Modbus RTU Protocol

Component Type: Protocol

Status: Implemented

Overview

CProtoModbusRtu is a Modbus RTU slave protocol object that maps FreeModbus callbacks to Dawn IO.

Implementation

Supported register group types: MODBUS_TYPE_COIL, MODBUS_TYPE_COIL_PACKED, MODBUS_TYPE_DISCRETE, MODBUS_TYPE_DISCRETE_PACKED, MODBUS_TYPE_INPUT, MODBUS_TYPE_HOLDING.
Overlap in the same register class is rejected at configuration (assert + error return).
MODBUS_TYPE_COIL_PACKED uses Modbus bit encoding for bool IO: 0 -> bit 0, nonzero -> bit 1.
MODBUS_TYPE_INPUT and MODBUS_TYPE_HOLDING support mixed-width IO (16/32/64-bit) inside one group.
Register shadow buffer is allocated only for notify groups. Non-notify groups read directly from IO.

Register Management

The mapping between Dawn IO objects and Modbus registers is handled by the CProtoModbusRegs base class. This logic is shared across all Modbus transport implementations (RTU, TCP).

Configuration

Kconfig

CONFIG_DAWN_PROTO_MODBUS: enables Modbus protocol support.
CONFIG_DAWN_PROTO_MODBUS_RTU: enables the RTU transport.
CONFIG_DAWN_PROTO_MODBUS_RTU_PATH: default RTU device path.
CONFIG_DAWN_PROTO_MODBUS_RTU_BAUD: default RTU baudrate.
CONFIG_DAWN_PROTO_MODBUS_RTU_ADDR: default Modbus slave address.
CONFIG_DAWN_PROTO_MODBUS_RTU_PARITY: default RTU parity mode.

YAML

protocols:
  - id: modbus1
    type: modbus_rtu
    config:
      path: "/dev/ttyS1"
      registers:
        - type: coil
          notify: 0
          start: 0
          bindings:
            - coil1
            - coil2
        - type: discrete_packed
          notify: 1
          start: 100
          bindings:
            - di1
            - di2
            - di3
        - type: holding
          notify: 0
          start: 1000
          bindings:
            - holding1
        - type: input
          notify: 0
          start: 2000
          bindings:
            - input1

Supported fields:

config.path: serial device path.
config.registers: register group definitions.

Supported registers[*].type values:

coil
discrete
coil_packed
discrete_packed
input
holding

Supported registers[*] fields:

type: register group type.
notify: enable change notification shadowing for the group.
start: first register or bit address.
bindings: IOs mapped into the group.

External Control

ControlIO: supported.

CProtoModbusRtu supports runtime start/stop control through CIOControl. When stopped, Modbus polling/service loop is inactive. When started again, register servicing resumes.

TriggerIO: not supported.

Brainstorming & Future Ideas

Only one Modbus instance can be active at a time (global FreeModbus callback limitation).
Write NuttX Modbus support from scratch with multiple instances support and ready for Modbus TCP