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 main reasons for the use of Modbus in the industrial environment are:
- developed with industrial applications in mind
- openly published and royalty-free
- easy to deploy and maintain
- moves raw bits or words without placing many restrictions on vendors
Modbus enables communication among many (approximately 240) devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.
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. The Modbus Organization is an association of users and suppliers of Modbus compliant devices that seeks to drive the adoption and evolution of Modbus.
Communication and devices
Each device intended to communicate using Modbus is given a unique address. In serial and MB+ networks, only the node assigned as the Master may initiate a command. On Ethernet, any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcastable commands sent to node 0 which are acted on but not acknowledged). All Modbus commands contain checksum information, to allow the recipient to detect transmission errors. The basic Modbus commands can instruct an RTU to change the value in one of its registers, control or read an I/O port, and command the device to send back one or more values contained in its registers.
There are many modems and gateways that support Modbus, as it is a very simple protocol and often copied. Some of them were specifically designed for this protocol. Different implementations use wireline, wireless communication, such as in the ISM band, and even short message service (SMS) or General Packet Radio Service (GPRS). One of the more common designs of wireless networks makes use of Mesh networking. Typical problems that designers have to overcome include high latency and timing issues.
All Modbus variants choose one of the following frame formats.
|Start||28||At least 3 1⁄2 character times of silence (mark condition)|
|Function||8||Indicates the function code; e.g., read coils/inputs|
|Data||n × 8||Data + length will be filled depending on the message type|
|End||28||At least 3 1⁄2 character times of silence between frames|
|Start||1||Starts with colon (
|Function||2||Indicates the function codes like read coils / inputs|
|Data||n||Data + length will be filled depending on the message type|
|End||2||Carriage return – line feed (CR/LF) pair (ASCII values of 0x0D & 0x0A)|
|Transaction identifier||2||For synchronization between messages of server & client|
|Protocol identifier||2||Zero for Modbus/TCP|
|Length field||2||Number of remaining bytes in this frame|
|Unit identifier||1||Slave address (255 if not used)|
|Function code||1||Function codes as in other variants|
|Data bytes||n||Data as response or commands|
Unit identifier is used with Modbus/TCP devices that are composites of several Modbus devices, e.g. on Modbus/TCP to Modbus RTU gateways. In such case, the unit identifier tells the Slave Address of the device behind the gateway. Natively Modbus/TCP-capable devices usually ignore the Unit Identifier.
The byte order is Big-Endian (first byte contains MSB).
Supported function codes
The various reading, writing and other operations are categorised as follows. The most primitive reads and writes are shown in bold. A number of sources use alternative terminology, for example Force Single Coil where the standard uses Write Single Coil.
|Function type||Function name||Function code|
|Data Access||Bit access||Physical Discrete Inputs||Read Discrete Inputs||2|
|Internal Bits or Physical Coils||Read Coils||1|
|Write Single Coil||5|
|Write Multiple Coils||15|
|16-bit access||Physical Input Registers||Read Input Registers||4|
|Internal Registers or Physical Output Registers||Read Holding Registers||3|
|Write Single Register||6|
|Write Multiple Registers||16|
|Read/Write Multiple Registers||23|
|Mask Write Register||22|
|Read FIFO Queue||24|
|File Record Access||Read File Record||20|
|Write File Record||21|
|Diagnostics||Read Exception Status||7|
|Get Com Event Counter||11|
|Get Com Event Log||12|
|Report Slave ID||17|
|Read Device Identification||43|
|Other||Encapsulated Interface Transport||43|
Almost all implementations have variations from the official standard. Different varieties might not communicate correctly between equipment of different suppliers. Some of the most common variations are:
- Data types
- Floating point IEEE
- 32-bit integer
- 8-bit data
- Mixed data types
- Bit fields in integers
- Multipliers to change data to/from integer. 10, 100, 1000, 256 ...
- Protocol extensions
- 16-bit slave addresses
- 32-bit data size (1 address = 32 bits of data returned.)
- Word swapped data
- Since Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported.
- No standard way exists for a node to find the description of a data object, for example, to determine if a register value represents a temperature between 30 and 175 degrees.
- Since Modbus is a master/slave protocol, there is no way for a field device to "report by exception" (except over Ethernet TCP/IP, called open-mbus)- the master node must routinely poll each field device, and look for changes in the data. This consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link.
- Modbus is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station (once again Ethernet TCP/IP proving the exception).
- Modbus transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the transmission.
- Modbus protocol itself provides no security against unauthorized commands or interception of data.
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