Modbus RTU
Modbus RTU
The Modbus RTU driver is one of the oldest and most widely adopted standards in industrial automation. It is used to communicate with devices that operate over serial communication using the RS485 or RS232 physical layer (Drives, Energy Analyzers, Sensors, PLCs, etc.).
Connection Settings
When you select the protocol name in the tree view (e.g., [1] Network), these settings define the properties of your computer’s serial port.
Warning: The settings here (Baudrate, Parity, Stop Bit) must exactly match the configuration of the field device. Otherwise, communication cannot be established.
| Parameter | Description | Example |
| Serial Port | The physical COM port to which the PLC is connected on your PC. (Check via Device Manager.) | COM1 |
| Baudrate | Communication speed (e.g., 9600, 19200, 115200). Incorrect selection prevents communication. | 115200 |
| Parity | Parity bit control (None, Odd, Even). | None |
| Stop Bits | Number of stop bits (One, Two). | One |
| Data Bits | Number of data bits (usually 8). | 8 |
| Delay (ms) | Waiting time between two requests to prevent collisions on RS485 lines. | 100 |
| Timeout (ms) | May need to be increased based on cable length and baudrate. | 250 |
| Retry Count | Number of retries before the system enters the Fault state in case of communication errors. | 3 |
Device Settings
This panel appears when you select Device in the tree view. It defines a specific physical device (Slave) on the RS485 line and how data exchange with it will be handled.
| Parameter | Description |
| Address | Station Number (Slave ID). Each device on the line must have a unique ID. |
| Symbol | Symbolic name assigned to the device (e.g., Device). |
| Enable | Determines whether the device is active. You can define conditions to dynamically enable/disable communication. Multiple conditions can be added; if any condition is true, the device becomes active. Leave blank to keep the device always active. |
| Skip After Failure | Checked: If the device does not respond (Timeout), the system stops querying the device and moves to the next one.Unchecked: The system continues attempting to read each tag of the device even after an error. (Recommended to check this to prevent system slowdown if a device on the RS485 line fails.) |
| Byte Ordering | Byte order (Endianness) for merging 32-bit values (Real/Float, DInt). If the value appears incorrect or unrealistic, try different modes (LittleEndian, BigEndian, Swap Bytes, etc.). |
| Use Func. Code 5 | Checked: FC 05 (Write Single Coil) is used for bit writes. Unchecked: FC 15 (Write Multiple Coils) is used. (Some older devices do not support FC 15.) |
| Use Func. Code 6 | Checked: FC 06 (Write Single Register). Unchecked: FC 16 (Write Multiple Registers). |
| Discrete Max. Block | Maximum number of bits (Coils/Inputs) to read in a single request. Default 2040 is ideal. Can be reduced if addresses are highly scattered. |
| Analog Max. Block | Maximum number of words (Registers) to read in a single request. Modbus standard allows up to 125. |
| Connected | Selected bit becomes True when the device is successfully connected. |
| Fault | Selected bit becomes True when a communication error occurs. |
| Comment | A descriptive note for the device. |
RS485 Performance Management
Modbus RTU (Serial Communication) is slower and more sensitive compared to Ethernet. To maximize system performance:
- Enable “Skip After Failure”
If a device loses power or its cable is disconnected, SCADA will not waste time trying to query it. Other devices (e.g., Device 2, Device 3) continue running without slowdown.
- Use the Enable Feature
If your network has 20 devices but the operator is viewing only Device 1, disable the other 19 devices using the Enable property.
This increases the data update speed of Device 1 by up to 20×.
Addressing Table
Wise SCADA uses standard Modbus addressing notation.
| Address Range | Data Type | Description | Function Codes (FC) |
| 000001 – 065536 | Discrete Output | Coils (Bit): Read/write digital outputs/relays. | Read: 01 — Write: 05, 15 |
| 100001 – 165536 | Discrete Input | Contacts (Bit): Read-only digital inputs. | Read: 02 |
| 300001 – 365536 | Analog Input | Input Registers (Word): Read-only 16-bit analog values. | Read: 04 |
| 400001 – 465536 | Analog Output | Holding Registers (Word): Read/write 16-bit memory areas. The most commonly used type. | Read: 03 — Write: 06, 16 |