Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices.
Modbus is popular in industrial environments because it is openly published and royalty-free. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions - other than the datagram (packet) size - on the format of the data to be transmitted.
Modbus supports communication to and from multiple devices connected to the same cable or Ethernet network. For example, there can be a device that measures temperature and another device to measure humidity connected to the same cable, both communicating measurements to the same computer.
Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems in the electric power industry. Many of the data types are named from industrial control of factory devices, such as Ladder logic because of its use in driving relays: A single physical output is called a coil, and a single physical input is called a discrete input or a contact.
The development and update of Modbus protocols have 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 advocates for the continued use of the technology. Modbus Organization, Inc. is a trade association for the promotion and development of Modbus protocol.
- 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 whether a register value represents a temperature between 30 and 175 degrees.
- Since Modbus is a client/server (formerly master/slave) protocol, there is no way for a field device to "report an exception" (except over Ethernet TCP/IP, called open-mbus) – the client 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 Parent station (once again, Ethernet TCP/IP is an 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.[clarification needed]
- Modbus protocol itself provides no security against unauthorized commands or interception of data.
Modbus object types
The following is a table of object types provided by a Modbus server device to a Modbus client device:
|Object type||Access||Size||Address Space|
|Coil||Read-write||1 bit||0x0000 – 0xFFFF|
|Discrete input||Read-only||1 bit||0x0000 – 0xFFFF|
|Input register||Read-only||16 bits||0x0000 – 0xFFFF|
|Holding register||Read-write||16 bits||0x0000 – 0xFFFF|
- Modbus RTU (Remote Terminal Unit) — This is used in serial communication and is the most common implementation available for Modbus. Modbus RTU makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods.
- Modbus ASCII — This is used in serial communication and makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon (":") and trailing newline (CR/LF).
- Modbus TCP/IP or Modbus TCP — This is a Modbus variant used for communications over TCP/IP networks, connecting over port 502. It does not require a checksum calculation, as lower layers already provide checksum protection.
- Modbus over TCP/IP or Modbus over TCP or Modbus RTU/IP — This is a Modbus variant that differs from Modbus TCP in that a checksum is included in the payload as with Modbus RTU.
- Modbus over UDP — Some have experimented with using Modbus over UDP on IP networks, which removes the overheads required for TCP.
- Modbus Plus (Modbus+, MB+ or MBP) — Modbus Plus is proprietary to Schneider Electric and unlike the other variants, it supports peer-to-peer communications between multiple clients. It requires a dedicated co-processor to handle fast HDLC-like token rotation. It uses twisted pair at 1 Mbit/s and includes transformer isolation at each node, which makes it transition/edge-triggered instead of voltage/level-triggered. Special hardware is required to connect Modbus Plus to a computer, typically a card made for the ISA, PCI or PCMCIA bus.
- Pemex Modbus — This is an extension of standard Modbus with support for historical and flow data. It was designed for the Pemex oil and gas company for use in process control and never gained widespread adoption.
- Enron Modbus — This is another extension of standard Modbus developed by Enron Corporation with support for 32-bit integer and floating-point variables and historical and flow data. Data types are mapped using standard addresses. The historical data serves to meet an American Petroleum Institute (API) industry standard for how data should be stored.
Data model and function calls are identical for the first 4 variants of protocols; only the encapsulation is different. However the variants are not interoperable, nor are the frame formats.
Communications and devices
Each device communicating (i.e., transferring data) on a Modbus is given a unique address.
On Modbus RTU, Modbus ASCII and Modbus Plus (which are all RS-485 single cable multi-drop networks), only the node assigned as the Client may initiate a command. All other devices are servers and respond to requests and commands.
For the protocols using Ethernet such as Modbus TCP, any device can send out a Modbus command thus all can act as a client, although normally only one device acts as a Client.
There are many modems and gateways that support Modbus, as it is a very simple and often copied protocol. 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.
Modbus commands can instruct a Modbus device to:
- change the value in one of its registers, that is written to Coil and Holding registers.
- read an I/O port: Read data from a Discrete and Coil ports,
- command the device to send back one or more values contained in its Coil and Holding registers.
A Modbus command contains the Modbus address of the device it is intended for (1 to 247). Only the addressed device will respond and 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.
A Modbus "frame" consists of an Application Data Unit (ADU), which encapsulates a Protocol Data Unit (PDU):
- ADU = Address + PDU + Error check,
- PDU = Function code + Data.
The byte order for values in Modbus data frames is most significant byte of a multi-byte value is sent before the others. All Modbus variants use one of the following frame formats.
Modbus RTU frame format (primarily used on asynchronous serial data lines like RS-485/EIA-485)
|Start||28||At least 3½ character times of silence (mark condition)|
|Function||8||Indicates the function code; e.g., read coils/holding registers|
|Data||n × 8||Data + length will be filled depending on the message type|
|CRC||16||Cyclic redundancy check|
|End||28||At least 3½ character times of silence between frames|
Note about the CRC:
- Polynomial: x16 + x15 + x2 + 1 (CRC-16-ANSI also known as CRC-16-IBM, normal hexadecimal algebraic polynomial being
- Initial value: 65,535.
- Example of frame in hexadecimal:
01 04 02 FF FF B8 80(CRC-16-ANSI calculation from
80B8, which is transmitted least significant byte first).
Modbus ASCII frame format (primarily used on 7- or 8-bit asynchronous serial lines)
|Start||1||Starts with colon |
|Function||2||Indicates the function codes like read coils / inputs|
|Data||n × 2||Data + length will be filled depending on the message type|
|LRC||2||Checksum (Longitudinal redundancy check)|
|End||2||Carriage return – line feed (CR/LF) pair (ASCII values of |
Address, function, data, and LRC are all capital hexadecimal readable pairs of characters representing 8-bit values (0–255). For example, 122 (7 × 16 + 10) will be represented as
LRC is calculated as the sum of 8-bit values (excluding the start and end characters), negated (two's complement) and encoded as an 8-bit value. Example: if address, function, and data encode as 247, 3, 19, 137, 0, and 10, their sum is 416. Two's complement (−416) trimmed to 8 bits is 96 (e.g. 256 × 2 − 416), which will be represented as
60 in hexadecimal. Hence the following frame:
:F7031389000A60<CR><LF>. It is specified for use only as a checksum: because it calculated on the encoded data rather than the transmitted characters, its 'Longitudinal' characteristic is not available for use with parity bits to locate single-bit errors.
|Transaction identifier||2||For synchronization between messages of server and client|
|Protocol identifier||2||0 for Modbus/TCP|
|Length field||2||Number of remaining bytes in this frame|
|Unit identifier||1||Server 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 Server Address of the device behind the gateway. Natively Modbus/TCP-capable devices usually ignore the Unit Identifier.
Available function/command codes
The various reading, writing and other operations are categorized 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.
Prominent entities within a Modbus server are:
- Coils: readable and writable, 1 bit (off/on)
- Discrete Inputs: readable, 1 bit (off/on)
- Input Registers: readable, 16 bits (0 to 65,535), essentially measurements and statuses
- Holding Registers: readable and writable, 16 bits (0 to 65,535), essentially configuration values
|Function type||Function name||Function code||Comment|
|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 Multiple Holding Registers||3|
|Write Single Holding Register||6|
|Write Multiple Holding 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||serial only|
|Get Com Event Counter||11||serial only|
|Get Com Event Log||12||serial only|
|Report Server ID||17||serial only|
|Read Device Identification||43|
|Other||Encapsulated Interface Transport||43|
Format of data of requests and responses for main function codes
Requests and responses follow frame formats described above. This section gives details of data formats of most used function codes.
Function code 1 (read coils) and function code 2 (read discrete inputs)
- Address of first coil/discrete input to read (16-bit)
- Number of coils/discrete inputs to read (16-bit)
- Number of bytes of coil/discrete input values to follow (8-bit)
- Coil/discrete input values (8 coils/discrete inputs per byte)
Value of each coil/discrete input is binary (0 for off, 1 for on). First requested coil/discrete input is stored as least significant bit of first byte in reply.
If number of coils/discrete inputs is not a multiple of 8, most significant bit(s) of last byte will be stuffed with zeros.
For example, if eleven coils are requested, two bytes of values are needed. Suppose states of those successive coils are on, off, on, off, off, on, on, on, off, on, on, then the response will be
02 E5 06 in hexadecimal.
Because the byte count returned in the reply message is only 8 bits wide and the protocol overhead is 5 bytes, a maximum of 2008 (251 x 8) discrete inputs or coils can be read at once.
Function code 5 (force/write single coil)
- Address of coil (16-bit)
- Value to force/write: 0 for off and 65,280 (FF00 in hexadecimal) for on
Normal response: same as request.
Function code 15 (force/write multiple coils)
- Address of first coil to force/write (16-bit)
- Number of coils to force/write (16-bit)
- Number of bytes of coil values to follow (8-bit)
- Coil values (8 coil values per byte)
Value of each coil is binary (0 for off, 1 for on). First requested coil is stored as least significant bit of first byte in request.
If number of coils is not a multiple of 8, most significant bit(s) of last byte should be stuffed with zeros. See example for function codes 1 and 2.
- Address of first coil (16-bit)
- number of coils (16-bit)
Function code 4 (read input registers) and function code 3 (read holding registers)
- Address of first register to read (16-bit)
- Number of registers to read (16-bit)
- Number of bytes of register values to follow (8-bit)
- Register values (16 bits per register)
Because the maximum length of a Modbus PDU is 253 (inferred from the maximum Modbus APU length of 256 on RS485), only up to 125 registers can be requested at once.
Function code 6 (preset/write single holding register)
- Address of holding register to preset/write (16-bit)
- New value of the holding register (16-bit)
Normal response: same as request.
Function code 16 (preset/write multiple holding registers)
- Address of first holding register to preset/write (16-bit)
- Number of holding registers to preset/write (16-bit)
- Number of bytes of register values to follow (8-bit)
- New values of holding registers (16 bits per register)
Because register values are 2-bytes wide and only 127 bytes worth of values can be sent, only 63 holding registers can be preset/written at once.
- Address of first preset/written holding register (16-bit)
- Number of preset/written holding registers (16-bit)
For a normal response, server repeats the function code. Should a server want to report an error, it will reply with the requested function code plus 128 (hex
0x80) (3 becomes 131 = hex
0x83), and will only include one byte of data, known as the exception code.
Main Modbus exception codes
|1||Illegal Function||Function code received in the query is not recognized or allowed by server|
|2||Illegal Data Address||Data address of some or all the required entities are not allowed or do not exist in server|
|3||Illegal Data Value||Value is not accepted by server|
|4||Server Device Failure||Unrecoverable error occurred while server was attempting to perform requested action|
|5||Acknowledge||Server has accepted request and is processing it, but a long duration of time is required. This response is returned to prevent a timeout error from occurring in the client. client can next issue a Poll Program Complete message to determine whether processing is completed|
|6||Server Device Busy||Server is engaged in processing a long-duration command. client should retry later|
|7||Negative Acknowledge||Server cannot perform the programming functions. Client should request diagnostic or error information from server|
|8||Memory Parity Error||Server detected a parity error in memory. Client can retry the request, but service may be required on the server device|
|10||Gateway Path Unavailable||Specialized for Modbus gateways. Indicates a misconfigured gateway|
|11||Gateway Target Device Failed to Respond||Specialized for Modbus gateways. Sent when server fails to respond|
Coil, discrete input, input register, holding register numbers and addresses
The Modbus Organization mentions the following in the Modbus Application Protocol v1.1b:
The Modbus application protocol defines precisely PDU addressing rules.
In a Modbus PDU, each data is addressed from 0 to 65535.
It also defines clearly a MODBUS data model composed of 4 blocks that comprises several elements numbered from 1 to n.
In the Modbus data model, each element within a data block is numbered from 1 to n
Some conventions govern how Modbus entities (coils, discrete inputs, input registers, holding registers) are referenced.
It is important to make a distinction between entity number and entity address:
- Entity numbers combine entity type and entity location within their description table.
- Entity address is the starting address, a 16-bit value in the data part of the Modbus frame. As such its range goes from 0 to 65,535 (0000 to FFFF in the packets)
In the traditional convention, entity numbers start with a single digit representing the entity type, followed by four digits representing the entity location:
- coils numbers start with 0 and span from 00001 to 09999,
- discrete input numbers start with 1 and span from 10001 to 19999,
- input register numbers start with 3 and span from 30001 to 39999,
- holding register numbers start with 4 and span from 40001 to 49999.
For data communications, the entity location (1 to 9,999) is translated into a 0-based entity address (0 to 9,998) by subtracting 1. For example, in order to read holding registers starting at number 40001, the data frame will contain function code 4 (as seen above) and address 0. For holding registers starting at number 40100, address will be 99 etc.
- coil numbers span from 000001 to 065536,
- discrete input numbers span from 100001 to 165536,
- input register numbers span from 300001 to 365536,
- holding register numbers span from 400001 to 465536.
When using the extended referencing, all number references 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 must appear as #040001.
Another way to note the data adresses is to use the hexadecimal value, which clarifies the use of the 4 digits in the traditional convention mentioned previously.
- coil numbers span from 0x0000 to 0xFFFF,
- discrete input numbers span from 1x0000 to 1xFFFF,
- input register numbers span from 3x0000 to 3xFFFF,
- holding register numbers span from 4x0000 to 4xFFFF.
The advantage of this notation is that when user needs to analyze the Modbus packets, the address matches.
Another de facto protocol closely related to Modbus appeared after it, and was defined by PLC brand April Automates, the result of a collaborative effort between French companies Renault Automation and Merlin Gerin et Cie in 1985: JBUS. Differences between Modbus and JBUS at that time (number of entities, server stations) are now irrelevant as this protocol almost disappeared with the April PLC series which AEG Schneider Automation bought in 1994 and then made obsolete. However the name JBUS has survived to some extent.
JBUS supports function codes 1, 2, 3, 4, 5, 6, 15, and 16 and thus all the entities described above. However numbering is different with JBUS:
- Number and address coincide: entity #x has address x in the data frame.
- Consequently, entity number does not include the entity type. For example, holding register #40010 in Modbus will be holding register #9, located at address 9 in JBUS.
- Number 0 (and thus address 0) is not supported. Server should not implement any real data at this number and address and it can return a null value or throw an error when requested.
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
- IEEE floating-point number
- 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 server addresses
- 32-bit data size (1 address = 32 bits of data returned)
- Word-swapped data
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