Modbus is a widely used communication protocol in industrial automation systems. It supports various transmission modes such as ASCII, RTU, and TCP, but does not define the physical layer. The protocol specifies message structures that controllers can recognize and use regardless of the network they are communicating over. Originally implemented by Modicon controllers using RS232C for serial communication, Modbus has since evolved to support multiple physical interfaces like RS-232, RS-485, and even wireless and fiber-based media.
The protocol operates in a master-slave architecture, where the master initiates requests and the slave responds. This allows for bidirectional data exchange—both reading and writing. Modbus is known for its simplicity, openness, and standardization, making it a popular choice across industries. Over 400 manufacturers support Modbus, with more than 600 products incorporating the protocol.
One of the key features of Modbus is its simple and compact frame format, which makes it easy to implement and maintain. This ease of use has contributed to its widespread adoption in both legacy and modern control systems.
**Function Code 02: Read Input Status**
This function code is used to read the status of one or more digital input points. For example, a host might request the status of inputs DI1 through DI4 at address 01. The communication involves a request from the master and a response from the slave, containing the current state of the inputs.
**Function Code 01: Read Output Status**
Used to read the status of digital outputs, this function allows the master to query the current state of outputs like DO1 and DO2. The response from the slave provides the real-time status of these outputs.
**Function Code 03: Read Multiple Registers**
This function is used to read multiple 16-bit registers from a device. For instance, a host may request data from three registers starting at address 0116. The response includes the values stored in those registers, allowing the master to access multiple data points efficiently.
**Function Code 05: Write Single Coil (Remote Control)**
This function allows the master to control a single digital output. For example, the host can send commands like "FF00" to close a relay or "0000" to open it. The slave responds with a confirmation that the command was executed successfully.
**Function Code 06: Write Single Register**
This function enables the master to write a single 16-bit value to a register on the slave device. For example, the host could write the value 07D0 to a specific register. The slave confirms the successful write, ensuring accurate data updates.
**Function Code 10: Write Multiple Registers**
This function allows the master to write multiple 16-bit registers at once. The maximum number of registers that can be written in one transaction is 60, making it efficient for bulk data updates. The slave confirms the operation, ensuring all data is correctly stored.
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