Overview of MODBUS Protocol Khorasan Regional Electric Company

Overview of MODBUS Protocol Khorasan Regional Electric Company Ministry of Energy of Iran Presented By: Hanieh Esmaeeli November 8 th. 2012 h. esmaeely@gmail. com

What is MODBUS protocol? � Was used in Process control by Modicon (Schneider Electric) in 1979 � data transmission between controller and sensors via RS-232 � Was promoted to RS-485 : Velocity TCP/IP Multi drop Net Master / Slave Open Protocol 2 Distance

Introduction � Open protocol � simple master–slave protocol � severely limited for electrical substation communication. The basic protocol for PLC connection � used by local serial connection, is not suited for Telemetry De facto � Optional interface: RS-232, RS-422, RS-485 WWW. MODBUS. org � 20 – 30 supplier � Support group for vender and user 3

introduction � applies to layers 1, 2 and 7 of the OSI stack relatively slow in comparison with other buses ü wide acceptance among instrument manufacturers and users • Fixed characteristics: frame format, frame, sequences, handling of communications errors , exception conditions , the functions performed • Stable characteristics: transmission medium, transmission characteristics, and transmission mode, 4

Modbus Addressing � To communicate with a slave device, the master sends a message containing q Device Address q Function Code q Data q Error Check 5

Device Address Function Code Data Error Check 6 • Number from 0 to 247 • broadcast messages: slave device responds to a MODBUS message • command that the slave device is to execute • 1 -255 • defines addresses in the device’s memory map for read functions • 16 -bit numeric value representing the Cyclic Redundancy Check (CRC). • CRC values do not match, the device asks • for a retransmission of the message

Data field 7

The MODBUS protocol provides frames for the transmission of messages between master and slaves 8

Modbus infrastructure �Serial Modbus �Modbus Plus �Ethernet TCP/IP 9

Modbus Serial RTU – compact and faster ASCII – readable 10 • referred to as Modbus-B for Modbus binary • typical message that is about twice the length of the equivalent RTU message

RTU Mode �the same baud rate in messaging make effective characters, so operating burden increase �RTU has 1 byte string and ASCII has 4 bit string ASCII is slower than others 11

Serial Modbus Operation � host device transmits a command, attached device(s) respond. Each device has a unique address assigned to it. Each device is configured for the same protocol emulation of Modbus. Mast er/Sl struc ave ture 12

TCP/IP Modbus � Evolution of Serial Modbus � Data Transportation Mechanism: Ethernet 13

Modbus problems �Output are related to physical interface �Band width moderating �Limited IED which are in connection 14

Modbus Plus � Modbus characteristics developed after 1980 � Main orders are common � Mobus AEG invent this new method 15

Modbus plus Performance theory 16

17

Typical Applications of Modbus Plus in Substation Automation v Interlocking between PLCs (e. g. bus restoration) v Control and monitoring of protective relay I/O from PLCs v Sharing of analog data among IEDs v Acquisition of data by a network-resident host computer v Transfer of status and control between power plant and substation PLCs v Uploading and downloading of PLC programs v Download of configurations and settings to protective relays and other IEDs 18