Overview
The modbus-rtu node provides serial communication with Modbus RTU devices
over RS-485 or RS-232 interfaces. It supports master and slave modes with configurable baud rates, parity,
and CRC-16 error checking for reliable data exchange with industrial sensors, actuators, and controllers.
Serial Interface Wiring
RS-485 (2-Wire)
RS-232 (Point-to-Point)
Properties
| Property | Type | Required | Default | Description |
|---|---|---|---|---|
| port | string | Yes | - | Serial port path (e.g., /dev/ttyUSB0, COM3) |
| baudRate | number | No | 9600 | Serial baud rate (9600, 19200, 38400, 57600, 115200) |
| dataBits | number | No | 8 | Data bits per frame (7 or 8) |
| parity | string | No | "none" | "none", "even", or "odd" |
| stopBits | number | No | 1 | Stop bits (1 or 2) |
| slaveId | number | Yes | - | Slave device address (1-247) |
| function | string | Yes | - | Modbus function code (e.g., "readInputRegisters") |
| timeout | number | No | 3000 | Response timeout in milliseconds |
Inputs
{
"payload": {
"function": "readInputRegisters",
"address": 30001,
"quantity": 4
}
}{
"payload": {
"function": "writeSingleCoil",
"address": 1,
"value": true
}
}Outputs
{
"payload": {
"slaveId": 1,
"function": "readInputRegisters",
"address": 30001,
"values": [2450],
"crc": "valid"
},
"port": "/dev/ttyUSB0",
"baudRate": 9600
}{
"error": {
"code": "0x02",
"message": "Illegal data address",
"slaveId": 1
}
}Use Cases
Sensor Networks
Connect temperature, humidity, and pressure transmitters on RS-485 multidrop networks spanning hundreds of meters.
Motor Drives
Control variable frequency drives (VFDs) and servo controllers via serial Modbus for speed and torque adjustments.
Legacy Equipment
Interface with older PLCs and industrial equipment that only support RS-485/RS-232 serial communication.
Remote IO
Read and control remote I/O modules distributed across a facility using daisy-chained RS-485 wiring.
Example: RS-485 Sensor Network
Read temperature from multiple sensors on an RS-485 bus.
[
{
"id": "rtu-sensor-1",
"type": "modbus-rtu",
"port": "/dev/ttyUSB0",
"baudRate": 9600,
"parity": "none",
"slaveId": 1,
"function": "readInputRegisters",
"address": 0,
"quantity": 2
},
{
"id": "rtu-sensor-2",
"type": "modbus-rtu",
"port": "/dev/ttyUSB0",
"baudRate": 9600,
"parity": "none",
"slaveId": 2,
"function": "readInputRegisters",
"address": 0,
"quantity": 2
}
]Tips & Best Practices
Line Termination: Add 120-ohm termination resistors at both ends of the RS-485 bus to prevent signal reflections.
Biasing Resistors: Use pull-up (A) and pull-down (B) biasing resistors to define bus idle state and prevent floating.
Turnaround Delay: Allow sufficient turnaround time (3.5 character times) between request and response for reliable communication.
USB Adapters: Use quality USB-to-RS-485 adapters (FTDI or CH340 chipset). Avoid generic adapters that may cause timing issues.