Modbus

thumb|Logo of Modbus-IDA, the older name of the industry consortium

Modbus is a fieldbus, an industrial computer networking protocol. It was originally designed for use with programmable logic controllers (PLCs), but has become a de facto standard communication protocol for communication between industrial electronic devices in a wide range of buses and networks.

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 on the format of the data to be transmitted.

The Modbus protocol uses serial communication lines, Ethernet, or the Internet protocol suite as a transport layer. 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, via Modbus.

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. 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-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.

It was originally published in 1979 by Modicon (bought by Schneider Electric in 1997). In 2004, they transferred the rights to the Modbus Organization which is a trade association of users and suppliers of Modbus-compliant devices that advocates for the continued use of the technology.

Protocol description

thumb|alt=MODBUS communication stack|MODBUS communication stack

Modbus standards or buses include:

TCP/IP over Ethernet
Asynchronous serial communication in a wide range of standards, technologies: EIA/TIA-232-E, EIA-422, EIA/TIA-485-A, fiber, radio frequency,...
MODBUS PLUS, a high speed token passing network.

thumb|Architecture of a network for Modbus communication

To support Modbus communication on a network, many modems and gateways incorporate proprietary designs (refer to the diagram: Architecture of a network for Modbus communication). Implementations may deploy either wireline or wireless communication, such as in the ISM radio band, and even Short Message Service (SMS) or General Packet Radio Service (GPRS).

PDU and ADU

Modbus defines a client which is an entity that initiates a transaction to request any specific task from its request receiver. The client's "request receiver", which the client has initiated the transaction with, is then called the server. For example, when a microcontroller connects to a sensor to read its data by Modbus on a wired network, e.g RS485 bus, the MCU in this context is the client and the sensor is the server. In former terminology, the client was named master and the server named slave.

Modbus defines a protocol data unit (PDU) independently to its lower layer protocols in its protocol stack. Mapping MODBUS protocol on specific buses or networks requires some additional fields, defined as the application data unit (ADU). The ADU is formed by a client inside a Modbus network when the client initiates a transaction. Contents are:

PDU = Function code + data
ADU = Additional address + PDU + error check

The ADU is officially called a Modbus frame by the Modbus Organization, although frame is used as the data unit in the data-link layer in the OSI and TCP/IP model (while Modbus is an application layer protocol).

PDU max size is 253 bytes. ADU max size on RS232/RS485 network is 256 bytes, and with TCP is 260 bytes.

For data encoding, Modbus uses a big-endian representation for addresses and data fields. Thus, for a 16-bit value, the most significant byte is sent first. For example, when a 16-bit register has value 0x1234, byte 0x12 is sent before byte 0x34.

Function code is 1 byte which gives the code of the function to execute. Function codes are integer values, ranging from 1 to 255, and the range from 128 to 255 is for exception responses.

The data field of the PDU has the address from 0 to 65535 (not to be confused with the address of the Additional address field of ADU). The data field of the PDU can be empty, and then has a size of 0. In this case, the server will not request any information and the function code defines the function to be executed. If there is no error during the execution process, the data field of the ADU response from server to client will include the data requested, i.e. the data the client previously received. If there is any error, the server will respond with an exception code.

Modbus transaction and PDU

A Modbus transaction between client and server includes:

Step 1: Client initiates a request with PDU = Function code + data request
Step 2: Server receives the request from client. Server will then read/parse the function code, get the address of the data field of the PDU, then get this data field value and finally perform the action based on the function code. If there is no error during those steps, the server will respond with PDU = Function code + data response. As long as there is no error during those steps, the server's responding function code will also be the function code sent from the client. If there is any error during those steps, the server will respond with PDU = Exception Function code + Exception code (Reference to PDU mb_excep_rsp_pdu defined below).
Step 3: Client receives the response and ends the transaction.

Based on that, Modbus defines 3 PDU types:

MODBUS Request PDU, mb_req_pdu
MODBUS Response PDU, mb_rsp_pdu
MODBUS Exception Response PDU, mb_excep_rsp_pdu

:mb_req_pdu = Function code (1 byte) + request data (n bytes)

::request data field's size depends on the function code and usually includes values like variable values, data offset, and sub-function codes.

:mb_rsp_pdu = Function code (1 byte) + response data (n bytes)

::As in mb_req_pdu, response data field's size depends on the function code and usually includes values like variable values, data offset, and sub-function codes.

:mb_excep_rsp_pdu = Exception Function code (1 byte) + exception code (1 byte)

::Exception Function code = Function code (1 byte) + 0x80. Exception Function code is equal to the Function code, except that its MSB is set to 1.

::Exception code (1 byte) of mb_excep_rsp_pdu is defined in the MODBUS Exception Codes table.

Modbus data model

Modbus defines its data model based on a series of tables of four primary types:

{| class="wikitable"

! Primary tables

! Access

! Size

!Features

| Discrete input

| R

|1 bit (0–1)

|Read on/off value

| Coil (discrete output)

| R/W

| 1 bit (0–1)

|Read/Write on/off value

|Input register

|16 bit words (0–65,535)

|Read measurements and statuses

|Holding register

|R/W

|16 bit words (0–65,535)

|Read/Write configuration values

For each of the primary tables, the protocol allows individual selection of 65536 data items,

and the operations of read or write of those items are designed to span multiple consecutive

data items up to a data size limit which is dependent on the transaction function code.

Function code

Modbus defines three types of function codes: Public, User-Defined and Reserved.

Public function codes

{| class="wikitable"

! colspan="3" |Function type||Function name||Function code

!Comment

| rowspan="13" |Data Access|| rowspan="4" |Bit access||Physical Discrete Inputs||Read Discrete Inputs||2

| rowspan="3" |Internal Bits or Physical Coils||Read Coils||1

|Write Single Coil||5

|Write Multiple Coils||15

| rowspan="7" |16-bit access||Physical Input Registers||Read Input Registers||4

| rowspan="6" |Internal Registers or Physical Output Registers||Read Multiple Holding Registers||3

|Write Single Holding Register||6

|Write Multiple Holding Registers||16

| > 1

|Read/Write Multiple Registers||23

|Mask Write Register||22

|Read FIFO Queue||24

| colspan="2" rowspan="2" |File Record Access||Read File Record||20

|Write File Record||21

| colspan="3" rowspan="6" |Diagnostics||Read Exception Status||7

|serial only

|Diagnostic||8

|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

| colspan="3" |Other||Encapsulated Interface Transport||43

Note: Some sources use terminology that differs from the standard; for example Force Single Coil instead of Write Single Coil.

Function code 01 (read coils) as an example of public function code

Function code 01 (read coils) allows reading the state from 1 to 2000 coils of a remote device. mb_req_pdu (request PDU) will then have 2 bytes to indicate the address of the first coil to read (from 0x0000 to 0xFFFF), and 2 bytes to indicate the number of coils to read. mb_req_pdu defines coil address by index 0, i.e the first coil has address 0x0. On a successful execution, mb_rsp_pdu will return one byte to note the function code (0x01), followed by one byte to indicate the number of data bytes it is returning (n), which will be the number of coils requested by mb_req_pdu, divided by 8 bits per byte, and rounded up. The remainder of the response will be the specified number (n) of data bytes. That is, the mb_req_pdu and mb_rsp_pdu of function code 01 will take the following form:

:mb_req_pdu:

:* Function code: 0x01 (1 byte)

:* Starting Address (1st coil address to read): From 0x0000 to 0xFFFF (2 bytes)

:* Quantity of coils to read: Range from 1 to 2000 (0x7D0) (2 bytes)

:mb_rsp_pdu:

:* Function code: 0x01 (1 byte)

:* Byte count: 1 byte (n=quantity of coils/8, rounded up)

:* Coil Status: n bytes

For instance, mb_req_pdu and mb_rsp_pdu to read coils status from 20-38 will be:

:mb_req_pdu:

:* Function code: 0x01

:* Starting Address High byte: 0x00

:* Starting Address Low byte: 0x13

:* Quantity of Outputs High byte: 0x00

:* Quantity of Outputs Low byte: 0x13

::Starting Address (2 bytes) is 0x0013, (or 19 in decimal) which is the 20th coil.

::Quantity of Outputs (2 bytes) is 0x0013, (or 19 in decimal) which corresponds to 19 values of status of coils 20th to 38th.

:mb_rsp_pdu:

:* Function code: 0x01

:* Byte Count: 0x03

:* Outputs status 27-20: 0xCD

:* Outputs status 35-28: 0x6B

:* Outputs status 38-36: 0x05

::As 19 coils (20-38) are required, 3 bytes is used to indicate the coil's state. So that Byte Count is 0x03. States of coil from 20 to 27 is 0xCD, which is 1100 1101 in binary. So coil 27 is MSb, and coil 20 is LSb. Same for coil 28 to 35. With coil from 36 to 38, the state will be 0x05, which is 0000 0101. Coil 38 state is the 3rd bit (count from the right), i.e 1, coil 37 is 0, and coil 36 state is LSb bit, i.e. 1. 5 left bits are all 0.

User-defined function codes

User-Defined Function Codes are function codes defined by users. Modbus gives two range of values for user-defined function codes: 65 to 72 and 100 to 110. Obviously, user-defined function codes are not unique.

Reserved function codes

Reserved Function Codes are function codes used by some companies for legacy product and are not available for public use.

Exception responses

When a client sends a request to a server, there can be four possible events for that request:

If server receives the request and execute successfully, server will return a normal response.
If server cannot receive the request as having communication channel error, server will not respond anything to the client. Client will then have the timeout request error.
If server receives the request and detect an error on the communication channel (e.g parity, LRC, CRC), server will not response anything to the client. Client will then have the timeout request error.
If server receives the request and is unable to execute it (e.g client requests to read a non-existent register), server will return an exception response to client to indicate the nature of the error.

Exception response message includes two other fields when compared to a normal response message:

Function Code: Function code's MSB bit of Exception is 1. This will make this function code 0x80 higher than then request message function code.
Data: Server returns the exception code inside the Data field. This field defines the nature of the error.

All Modbus exception code:

{| class="wikitable"

! Code !! Text !! Details

| 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

| 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

Modbus over Serial Line protocol

Modbus standard also defines Modbus over Serial Line, a protocol over the data link layer of the OSI model for the Modbus application layer protocol to be communicated over a serial bus. Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. With Modbus protocol on the application layer, client/server model is used for the devices on the communication channel. With Modbus over Serial Line, client role is implemented by master, and the server role is implemented by slave.

The ADU for Modbus TCP is officially called Modbus TCP/IP ADU by the Modbus organization and is also called Modbus TCP frame by other parties.

Other Modbus protocol versions

Besides the widely used Modbus RTU, Modbus ASCII and Modbus TCP, there are many variants of Modbus protocols:

Modbus Plus (Modbus+, MB+, or MBP) – Modbus Plus is proprietary to Schneider Electric, though it is unpublished rather than patented, and unlike the other variants, it supports peer-to-peer communications between multiple clients. Despite the name, Modbus Plus is not a variant of Modbus. It is a different protocol, involving token passing. 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. Modbus Plus is normally implemented using a custom chipset available only to partners of Schneider.
Pemex Modbus – 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 – another extension of standard Modbus developed by Enron 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 models and function calls are identical for the first four variants listed above; only the encapsulation is different. However the variants are not interoperable, nor are the frame formats.

JBUS mapping

Another de facto protocol closely related to Modbus appeared later, and was defined by PLC maker 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, although numbering is different:

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, at address 9 in JBUS.
Number 0 (and thus address 0) is not supported. The 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.

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 learn that 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 get data by the event handler mechanism (except over Ethernet TCP/IP, called open-mbus) as 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 (again, Ethernet TCP/IP is an exception).
Modbus protocol itself provides no security against unauthorized commands or interception of data.

Protocol description

PDU and ADU

Modbus transaction and PDU

Modbus data model

Function code

Public function codes

Function code 01 (read coils) as an example of public function code

User-defined function codes

Reserved function codes

Exception responses

Modbus over Serial Line protocol

Other Modbus protocol versions

JBUS mapping

Limitations

See also

References

Works cited

External links

Official

Other