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Modbus Protocol PDF format version of the MODBUS Protocol The original was found at: http://www.http://www.modicon.com/techpubs/toc7.html (In case of any discrepancies, that version should be considered accurate.) Hope you find this useful! Spehro Pefhany, January 2000 3-1750 The Queensway Suite 1298 Toronto ON Canada M9C 4H5 (905) 271-4477 fax: (905) 271-9838 e-mail: info@trexon.com

Modbus Protocol Chapter 1 Modbus Protocol Chapter 2 Data and Control Functions Chapter 3 Diagnostic Subfunctions Chapter 4 Exception Responses Chapter 5 Application Notes Chapter 6 LRC / CRC Generation http://www.modicon.com/techpubs/toc7.html [1/11/2000 10:32:59 PM]

Modbus Protocol Chapter 1 Modbus Protocol V Introducing Modbus Protocol V Two Serial Transmission Modes V Modbus Message Framing V Error Checking Methods 1.1 Introducing Modbus Protocol Modicon programmable controllers can communicate with each other and with other devices over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus industrial networks, and standard networks such as MAP and Ethernet. Networks are accessed by built-in ports in the controllers or by network adapters, option modules, and gateways that are available from Modicon. For original equipment manufacturers, Modicon ModConnect partner programs are available for closely integrating networks like Modbus Plus into proprietary product designs. The common language used by all Modicon controllers is the Modbus protocol. This protocol defines a message structure that controllers will recognize and use, regardless of the type of networks over which they communicate. It describes the process a controller uses to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported. It establishes a common format for the layout and contents of message fields. The Modbus protocol provides the internal standard that the Modicon controllers use for parsing messages. During communications on a Modbus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the controller will construct the reply message and send it using Modbus protocol. On other networks, messages containing Modbus protocol are imbedded into the frame or packet structure that is used on the network. For example, Modicon network controllers for Modbus Plus or MAP, with associated application software libraries and drivers, provide conversion between the imbedded Modbus message protocol and the specific framing protocols those networks use to communicate between their node devices. This conversion also extends to resolving node addresses, routing paths, and error-checking methods specific to each kind of network. For example, Modbus device addresses contained in the Modbus protocol will be converted into node addresses prior to transmission of the messages. Error-checking fields will also be applied to message packets, consistent with each http://www.modicon.com/techpubs/intr7.html (1 of 5) [1/11/2000 10:36:08 PM]

Modbus Protocol network's protocol. At the final point of delivery, however-for example, a controller-the contents of the imbedded message, written using Modbus protocol, define the action to be taken. Figure 1 shows how devices might be interconnected in a hierarchy of networks that employ widely differing communication techniques. In message transactions, the Modbus protocol imbedded into each network's packet structure provides the common language by which the devices can exchange data. Figure 1 Overview of Modbus Protocol Application 1.1.1 Transactions on Modbus Networks Standard Modbus ports on Modicon controllers use an RS-232C compatible serial interface that defines connector pinouts, cabling, signal levels, transmission baud rates, and parity checking. Controllers can be networked directly or via modems. Controllers communicate using a master-slave technique, in which only one device (the master) can initiate transactions (queries). The other devices (the slaves) respond by supplying the requested data to the master, or by taking the action requested in the query. Typical master devices include host processors and programming panels. Typical slaves include programmable http://www.modicon.com/techpubs/intr7.html (2 of 5) [1/11/2000 10:36:08 PM]

Modbus Protocol controllers. The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves return a message (response) to queries that are addressed to them individually. Responses are not returned to broadcast queries from the master. The Modbus protocol establishes the format for the master's query by placing into it the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error-checking field. The slave's response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurred in receipt of the message, or if the slave is unable to perform the requested action, the slave will construct an error message and send it as its response. 1.1.2 Transactions on Other Kinds of Networks In addition to their standard Modbus capabilities, some Modicon controller models can communicate over Modbus Plus using built-in ports or network adapters, and over MAP, using network adapters. On these networks, the controllers communicate using a peer-to-peer technique, in which any controller can initiate transactions with the other controllers. Thus a controller may operate either as a slave or as a master in separate transactions. Multiple internal paths are frequently provided to allow concurrent processing of master and slave transactions. At the message level, the Modbus protocol still applies the master-slave principle even though the network communication method is peer-to-peer. If a controller originates a message, it does so as a master device, and expects a response from a slave device. Similarly, when a controller receives a message it constructs a slave response and returns it to the originating controller. 1.1.3 The Query-Response Cycle Figure 2 Master-Slave Query-Response Cycle The Query The function code in the query tells the addressed slave device what kind of action to perform. The data bytes contain any additional information that the slave will need to perform the function. For example, function code 03 will query the slave to read holding registers and respond with their contents. The data field must contain the information telling the slave which http://www.modicon.com/techpubs/intr7.html (3 of 5) [1/11/2000 10:36:08 PM]

Modbus Protocol register to start at and how many registers to read. The error check field provides a method for the slave to validate the integrity of the message contents. The Response If the slave makes a normal response, the function code in the response is an echo of the function code in the query. The data bytes contain the data collected by the slave, such as register values or status. If an error occurs, the function code is modified to indicate that the response is an error response, and the data bytes contain a code that describes the error. The error check field allows the master to confirm that the message contents are valid. 1.2 Two Serial Transmission Modes Controllers can be setup to communicate on standard Modbus networks using either of two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode, etc), during configuration of each controller. The mode and serial parameters must be the same for all devices on a Modbus network. The selection of ASCII or RTU mode pertains only to standard Modbus networks. It defines the bit contents of message fields transmitted serially on those networks. It determines how information will be packed into the message fields and decoded. On other networks like MAP and Modbus Plus, Modbus messages are placed into frames that are not related to serial tranasmission. For example, a request to read holding registers can be handled between two controllers on Modbus Plus without regard to the current setup of either controller's serial Modbus port. 1.2.1 ASCII Mode When controllers are setup to communicate on a Modbus network using ASCII (American Standard Code for Information Interchange) mode, each eight-bit byte in a message is sent as two ASCII characters. The main advantage of this mode is that it allows time intervals of up to one second to occur between characters without causing an error. Coding System V Hexadecimal, ASCII characters 0 ... 9, A ... F V One hexadecimal character contained in each ASCII character of the message Bits per Byte V 1 start bit V 7 data bits, least significant bit sent first V 1 bit for even / odd parity-no bit for no parity V 1 stop bit if parity is used-2 bits if no parity Error Check Field http://www.modicon.com/techpubs/intr7.html (4 of 5) [1/11/2000 10:36:08 PM]

Modbus Protocol V Longitudinal Redundancy Check (LRC) 1.2.2 RTU Mode When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit) mode, each eight-bit byte in a message contains two four-bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII for the same baud rate. Each message must be transmitted in a continuous stream. Coding System V Eight-bit binary, hexadecimal 0 ... 9, A ... F V Two hexadecimal characters contained in each eight-bit field of the message Bits per Byte V 1 start bit V 8 data bits, least significant bit sent first V 1 bit for even / odd parity-no bit for no parity V 1 stop bit if parity is used-2 bits if no parity Error Check Field V Cyclical Redundancy Check (CRC) 1.3 Modbus Message Framing In either of the two serial transmission modes (ASCII or RTU), a Modbus message is placed by the transmitting device into a frame that has a known beginning and ending point. This allows receiving devices to begin at the start of the message, read the address portion and determine which device is addressed (or all devices, if the message is broadcast), and to know when the message is completed. Partial messages can be detected and errors can be set as a result. On networks like MAP or Modbus Plus, the network protocol handles the framing of messages with beginning and end delimiters that are specific to the network. Those protocols also handle delivery to the destination device, making the Modbus address field imbedded in the message unnecessary for the actual transmission. (The Modbus address is converted to a network node address and routing path by the originating controller or its network adapter.) 1.3.1 ASCII Framing In ASCII mode, messages start with a colon ( : ) character (ASCII 3A hex), and end with a carriage return-line feed (CRLF) pair (ASCII 0D and 0A hex). The allowable characters transmitted for all other fields are hexadecimal 0 ... 9, A ... F. Networked devices monitor the network bus continuously for the colon character. Wh http://www.modicon.com/techpubs/intr7.html (5 of 5) [1/11/2000 10:36:08 PM]

Chapter 2 Data and Control Functions V Modbus Function Formats V V V V V V V V V V V V V V Function Codes Read Coil Status Read Input Status Read Holding Registers Read Input Registers Force Single Coil Preset Single Register Read Exception Status Fetch Comm Event Counter Fetch Comm Event Log Force Multiple Coils Preset Multiple Registers Report Slave ID Read General Reference V Write General Reference V Mask Write 4x Register V Read / Write 4x Registers http://www.modicon.com/techpubs/dcon7.html (1 of 36) [1/11/2000 10:41:03 PM]

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