MODBUS ASCII (string) and MODBUS RTU.

The MODBUS protocol library provided by 200SMART supports MODBUS RTU communication.

MODBUS RTU sends data in hexadecimal format.

MODBUS ASCII sends data in ASCII code. Sending one data using RTU requires only one byte, while using ASCII code requires two bytes, affecting the transmission time.

There are many communication methods for PLC and inverters, but the commonly used and preferred method is MODBUS communication, which is widely applied in industrial control. In this article, we will share how to control an inverter with a PLC using MODBUS RTU.

2. Detailed Explanation of MODBUS Library Commands

Library commands are divided into two types: master library and slave library.

The master station is when the PLC acts as the MODBUS master to actively read data from other devices; the slave station is when the PLC acts as the MODBUS slave, and other devices read data. In this article, we share the scenario of the PLC as the master and the inverter as the slave.

STEP7-MinWIN SMART software comes with MODBUS communication library commands.

Initialization command:

Control command:

3. Communication Format (Protocol) of MODBUS RTU

The communication protocol consists of: station number, function code, data address, data content, checksum, and end character.

Checksum: N=None (no checksum), E=Even (even checksum), O=Odd (odd checksum).

Determine whether the number of bits with a value of 1 is odd or even (for even checksum, if the number of 1s in 16#03 is even, the checksum is 1; if odd, the checksum is 0. For odd checksum, if the number of 1s is even, the checksum is 0; if odd, the checksum is 1).

The function code specifies whether to read or write operations on the slave device, and also specifies the type of MODBUS register address. Common function codes include:

4. Hardware Connection for PLC and Inverter Communication

The communication terminals on the inverter are marked as A/B RS585+/RS485- or RJ11 port; find the relevant inverter manual to view the wiring definitions for the communication ports. Below is the wiring definition for the Delta VFD-M series inverter communication port.

5. Setting Communication Parameters for the Inverter

1. To achieve normal communication between the inverter or instrument and the PLC, their interfaces and protocols must be consistent, and the parameter settings must also be consistent.

P00 set to 03 (frequency command)

P01 set to 03 (run command)

P88 set to 03 (communication address, between 0-254)

P89 set to 02 (baud rate selection)

P92 set to 04 (data format, 200SMART does not support 2 stop bits, so only 04/05 can be selected)

P157 set to 01 (the inverter defaults to MODBUS mode)

2. Check the manual for start/stop, frequency set, running frequency, voltage, current, and other feedback parameter addresses.

Information frame format description

Indicates that reading data from addresses 2103, 2104 is placed in addresses 17, 70, and 00, 00.

Parameter address definitions for the communication protocol:

Running command address 2000H, frequency command address 2001H

Output frequency address 2103H, output current address 22104H

Output voltage address 2106H, inverter temperature address 210DH

6. Writing MODBUS Communication Program

At the beginning of the program, you can add a section to initialize and reset all bits.

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