Modbus RTU Driver

dc3-driver-modbus-rtu connects Modbus RTU slave devices to IoT DC3 — it reaches slaves over a serial port, periodically reads coil/register values, and supports commands that write values to coils and holding registers.

Modbus RTU is the serial-link variant of the Modbus protocol, running over RS-485/RS-232 buses, and is one of the most common protocols on the plant floor (heavily used by PLCs, energy meters, VFDs, and sensors). It shares the exact same function codes and address model as the Modbus TCP Driver; the only difference is the physical layer: RTU runs over a serial port (baud rate, data bits, parity, etc.) rather than IP/port. This driver acts as the Modbus master, connecting through one serial port to one or more slaves on the same bus, then reads and writes values according to the function code and address configured on each Point.

• Driver name / code: Modbus RTU Driver / ModbusRtuDriver
• Type: DRIVER_CLIENT (actively connects to slaves)

Driver configuration (device-level driver-attribute)

When onboarding a Modbus RTU device, fill in these attributes on the Device. These are serial-port parameters and must match the slave's serial settings exactly, or the connection will fail:

Attribute	code	Type	Default	Description
Port	port	STRING	/dev/ttyUSB0	Serial port name (e.g. /dev/ttyUSB0, COM3)
Baud Rate	baudRate	INT	9600	Serial baud rate (e.g. 9600, 19200, 115200)
Data Bits	dataBits	INT	8	Data bits (7 or 8)
Stop Bits	stopBits	INT	1	Stop bits (1 or 2)
Parity	parity	INT	0	Parity (0=None, 1=Odd, 2=Even, 3=Mark, 4=Space)

Point configuration (point-attribute)

Fill in these on each Point being collected:

Attribute	code	Type	Default	Description
Slave ID	slaveId	INT	1	Modbus slave unit ID
Function Code	functionCode	INT	1	Read function code [1, 2, 3, 4]
Offset	offset	INT	0	Register/coil address offset

The function code decides what gets read

Reading uses 01 (coil) / 02 (discrete input) / 03 (holding register) / 04 (input register). A Point's data type (Point's pointTypeFlag) must match the data width returned by the function code — the driver picks the register width from the Point type: LONG→4-byte int, FLOAT→4-byte float, DOUBLE→8-byte float, everything else 2-byte int.

Write command configuration (command-attribute)

Writable Points also need these on the write command:

Attribute	code	Type	Default	Description
Slave ID	slaveId	INT	1	Modbus slave unit ID
Function Code	functionCode	INT	6	Write function code [5, 6, 15, 16]
Offset	offset	INT	0	Register/coil address offset
Value Template	valueTemplate	STRING	${value}	Value template rendered with command params

Collection & health

• Collection cycle: default cron 0/30 * * * * ? (one read every 30 seconds).
• Health / online: device health check default cron 0/15 * * * * ?, lease timeout 45 seconds — see Device for the online-state mechanism.

Minimal onboarding example

Onboard one Modbus RTU slave on /dev/ttyUSB0, baud rate 9600, unit ID 1:

1. Create a Device with Modbus RTU Driver, and set driver attributes port=/dev/ttyUSB0, baudRate=9600, dataBits=8, stopBits=1, parity=0.
2. On the Profile bound to the device, add a temperature Point (pointTypeFlag=FLOAT, READ_ONLY), with point attributes slaveId=1, functionCode=3, offset=0.
3. Start the driver; within 30 seconds the collected value shows up in PointValue.

Common pitfalls

One wrong serial parameter and nothing connects

All five of port, baudRate, dataBits, stopBits, parity must match the slave's serial settings one by one (unlike TCP, RTU has no handshake negotiation — a mismatch just yields garbage or timeouts). After 3 consecutive connection failures the driver enters a 60-second backoff and reports the device offline throughout; once you fix the parameters or cabling, you must wait out the backoff before it reconnects.

Multiple slaves on one bus share a single serial port

An RS-485 bus can carry multiple slaves; they share the same port and are distinguished by the Point's slaveId. The driver maintains one dedicated serial connection per Device, keyed by device, so give every slave on the same physical bus the same port and address them via slaveId — do not assign a different serial port per slave.

Write commands only support coils and holding registers

The write function code defaults to 6, but the driver actually handles only two write targets: 01 (write coil) and 03 (write holding register). Codes like 15/16 are ignored and the write fails. As with reads, offset is the protocol-level 0-based offset, not the 40001-style convention.

Further reading

• Driver — the generic driver model and registration mechanism
• Attributes & config — where attributes like port / slaveId come from across the three layers
• Device onboarding — a full onboarding walkthrough
• Modbus TCP Driver — the Ethernet flavor of Modbus