When you need to use Modbus on a RS232 port, there are a number of things that you need to know about the protocol. These include the application header and function codes. In addition, you will want to understand the unit identifiers and test messages.
Application header
Modbus is a network protocol used in industrial applications. It uses the Internet protocol suite as the transport layer. This makes it simple to deploy and maintain. It is also used in edge computing and mission-critical systems.
The protocol supports a variety of networks and communication types. For example, it uses Ethernet as the transport layer, but it can also support serial lines. When using an RTU, a message is simplified into a Protocol Data Unit (PDU).
PDUs are broken into two basic components. One is the data and the other is the function code. A request includes an Application Data Unit (ADU). In addition to the ADU, it also contains error checking information.
Each ADU is a set of instructions that enables a Modbus device to send a message. Those instructions include a protocol identifier, a unit identifier, an offset from the start address, and a cyclic redundancy check.
A cyclic redundancy check is calculated before a message is sent. It is designed to detect accidental changes to data. Depending on the way it is performed, the contents of the error check field vary.
Modbus messages follow the same basic structure. The first byte is the Slave Address, which identifies the instrument. Next is the Function Code, which specifies what type of message the slave wants to send.
The last byte contains the Error Check Characters. These are a result of the Longitudinal Redundancy Check. Depending on the method the master is using, the length of the error check characters may vary.
A Modbus device can be programmed to send data in either ASCII or RTU. There are also a number of options for endianness.
Function code
Modbus is an industrial protocol that enables communication between a PLC or controller and an operator at a remote location. It uses a command-response protocol, a set of rules and a 7-byte header.
Each Modbus packet contains a function code and data. Function codes are user-defined or public codes, and can be used to access a wide variety of data models. Depending on the type of data model, function codes can be used to read or write. Some function codes are more specialized, while others are more general. The Modbus standard allows for the development of additional function codes.
In addition to the function code, each Modbus message includes a CRC, or cyclic redundancy check. A CRC is a two-byte checksum value that is a part of the Modbus protocol.
A function code specifies the type of message that is sent, and its format. A common example of a function code is the MRX instruction, which is a common method of writing a single register to a Modbus device. Another is the Write Single Register instruction, which writes a single register to a PLC.
Modbus function codes are defined in the Modbus PDU specification. These function codes can be implemented over several different network protocols. Most commonly, serial and TCP/IP protocols are used.
When sending data over the Modbus network, masters initiate a request by sending a command packet to a slave. Slaves then respond to the master with a response packet, which contains any data that the master has requested.
When a master sends a request, the message includes a function code and a data address. This data address is a unique 8-bit address that identifies an instrument or device.
Data
Modbus is a protocol that enables two or more devices to communicate over a serial channel. The data is sent in blocks of eight bits called bytes. Each byte contains information about the device that is sending the data.
Depending on the type of network and device, additional fields may be included in the application data unit. For example, a function code contains information about what the action to be performed is.
To make an accurate comparison of decoded frames with data, a decoder needs to have a large enough memory length. Decoding a single frame also requires enough sampling resolution.
A Modbus command can be used to record several analog output values. This information is stored in a holding register. This register can be in hexadecimal or BCD format.
A Modbus command also includes checksum information. Checksums help the receiver detect errors. If a byte is missing or the correct bit is not included in the message, an error will be generated.
When a request is received, the receiving device calculates a CRC (Cyclic Redundancy Check) for each byte in the message. Each byte is also compared against the CRC.
In addition to the bytes, the Modbus data model also contains a byte of data known as an exception code. This byte is usually not present in certain requests.
Modbus also uses a hexadecimal format for the data. The hexadecimal value helps to clarify the four digits in a conventional convention.
The Application Data Unit consists of a set of elements, each numbered from 1 to n. These elements can include discrete and register addresses, as well as other data fields.
Some devices allow the user to define the map that they want. Other devices are built with a predetermined map from the manufacturer.
Unit identifier
When using Modbus over a serial line, it’s important to understand the unit identifier. This is a number that enables the gateway to distinguish between slaves with the same IP address. It is also a way to differentiate between Modbus and TCP devices.
A Modbus packet consists of an application header, a function code, and one or more data packets. Depending on the device, it may allow you to set other parameters such as transmit delays and byte-to-byte timeouts.
The data packet contains a message, a function code, and data from the Modbus device. The CRC (checksum) helps verify the amount of bytes that are being sent. In general, you’ll be reading data in 16-bit integers and registers.
If you’re using Modbus over a serial line, the most common way to read or write data is through a 16-bit input register. You can read and write to a single register, or you can read and write to a pair of registers.
There are other data types, such as the discrete input and the discrete output. Discrete inputs are 1-bit. Discrete outputs are typically read-write. Some Modbus devices have several bits for each type. For example, a single bit for contact is known as a contact, while a coil is associated with a relay output.
The unit identifier is an important part of the protocol, but it’s often not taken advantage of by native Modbus/TCP-compatible devices. For instance, when using a Modbus bridge, the unit identifier is the Server Address of the device behind the gateway.
As a rule of thumb, the unit identifier is the most significant byte in a Modbus data packet. Usually, the most significant byte is the first byte to be received, but if more than one byte specifies an item, the most significant byte is the first to be transmitted.
Test messages
The Modbus standard is a serial communication protocol that uses Ethernet TCP / IP networks. It has been widely used in process control systems.
The Modbus protocol is built on a master-slave architecture. In this type of architecture, the host initiates an exchange with a slave.
A Modbus message consists of a function code, a Slave Address, and a data packet. The function code is one byte and specifies the type of message to be sent. There is also an error check field. This is an optional field that may be added to the data packet.
The Slave Address identifies the instrument, and the Data Field is an optional field that may contain additional information about the action defined by the function code. Depending on the type of bus, the data field may also include actual data bytes.
Each data packet begins with the device address and then includes a parameter. The parameter may be a discrete or register address.
The Application Data Unit is the part of the Modbus message that contains the actual data. Depending on the type of network, the data unit may also include other fields. These fields may include the number of items to be handled, the quantity of a certain item, and the discrete or register address.
The Modbus protocol can be used over Ethernet TCP/IP, serial, and UDP networks. The most common protocols are the TCP/IP and the serial protocols.
In order to send messages to a Modbus slave, a master must first request diagnostic information from the slave. Typically, a slave will respond to a master’s request by indicating that the request was successful. If an error occurs, a “oops” message is sent.
