Modbus Hans-Petter Halvorsen

Transcription

1 Modbus Hans-Petter Halvorsen

2 What is Modbus? Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a de facto standard communication protocol, and it is now a commonly available means of connecting industrial electronic devices The development and update of Modbus protocols has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to that organization Modbus became the first widely accepted fieldbus standard.

3 Modbus The nice thing of the Modbus standard is the flexibility, but at the same time the easy implementation of it. Not only intelligent devices like microcontrollers, PLCs etc. are able to communicate with Modbus, also many intelligent sensors are equipped with a Modbus interface to send their data to host systems. While Modbus was previously mainly used on wired serial communication lines, there are also extensions to the standard for wireless communications and TCP/IP networks.

4 Modbus Modbus is a master-slave system, where the master communicates with one or multiple slaves. The master typically is a PLC (Programmable Logic Controller), PC, DCS (Distributed Control System) or RTU (Remote Terminal Unit) Modbus RTU slaves are often field devices, all of which connect to the network in a multi- drop configuration When a MODBUS RTU master wants information from a device, the master sends a message that contains the device s address, data it wants, and a checksum for error detection. Every other device on the network sees the message, but only the device that is addressed responds. Using Modbus for Process Control and Automation (PDF):

5 The master typically is a PLC (Programmable Logic Controller), PC or DCS (Distributed Control System) Modbus Process Measurements PLC RTU RTU Slave SCADA DSC Master Slave PC A remote terminal unit (RTU) is a microprocessorcontrolled electronic device that interfaces objects in the physical world to a DCS or SCADA System Slave Sensors and Actuators Modbus RTU slaves are often field devices, all of which connect to the network in a multi- drop configuration

6 Master/Slave Modbus protocol is defined as a master/slave protocol, meaning a device operating as a master will poll one or more devices operating as a slave. This means a slave device cannot volunteer information; it must wait to be asked for it. The master will write data to a slave device s registers, and read data from a slave device s registers. A register address or register reference is always in the context of the slave s registers.

7 Master/Slave Master Request Response Slave The Modbus protocol follows a master and slave architecture where a master transmits a request to a slave and waits for the response.

8 Master/Slave Master Write Data Polling/Request Slave Registers Master Read Data Data Stored in Registers Modbus protocol is defined as a master/slave protocol, meaning a device operating as a master will poll one or more devices operating as a slave. This means a slave device cannot volunteer information; it must wait to be asked for it. The master will write data to a slave device s registers, and read data from a slave device s registers. A register address or register reference is always in the context of the slave s registers.

9 Modbus Register Types 1. Coil (Discrete Output) Coils are 1-bit registers, used to control discrete outputs, Read or Write 2. Discrete Input 1-bit registers 3. Input Register (Read Only) 16-bit data registers 4. Holding Register (Read/Write)

10 Register Addresses 0x = Coil, Address Range: x = Discrete Input, Address Range: x = Input Register, Address Range: x = Holding Register, Address Range: When using the extended referencing, all number references must be exactly six digits. This avoids confusion between coils and other entities. For example, to know the difference between holding register #40001 and coil #40001, if coil #40001 is the target, it must appear as #

11 Access Levels Memory Type Data Type Master Access Slave Access Coils Bit (Boolean) Read/Write Read/Write Discrete Input Bit (Boolean) Read-only Read/Write Input Register Unsigned Word Read-only Read/Write Holding Register Unsigned Word Read/Write Read/Write In SCADA systems, it is common for embedded devices to have certain values defined as inputs, such as gains or proportional integral derivative (PID) settings, while other values are outputs, like the current temperature or valve position. To meet this need, Modbus data values are divided into four ranges In many cases, sensors and other devices generate data in types other than simply Booleans and unsigned integers. It is common for slave devices to convert these larger data types into registers. For example, a pressure sensor may split a 32-bit floating point value across two 16-bit registers. Introduction to Modbus (National Instruments):

12 Register Referencing 40001:7 This is a commonly used notation for referencing individual bits in a register. This particular example references register (which is a Holding Register), bit 7. Bits are generally numbered starting at bit 0, which is the least significant or right most bit in the field of 16 bits found in a Modbus register.

13 Modbus ASCII Modbus Protocols Modbus RTU (Remote Terminal Unit) Modbus RTU uses RS-485 or RS-232 Modbus TCP/IP Modbus TCP uses Ethernet Modbus ASCII and Modbus RTU are simple serial protocols that use RS-232 or RS-485 to transmit data packets. Modbus TCP/IP follows the OSI Network Model and can be used in an ordinary Ethernet network

14 Modbus Communication RS-232 RS-485 TCP/IP Multi-drop network Master Slave Master Slave Master Slave Slave Master Slave Slave Master Slave

15 Modbus ASCII ASCII characters for protocol communication

16 Modbus RTU Binary representation of the data for protocol communication. Modbus RTU uses RS-485 or RS-232 Modbus RTU requires that you know or define baud rate, character format (8 bits no parity, etc.), and slave ID (aka slave address, unit number, unit ID). A mismatch in any of these will result in no communication.

17 Modbus TCP/IP Modbus TCP/IP follows the OSI Network Model and can be used in an ordinary Ethernet network Modbus TCP requires that you know or define IP addresses on the network Modbus TCP/IP uses Port 502 In Modbus TCP/IP we use the term Server/Client instead of Slave/Master Slave -> Server Master -> Client

18 Modbus RTU vs. Modbus TCP/IP The most basic difference between Modbus RTU and Modbus TCP/IP is that Modbus TCP/IP runs on an Ethernet physical layer and Modbus RTU is a serial protocol

19 Modbus RTU Frame Format

20 Modbus RTU Frame Format

21 Modbus ASCII/RTU Message Properties

22 Modbus in LabvIEW Hans-Petter Halvorsen, M.Sc.

23 Modbus in LabVIEW 3 ways to use Modbus in LabVIEW: Use a high-level OPC Server Use Modbus I/O Server Use the LabVIEW Modbus API LabVIEW Real-Time Module or LabVIEW DSC Module required Introduction to Modbus (National Instruments):

24 LabVIEW Modbus API

25 LabVIEW Example LabVIEW App #2 Master (Client) Master (Client) LabVIEW App #3 Write Data Polling/Request Read Data LabVIEW App #1 Slave (Server) Registers Data Stored in Registers In this Example we Create 3 different LabVIEW Applications

26 LabVIEW App #2 (Master) LabVIEW Example Write Data to Slave LabVIEW App #1 (Slave) LabVIEW App #3 (Master) Read Data from Slave Note! You need to start/run the Modbus Slave App before you start the Modbus Master Apps

27 This part is just for reading the internal Coil Data. It can be removed. It is just to see when new data from the Masters has been received Modbus Slave

28 Modbus Master (Write)

29 Modbus Master (Read)

30 LabVIEW Modbus Simulator The LabVIEW Modbus Simulator is integrated with LabVIEW Real- Time Module or LabVIEW DSC Module It can be used for test purpose, etc. The LabVIEW Modbus Simulator is a Modbus Slave (Server)

31 Find Modbus Examples with NI Example Finder

32 LabVIEW Modbus Simulator Example

33 References Modbus Organization: Modbus (Wikipedia): Introduction to Modbus (National Instruments): Connect LabVIEW to Any PLC With Modbus (National Instruments): Modbus Introduction to Modbus: Modbus TCP/IP: Modbus RTU: Using Modbus for Process Control and Automation (PDF): and_automation.pdf

34 Hans-Petter Halvorsen University of Southeast Norway Web: