Modbus

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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.[1] 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 between 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[2] since April 2004, when Schneider Electric transferred rights to that organization, signaling a clear commitment to openness.[3]
The Modbus Organization is an association formed of independent users and suppliers of Modbus compliant devices that seeks to drive the adoption of the Modbus communication protocol suite, and its evolution to address architectures for distributed automation systems across multiple market segments.[4]

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 ensure a command arrives undamaged. The basic Modbus commands can instruct an RTU to change a value in one of its registers, control or read an I/O port, as well as 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.

Frame format

All Modbus variants choose different frame formats.[1]

Modbus RTU Frame Format
Name Length
(bits)		 Function
Start 28 at least 3 1⁄2 character times of silence (mark condition)
Address 8 Station Address
Function 8 Indicates the function code, e.g. read coils / inputs
Data n * 8 Data + length will be filled depending on the message type
CRC 16 bits Checksum
End 28 at least 3 1⁄2 character times of silence between frames
Modbus ASCII Frame Format
Name Length
(char.)		 Function
Start 1 starts with colon ( : ) (ASCII hex value is 0x3A)
Address 2 Station Address
Function 2 Indicates the function codes like read coils / inputs
Data n Data + length will be filled depending on the message type
LRC 2 Checksum
End 2 carriage return – line feed(CR/LF) pair (ASCII values of 0x0D & 0x0A)
Modbus TCP Frame Format
Name Length
(bytes)		 Function
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.[5] 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.[6]

Modbus function codes
Function typeFunction nameFunction code
Data AccessBit accessPhysical Discrete InputsRead Discrete Inputs2
Internal Bits or Physical CoilsRead Coils1
Write Single Coil5
Write Multiple Coils15
16-bit accessPhysical Input RegistersRead Input Register4
Internal Registers or Physical Output RegistersRead Holding Registers3
Write Single Register6
Write Multiple Registers16
Read/Write Multiple Registers23
Mask Write Register22
Read FIFO Queue24
File Record AccessRead File Record20
Write File Record21
DiagnosticsRead Exception Status7
Diagnostic8
Get Com Event Counter11
Get Com Event Log12
Report Slave ID17
Read Device Identification43
OtherEncapsulated Interface Transport43

Implementations

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

Limitations

  • 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.[7]

Trade group

Modbus Organization, Inc. is a trade association for the promotion and development of Modbus protocol.[2]

Modbus Plus

Despite the name, Modbus Plus[8] is not a variant of Modbus. It is a different protocol, involving token passing.

It is a proprietary specification of Schneider Electric, though it is unpublished rather than patented. It is normally implemented using a custom chipset available only to partners of Schneider.


References

  1. ↑ 1.0 1.1 Drury, Bill (2009). Control Techniques Drives and Controls Handbook (PDF) (2nd ed.). Institution of Engineering and Technology. pp. 508–. (subscription required (help)). 
  2. ↑ 2.0 2.1 "Modbus home page". Modbus. Modbus Organization, Inc. Retrieved 2 August 2013. 
  3. ↑ "Modbus FAQ". Modbus. Modbus Organization, Inc. Retrieved 1 November 2012. 
  4. ↑ "About Modbus Organization". Modbus. Modbus Organization, Inc. Retrieved 8 November 2012. 
  5. ↑ "Modbus Application Protocol V1.1b3" (PDF). Modbus. Modbus Organization, Inc. Retrieved 2 August 2013. 
  6. ↑ Clarke, Gordon; Reynders, Deon (2004). Practical Modern Scada Protocols: Dnp3, 60870.5 and Related Systems. Newnes. pp. 47–51. ISBN 0-7506-5799-5. 
  7. ↑ Palmer; Shenoi, Sujeet, eds. (23–25 March 2009). "Critical Infrastructure Protection III". Third IFIP WG 11. 10 International Conference. Hanover, New Hampshire: Springer. p. 87. ISBN 3-642-04797-1. 
  8. ↑ "Modbus Plus - Modbus Plus Network - Products overview - Schneider Electric United States". Schneider-electric.com. Retrieved 2014-01-03. 

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