Modbus

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Modbus is a serial communication protocol originally published by Modicon (now Schneider Electric) in 1979 for use with a programmable logic controller (PLC). Modbus has become the de facto standard communication protocol and is now a publicly available tool for connecting industrial electronic devices. The main reasons for the use of Modbus in industrial environments are:

• developed with industry applications in mind,
• publicly published and royalty free,
• easy to implement and maintain,
• move raw bits or words without putting many restrictions on the vendor.

Modbus allows communication among many devices connected to the same network, for example, a system that measures temperature and humidity and communicates results to a computer. Modbus is often used to connect computer controllers with remote terminal units (RTUs) in surveillance and data acquisition control systems (SCADA). Many of the data types are named from their use in moving relays: single-bit physical output is called coil , and single-bit physical input is called discrete input or contact .

The development and renewal of the Modbus protocol has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to the organization. Modbus Organization is a user association and Modbus compatible device supplier that supports the continuous use of technology.

Video Modbus

Jenis objek modbus

The following is a table of object types provided by the Modbus slave device to the Modbus master device:

Maps Modbus

Protocol version

Modbus protocol version exists for serial port and for Ethernet and other protocols that support Internet protocol suite. There are many Modbus protocol variants:

• Modbus RTU - This is used in serial communication and utilizes a concise binary representation of data for protocol communications. The RTU format follows the command/data with a cyclic redundancy checksum check as a fault checking mechanism to ensure data reliability. Modbus RTU is the most common implementation available for Modbus. Modbus RTU messages must be transmitted continuously without any doubt between characters. Modbus messages are framed (separated) by silent periods.
• ASCII Modbus - This is used in serial communication and utilizes ASCII characters for protocol communications. The ASCII format uses a longitudinal redundancy checksum checksum. The ASCII Modbus message is framed by a colon (":") and follows a new line (CR/LF).
• TCP/IP Modbus or TCP Modbus - This is a Modbus variant used for communication over a TCP/IP network, connecting through port 502. This does not require checksum calculation, since the underlay already provides checksum protection.
• Modbus via TCP/IP or Modbus via TCP or Modbus RTU/IP - This is a different Modbus variant of TCP Modbus in that checksum is included in payload as with Modbus RTU.
• Modbus above UDP - Some have experimented with using Modbus via UDP on the IP network, which removes the overhead required for TCP.
Modbus Plus (Modbus, MB or MBP) - Modbus Plus is Schneider Electric's and unlike other variants, it supports peer-to-peer communication between multiple masters. A special co-processor is required to handle a token turn similar to HDLC. It uses twisted pair at 1 Mbit/s and includes isolation of the transformer on each node, making it a transition/edge-triggered rather than voltage/level-triggered. Special hardware is required to connect Modbus Plus to the computer, usually cards made for ISA, PCI or PCMCIA buses.
• Pemex Modbus - This is an extension of the standard Modbus with support for historical and stream data. It is designed for oil and gas companies Pemex for use in process control and never gets widespread adoption.
• Enron Modbus - This is another extension of the standard Modbus developed by Enron Corporation with support for 32-bit integers and floating-point variables as well as historical and stream data. Data types are mapped using standard addresses. Historical data serves to meet American Petroleum Institute (API) industry standards on how data should be stored.

The data model and function calls are identical for the first four variants of the protocol; only different encapsulation. However the variant can not be operated, nor is the frame format.

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Communications and devices

Every device that is meant to communicate using Modbus is given a unique address. In serial and MB networks, only nodes are designated as Master that can start commands. On Ethernet, any device can send Modbus commands, although usually only one master device does it. The Modbus command contains the Modbus address of the device intended for (1 to 247). Only the device in question will act on the command, although other devices may accept it (exceptions are specific commands that can be broadcast sent to node 0, acting but not recognized). All Modbus commands contain checksum information to enable the receiver to detect a transmission error. The basic Modbus command can instruct the RTU to change the value in one of its registers, control or read the I/O port, and instruct the device to send back one or more of the values ​​contained in its register.

There are many modems and gateways that support Modbus, because the protocol is very simple and often copied. Some of them are specifically designed for this protocol. Different implementations use wireline, wireless communications, as in ISM bands, and even Short Message Service (SMS) or General Packet Radio Service (GPRS). One of the more common wireless network designs utilizes mesh networks. Common problems that designers have to deal with include high latency and timing issues.

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Frame format

The Modbus frame consists of an Application Data Unit (ADU), which encapsulates the Protocol Data Unit (PDU):

• ADU = Address PDU Checking error,
• PDU = Data function code.

All Modbus variants choose one of the following frame formats.

A note about CRC:

• Polynomial: x ¹⁶ x ¹⁵ x ² 1 (CRC-16-ANSI is also known as CRC-16-IBM, hexadecimal hexadecimal hexadecimal polynomials to 8005 and flips A001 ).
• Initial value: 65,535.
• Example of a frame in hexadecimal: 01 04 02 FF FF B8 80 (Calculation CRC-16-ANSI from 01 to FF 80B8 , which is transmitted at most significant bytes first ).

Addresses, functions, data, and LRC are all set of capital-readable hexadecimal characters representing 8-bit values ​​(0-255). For example, 122 (7 ÃÆ'â € "16 10) will be represented as 7A .

The LRC is calculated as the number of 8-bit values, negated (complement two) and encoded as 8-bit values. Example: if address, function, and data are encoded as 247, 3, 19, 137, 0, and 10, their number is 416. Two complement (-416) trimmed into 8 bits is 96 (eg 256 ÃÆ'â € "2 - 416), which will be represented as 60 in hexadecimal. Therefore the following frames: : F7031389000A60 & lt; CR & gt; & lt; LF & gt; .

The unit identifier is used with a Modbus/TCP device that is a composite of some Modbus devices, e.g. on Modbus/TCP to the Modbus RTU gateway. In such case, the unit identifier tells the Slave Address of the device behind the gateway. Modbus/TCP-capable devices usually ignore the Unit Identifier.

The order of bytes for values ​​in the Modbus data frame is big-endian (MSB, the most significant byte of the first received value).

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Supported function code

Various readings, writing and other operations are categorized as follows. The most primitive readings and writings are shown in bold. Some sources use alternative terminology, such as Force Single Coil where the standard uses Write Single Coil .
The prominent entity in a Modbus slave is:

• Spool: read and write, 1 bit (off)
• Discrete Input: readable, 1 bit (off)
• Input Register: can be read, 16 bit (0 to 65,535), basically the measurement and status
• Holding Registers: can be read and written, 16 bits (0 to 65,535), basically the configuration value

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The format of request and response data for the main function code

Requests and responses follow the frame format described above. This section provides details of the data format of the most commonly used function code.

Code function 1 (read coil) and function code 2 ( read discrete input)

Request :

• Address of the first coil/discrete input to read (16-bit)
• Number of coil/discrete inputs to read (16-bit)

Normal response :

• Number of bytes of coil/discrete input value to follow (8-bit)
• Value of coil/discrete input (8 coils/discrete input per byte)

The value of each coil/discrete input is binary (0 for off, 1 for on). The first required discrete/discrete input is stored as the most significant bit of the first byte in response If the number of discrete coils/inputs is not a multiple of 8, the most significant bit of the last byte will be filled with zero.
For example, if eleven coils are requested, two byte values ​​are required. Suppose that the state of the consecutive coil is active, inactive, active, active, active, active, active, active, active, active , then the response will be 02 E5 06 in hexadecimal.

Code function 5 (forcing/writing single coil)

Request :

• The coil (16-bit) address
• Value for forced/write: 0 for off and 65.280 (FF00 in hexadecimal) for on

Normal response : same as request.

Code function 15 (forcing/writing multiple scrolls)

Request :

• First coil address to force/write (16-bit)
• Number of scrolls to be forced/written (16-bit)
• Number of bytes of coil value to follow (8-bit)
• Coil value (8 coils per byte)

The value of each coil is binary (0 for off, 1 for di). The requested coil is first stored as the most significant bit of the first byte requested.
If the number of coils is not a multiple of 8, the least significant (s) of the last byte must be zero. See examples for function codes 1 and 2.
normal response :

• The first (16-bit) coil address
• number of scrolls (16-bit)

Function code 4 (read the input register) and function code 3 (read holding register)

Requests :

• The first register address to read (16-bit)
• The number of registers to read (16-bit)

Normal response :

• The number of bytes of register to follow (8-bit)
• List value (16 bits per register)

Since the number of bytes for the register value is 8-bit wide, only 127 registers can be read at once.

Code function 6 (preset/write single holding register)

Requests :

• The address holds the register to preset/write (16-bit)
• New value of register holding (16-bit)

Respons normal : sama dengan permintaan.

Kode fungsi 16 (preset/tulis beberapa holding register)

Requests :

• The first holding register address to preset/write (16-bit)
• The number of holds the register to preset/write (16-bit)
• The number of bytes of register to be followed (8-bit)
• New value to hold a register (16 bits per register)

Since the register value is 2 bytes wide and only the value of 127 bytes can be sent, only 63 registers can be set at all.

Normal response :

• The first register/write address (16-bit)
• number of written presets/holding registers (16-bit)

Exception response

For normal responses, the slave repeats the function code. If a slave wants to report an error, he will reply with the requested function code plus 128 (hex 0x80 ) (3 becomes 131 = hex 0x83 ), and will only include one byte data, known as the exception code .

Main Modbus exclusion code

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Coil, discrete input, input list, holding register number and address

Some conventions govern how access to Modbus entities (coils, discrete inputs, input registers, holding registers) are referenced.

It is important to make a distinction between the number and entity address entities:

• The number entity combines the entity type and the location of the entity in the table of descriptions.
• Entity address is the start address, the 16-bit value in the data part of the Modbus frame. Thus the range starts from 0 to 65,535

In traditional standards, the number for that entity starts with a digit, followed by a 4-digit number in the range 1-9,999:

• coil number start with 0 and range from 0 0001 to 0 9999,
• discrete input number starts with 1 and ranges from 1 0001 to 1 9999,
• list of input numbers starts with 3 and ranges from 3 0001 to 3 9999,
• hold the list of numbers starting with 4 and the range from 4 0001 to 4 9999.

This translates to addresses between 0 and 9.998 in the data frame. For example, to read the hold on the register starting from number 40001, the corresponding address in the data frame will be 0 with function code 3 (as seen above). To hold a register from number 40100, address will be 99. Etc.

This limits the number of addresses to 9,999 for each entity. Reference de facto extends this to a maximum of 65,536. It only consists of adding a single digit to the previous list:

• spindle number span from 0 00001 to 0 65536,
• discrete input number span from 1 00001 to 1 65536,
• input register number span from 3 00001 to 3 65536,
• keep listing number span from 4 00001 to 4 65536.

When using extended references, all references numbers must have exactly 6 digits. This avoids confusion between coils and other entities. For example, to know the difference between holding register # 40001 and coil # 40001, if coil # 40001 is the target, it should appear as # 040001.

JBUS Mapping

Other de facto protocols closely associated with Modbus came later and were determined by the April Automates PLC brand, a result of the collaborative efforts of the French company Renault Automation and Merlin Gerin et Cie in 1985: JBUS . The difference between Modbus and JBUS at the time (number of entities, slave stations) is now irrelevant because the protocol was almost gone with the April PLC series purchased by AEG Schneider Automation in 1994 and then made it obsolete. But the name JBUS survived to some extent.

JBUS supports function codes 1, 2, 3, 4, 5, 6, 15, and 16 and thus all the entities described above. But the numbering is different from JBUS:

• Number and address coincide: entity # x has the address x in the data frame.
• As a result, entity numbers do not include entity types. For example, holding register # 40010 in Modbus will hold register # 9, which is located at address 9 on JBUS.
• The number 0 (and thus address 0) is not supported. Slaves should not implement any real data in these numbers and addresses and can either return a null value or remove an error when prompted.

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Implementations

Almost all implementations have variations of official standards. Different varieties may not communicate properly between different supplier equipment. Some of the most common variations are:

• Data type
  • IEEE floating-point number
  • 32-bit integers
  • 8-bit data
  • Mixed data types
  • Field of bits in integers
  • Multiplier to convert data to/from integer. 10, 100, 1000, 256...
• Protocol extensions
  • 16-bit slave address
  • 32-bit data size (1 address = 32 bit data returned)
  • Word exchange data

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Limitations

• Since Modbus was designed in the late 1970s to communicate with programmable logic controllers, the number of data types was limited to those understood by PLCs at the time. Large binary objects are not supported.
• There is no standard way for a node to find a description of a data object, for example, to determine whether the register value represents a temperature between 30 and 175 degrees.
• Since Modbus is a master/slave protocol, there is no way for field devices to "report with exceptions" (except via TCP/IP Ethernet, called open-mbus) - the master node must routinely poll each field device and look for data change. It spends bandwidth and network time in applications where bandwidth may be expensive, such as through radio links with low bit rates.
• Modbus is limited to handle 254 devices on a single data link, which limits the number of field devices that can be connected to the master station (again, TCP/IP Ethernet is the exception).
• Modbus transmission must be contiguous, limiting the type of remote communication device to devices that can support data to avoid loopholes in transmission.
• The modbus protocol itself does not provide security against unauthorized commands or data interception.

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Trading group

Modbus Organization, Inc. is the trade association for the promotion and development of the Modbus protocol.

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Modbus Plus

Despite its name, Modbus Plus is not a variant of Modbus. This is a different protocol, which involves token passing.

This is a Schneider Electric specification, although it is not published and is not patented. This is usually implemented using a special chipset that is only available to Schneider partners.

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References

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External links

Specifications

• Modbus organization with protocol specifications
• Modbus Protocol ; Modicon; 74 pages; 2000.

More

• Free Modbus Guide for Field Technicians

Source of the article : Wikipedia