Slide 1 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus training

Slide 2 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus Part 1 :Reference documents - WEB sites Part 2 :Definitions Part 3 : Modbus frame description Part 4 : Security of transmission Part 5 : Physical layer Part 6 : Main characteristics resume

Slide 3 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus User guide Reference : TSX DG MOD * Detailed description of the Modbus protocol. X-WAY communication user guide Reference : TSX DR NET * This guide gives common characteristics to X-WAY Schneider networks. Communication application specific functions PL7 Micro/Junior/Pro Reference : TLX DS COM PL7 42 * Volume 1 : Common communication application Volume 2 : Modbus bus Part 1 : Reference documents - WEB sites Schneider documents

Slide 4 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 WEB sites Modbus.org : Modicon site for Modbus users. Transparent Factory : Schneider Electric site for Transparent Factory users. Modbus Plus : Modbus Plus Planning and installation. Part 1 : Reference documents - WEB sites

Slide 5 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus protocol MODBUS Protocol is a messaging structure created by MODICON company to connect PLC to programming tools. It is now widely used to establish master-slave communication between intelligent devices. MODBUS is independent of the physical layer. It can be implemented using RS232, RS422, or RS485 or over a variety of media (e.g. fiber, radio, cellular, etc...). Part 2 : Définitions

Slide 6 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus Plus MODBUS PLUS is a higher speed network 1 Mbit/s token passing derivative that uses the MODBUS messaging structure. Application Presentation Session Transport Network Link Physical Token passing Modbus RS485 Part 2 : Définitions

Slide 7 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus TCP/IP MODBUS TCP/IP uses TCP/IP and Ethernet 10 Mbit/s or 100 Mbits/s to carry the MODBUS messaging structure. Application Préeentation Session Transport Network Link Physical CSMA / CD ETHERNET V2 ou Modbus TCP IP Part 2 : Définitions

Slide 8 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus ASCII and Modbus RTU The MODBUS protocol comes in 2 versions : ASCII transmission mode Each eight-bit byte in a message is sent as 2 ASCII characters. RTU transmission mode Each eight-bit byte in a message is sent as two four-bit hexadecimal characters. The main advantage of the RTU mode is that it achieves higher throughput. ASCII mode allows time intervals of up to 1 second to occur between characters without causing an error. Part 2 : Définitions

Slide 9 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Modbus frame structure AddressChecksumDataFunction The Modbus frame structure is the same for requests (master to slave messages) and responses (slave to master messages). Modbus ASCII Modbus RTU :CRLF 3A Hex 0D Hex 0A Hex Part 3 : Modbus frame description AddressChecksumDataFunction silence Silence >= 3,5 characters

Slide 10 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Address field AddressChecksumDataFunction Valid slave device addresses are in the range of decimal. The individual slave devices are assigned addresses in the range of Value 0 is reserved for broadcast messages (no response). Request : A master addresses a slave by placing the slave address in the address field of the message. Response : When the slave sends its response, it places its own address in this address field of the response to let the master know which slave is responding. Part 3 : Modbus frame description

Slide 11 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Valid codes are in the range of decimal. Request : The function code field tells the slave what kind of action to perform. Response : For a normal response, the slave simply echoes the original function code. For an exception response, the slave returns a code that is equivalent to the original function code with its most significant bit set to a logic 1. Function field AddressChecksumDataFunction Part 3 : Modbus frame description

Slide 12 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Valid codes are in the range of decimal. Request : The data field contains additional information which the slave must use to take the action defined by the function code. This can include items like register addresses, quantity of items to be handled, etc... Response : If no error occurs, the data field contains the data requested. If an error occurs, the field contains an exception code that the master application can use to determine the next action to be taken. Data field AddressChecksumDataFunction Part 3 : Modbus frame description

Slide 13 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Valid codes are in the range of decimal. Modbus RTU uses CRC : Cyclycal Reduncy Check (2 byte) Modbus ASCII uses LRC : Longitudinal Redundancy Check (1 bytes) Request : The checksum is calculated by the master and sends to the slave. Response : The checksum is re-calculated by the slave and compared to the value sent by the master. If a difference is detected, the slave will not construct a response to the master. Checksum field AddressChecksumDataFunction Part 3 : Modbus frame description

Slide 14 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Request : Frame exemple in RTU mode Function code = 3 : Read n words Slave Address CRC16 First word address Function code = 3 Number of words to read 1 byte 2 bytes Response : Slave Address CRC16 Number of bytes read Function code = 3 Value of the first word 1 byte 2 bytes Value of the last word 2 bytes Part 3 : Modbus frame description

Slide 15 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Function code exemples Part 3 : Modbus frame description CodeType 01Read n consecutive output bits 02Read n consecutive input bits 03Read n consecutive output words 04Read n consecutive input words 05Write 1 output bit 06Write 1 output word 07Read exception status 08Access diagnostic counters 11Read event counter 12Read connection events 15Write n output bits 16Write n output words 17Read identification

Slide 16 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Parity checking Even or odd can be optionally applied to each character. Frame checking LRC or CRC is applied to the entire message. Continuous stream The entire message frame must be transmitted as a continuous stream. If a silent interval (more than 1.5 character times RTU mode or 1 second ASCII mode) occurs before completion of the frame, the receiving device flushes the incomplete message and assumes that the next byte will be the address field of a new message. Error checking methods Part 4 : Security of transmission

Slide 17 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 The master is configured by the user to wait for a predetermined timeout interval before aborting the transaction. This interval is set to be long enough for any slave to respond normally. If the slave detects a transmission error, the message will not be acted upon. The slave will not construct a response to the master. Thus the timeout will expire and allow the master's program to handle the error. Error checking methods Part 4 : Security of transmission

Slide 18 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 RS485 physical layer Part 5 : Physical layer RS485 is the most physical layer used on Modbus. 32 devices included the master can be connected on the bus. This is a bus topology with line terminations.

Slide 19 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Main characteristics resume Topology: Bus with line terminations Maximum distance:With RS485 : 1300 m without repeater Data rate: bits/s maximum Max. no. of devices: With RS485 : 32 master included Part 6 : Main characteristics resume

Slide 20 / 20 Industrial Automation - Custumer View - Services PhW - Modbus_en 06/ 2002 Method of accessing the medium: Master slave Transmission method: Messaging Max. useful data size: 120 words Transmission security: LRC or CRC Start and stop delimiters Parity bit Continuous stream Main characteristics resume Part 6 : Main characteristics resume