C语言Modbus通讯协议编程：构建命令帧并通过串口发送

要使用C语言编写Modbus通讯协议的相关代码，构建要发送的Modbus命令帧，并将其通过串口发送给电脑设备，可以按照以下步骤进行：

引入必要的头文件：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

设置串口通信参数：

int set_serial_port(int fd, int baud_rate, int data_bits, int stop_bits, char parity)
{
    struct termios options;
    memset(&options, 0, sizeof(options));
    
    // 获取当前配置
    if (tcgetattr(fd, &options) != 0) {
        perror("tcgetattr");
        return -1;
    }
    
    // 设置波特率
    switch (baud_rate) {
        case 9600:
            cfsetispeed(&options, B9600);
            cfsetospeed(&options, B9600);
            break;
        case 115200:
            cfsetispeed(&options, B115200);
            cfsetospeed(&options, B115200);
            break;
        // 其他波特率自行添加
        default:
            fprintf(stderr, "Unsupported baud rate\n");
            return -1;
    }
    
    // 设置数据位
    options.c_cflag &= ~CSIZE; // 清除数据位设置
    switch (data_bits) {
        case 5:
            options.c_cflag |= CS5;
            break;
        case 6:
            options.c_cflag |= CS6;
            break;
        case 7:
            options.c_cflag |= CS7;
            break;
        case 8:
            options.c_cflag |= CS8;
            break;
        default:
            fprintf(stderr, "Unsupported data bits\n");
            return -1;
    }
    
    // 设置停止位
    switch (stop_bits) {
        case 1:
            options.c_cflag &= ~CSTOPB;
            break;
        case 2:
            options.c_cflag |= CSTOPB;
            break;
        default:
            fprintf(stderr, "Unsupported stop bits\n");
            return -1;
    }
    
    // 设置奇偶校验位
    options.c_iflag &= ~(INPCK | ISTRIP);
    switch (parity) {
        case 'N':
        case 'n':
            options.c_cflag &= ~PARENB;
            break;
        case 'E':
        case 'e':
            options.c_cflag |= PARENB;
            options.c_cflag &= ~PARODD;
            break;
        case 'O':
        case 'o':
            options.c_cflag |= PARENB;
            options.c_cflag |= PARODD;
            break;
        default:
            fprintf(stderr, "Unsupported parity\n");
            return -1;
    }
    
    // 应用配置
    if (tcsetattr(fd, TCSANOW, &options) != 0) {
        perror("tcsetattr");
        return -1;
    }
    
    return 0;
}

打开串口：

int open_serial_port(const char *device_path)
{
    int fd = open(device_path, O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd == -1) {
        perror("open");
        return -1;
    }
    
    fcntl(fd, F_SETFL, 0);
    
    return fd;
}

构建Modbus命令帧：

void build_modbus_frame(unsigned char *frame, unsigned char slave_address, unsigned char function_code, unsigned short address, unsigned short data)
{
    frame[0] = slave_address;
    frame[1] = function_code;
    frame[2] = (address >> 8) & 0xFF;
    frame[3] = address & 0xFF;
    frame[4] = (data >> 8) & 0xFF;
    frame[5] = data & 0xFF;
    
    // 计算CRC校验
    unsigned short crc = calculate_crc(frame, 6);
    
    frame[6] = crc & 0xFF;
    frame[7] = (crc >> 8) & 0xFF;
}

计算CRC校验：

unsigned short calculate_crc(unsigned char *data, int length)
{
    unsigned short crc = 0xFFFF;
    for (int i = 0; i < length; ++i) {
        crc ^= data[i];
        for (int j = 0; j < 8; ++j) {
            if (crc & 0x0001) {
                crc >>= 1;
                crc ^= 0xA001;
            } else {
                crc >>= 1;
            }
        }
    }
    return crc;
}

发送Modbus命令帧：

int send_modbus_frame(int fd, unsigned char *frame, int length)
{
    int bytes_written = write(fd, frame, length);
    if (bytes_written == -1) {
        perror("write");
        return -1;
    }
    return bytes_written;
}

使用C语言的串口接收函数，获取电压采样值：

int receive_voltage_sample(int fd, unsigned char *buffer, int buffer_length)
{
    int bytes_read = read(fd, buffer, buffer_length);
    if (bytes_read == -1) {
        perror("read");
        return -1;
    }
    return bytes_read;
}

这样，你就可以使用以上函数进行Modbus通讯了。具体的使用方法可以参考以下代码片段：

int main()
{
    const char *device_path = "/dev/ttyUSB0"; // 串口设备路径
    int baud_rate = 9600; // 波特率
    int data_bits = 8; // 数据位
    int stop_bits = 1; // 停止位
    char parity = 'N'; // 奇偶校验位
    
    int fd = open_serial_port(device_path);
    if (fd == -1) {
        fprintf(stderr, "Failed to open serial port\n");
        return 1;
    }
    
    if (set_serial_port(fd, baud_rate, data_bits, stop_bits, parity) == -1) {
        fprintf(stderr, "Failed to set serial port\n");
        close(fd);
        return 1;
    }
    
    unsigned char frame[8];
    unsigned char slave_address = 1; // 从机地址
    unsigned char function_code = 3; // 功能码
    unsigned short address = 0; // 寄存器地址
    unsigned short data = 0; // 数据
    
    build_modbus_frame(frame, slave_address, function_code, address, data);
    
    if (send_modbus_frame(fd, frame, sizeof(frame)) == -1) {
        fprintf(stderr, "Failed to send Modbus frame\n");
        close(fd);
        return 1;
    }
    
    unsigned char buffer[256];
    int bytes_read = receive_voltage_sample(fd, buffer, sizeof(buffer));
    if (bytes_read == -1) {
        fprintf(stderr, "Failed to receive voltage sample\n");
        close(fd);
        return 1;
    }
    
    // 处理接收到的电压采样值
    
    close(fd);
    
    return 0;
}

希望以上代码可以帮助到你。请根据实际情况修改串口相关参数和Modbus命令帧的内容。