Protocol Description
Overview
The MODBUS protocol is a widely used communication standard in industrial automation, facilitating master-slave data exchange. It is known for its simplicity and flexibility, making it suitable for a range of applications in connecting intelligent devices.
In MODBUS communication, messages sent from the master to a slave device contain key components: the slave's address, a command (such as "read register" or "write register"), the associated data, and an error-checking field (using LRC or CRC for checksum). As a messaging protocol, MODBUS is independent of the underlying physical layer, meaning it can be implemented over various mediums like RS485 or USB-to-serial interfaces.
USB interface
ADI-524 has a built-in USB to serial converter, you can plug USB cable and start device configuration as there is no need to install any additional drivers on most operating systems. However, if you find that you need to install drivers, you can download driver - .V1.00 driver.zip
The ADI-524 allows operation via only one interface at a time. Either RS485 or USB. DIP switch settings are necessary when using both interfaces
MODBUS RTU Framing
The ADI-524 module supports standard RTU Modbus protocol for getting the input status and setting/getting the relays status. RTU Modbus Protocol frame is illustrated below:
| Start | Address | Function | Data |
RC Check | End |
| ≥ 3.5 Char | 4 Bits |
8 Bits |
N*8 Bits | 16 Bits | ≥ 3.5 Char |
- Start Address: This identifies the specific device or slave on the network. Each device has a unique address to differentiate it from others.
- Function Code: Specifies the type of operation being requested. Common function codes include:
- 0x03: Read Holding Registers
- 0x06: Write Single Register
- 0x10: Write Multiple Registers - Starting Address: The address of the first register from which data will be read or to which data will be written
- Number of Registers: Indicates how many registers should be read or written, starting from the specified address.
- CRC (Cyclic Redundancy Check): This is a checksum used to verify the integrity of the data. It is calculated based on the previous fields and helps detect errors in the transmitted data.
Each part of the request plays an important role in ensuring the correctness of Modbus communication. The start address and function code define the operation, and the CRC checksum protects the accuracy of the message.
MODBUS Command examples
04 function code for reading value from input.
Modbus dear query message
| Command: read analog input 1 | ||
| Byte |
Byte description | Example (HEX) |
| 0 |
Slave ID | 01 |
| 1 |
Function code | 04 |
| 2 |
Starting address High | 00 |
| 3 |
Starting address Low | 00 |
| 4 |
Number of Registers High | 00 |
| 5 |
Number of Registers Low | 02 |
| 6 |
CRC AB | 71 |
| 7 |
CRC CD | CB |
| Start |
Slave address |
Function code 04 |
Starting Number |
CRC ABCD 71 CB |
End |
|||
| High | Low | High | Low | |||||
| 00 |
00 |
00 |
02 |
|||||
Modbus response
| Response: Analog Input - 13.40mA | ||
| Byte |
Byte description | Example (HEX) |
| 0 |
Slave ID | 01 |
| 1 |
Function code | 04 |
| 2 |
Byte Count | 04 |
| 3 |
Data 1 | 41 |
| 4 |
Data 2 |
6C |
| 5 |
Data 3 |
CC |
| 6 |
Data 4 |
CD |
| 7 |
CRC AB | BB |
| 8 |
CRC CD | 30 |
The number in 32-bit floating point HEX format 0x416CCCCD in decimal is: 13.40
| Start |
Slave address |
Function code 04 |
Number of bytes | Data | Error check
|
End |
|||
| First register |
Second register |
||||||||
| Data 1 |
Data 2 |
Data 3 |
Data 4 |
||||||