ModbusMaster

Asynchronous Modbus-RTU library for mbedOS 6.

This project is inspired by the Arduino ModbusMaster library by 4-20ma.

NOTE: Previous mbed 5 version can be found on the mbed5 branch.

Example

Read registers:

#include <mbed.h>
#include <ModbusMaster.h>

typedef ModbusMaster<16, 32> MBM;

EventQueue queue;
MBM modbus(&queue, PA_2, PA_3, 115200, 1);

int main() {
    modbus.readHoldingRegisters(1, 4, [](auto res){
        if(res != MBM::Result::success) {
            // Handle error
        } else {
            uint16_t* reg = modbus.getRegisters();
            reg[0]; // => register 1 value
            reg[1]; // => register 2 value
            reg[2]; // => register 3 value
            reg[3]; // => register 4 value
        }
    });
    queue.dispatch_forever();
    return 0;
}

API

ModbusMaster<>()

Create a new ModbusMaster instance.

Template args:

• size_t txBufSize Transmit buffer size
• size_t rxBufSize Receive buffer size

Constructor args:

• EventQueue* queue EventQueue used for processing response off IRQ context
• PinName txPin Target TX pin
• PinName rxPin Target RX pin
• int baud: Serial baud rate (also used for Modbus frame delimiting)
• uin8_t slaveId: Modbus slave ID
• std::chrono::milliseconds rxTimeout = 50ms: Modbus response timeout

void attachPreTransmit(Callback<void()> f)

Attach callback to be called before transmit. Can be used to toggle RS485 direction.

void attachPostTransmit(Callback<void()> f)

Attach callback to be called after transmit. Can be used to toggle RS485 direction.

void attachPostReceive(Callback<uint16_t(uint8_t* data, uint16_t len)> f)

Attach callback to be called after receive frame, before CRC calculation. (Can be used to modify received frame to workaround slave device quirks)

Callback function must return the buffer length.

void setSlaveId(uint8_t id)

Change modbus slave ID for subsiquent requests.

void setTimeout(std::chrono::milliseconds t)

Change receive timeout for subsiquent requests.

void setCrcCheck(bool c)

Change whether CRC check is performed on the received response. (Can be used for slave devices with incorrect CRCs)

uint8_t* getCoils()

Get coils result after readCoils or readDiscreteInputs. First result is on first byte LSB.

uint16_t* getRegisters()

Get register result after readHoldingRegisters or readInputRegisters. Registers values are converted to little-endien.

void readCoils(uint16_t addr, uint16_t num, Callback<void(Result)>)

Modbus Read Coils function.

void readDiscreteInputs(uint16_t addr, uint16_t num, Callback<void(Result)>)

Modbus Read Discrete Inputs function.

void writeSingleCoil(uint16_t addr, bool val, Callback<void(Result)>)

Modbus Write Single Coils function.

void writeMultipleCoils(uint16_t addr, uint16_t num, uint8_t* val, Callback<void(Result)>)

Modbus Write Multiple Coils function.

void readHoldingRegisters(uint16_t addr, uint16_t num, Callback<void(Result)>)

Modbus Read Holding Registers function.

void readInputRegisters(uint16_t addr, uint8_t num, Callback<void(Result)>)

Modbus Read Input Registers function.

void writeSingleRegister(uint16_t addr, uint16_t val, Callback<void(Result)>)

Modbus Write Single Register function.

void writeMultipleRegisters(uint16_t addr, uint16_t num, uint16_t* val, Callback<void(Result)>)

Modbus Write Multiple Registers function.