modbus的C语言代码

以下是一个简单的用C语言实现Modbus RTU通信的示例代码，用于读取一个保持寄存器的值：

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

#define SLAVE_ADDRESS 1
#define FUNCTION_READ_HOLDING_REGISTERS 0x03
#define REGISTER_ADDRESS 0x0000
#define DATA_LENGTH 0x0001

unsigned short crc16(unsigned char *buf, int len) {
    unsigned short crc = 0xFFFF;
    int i, j;

    for (i = 0; i < len; i++) {
        crc ^= buf[i];
        for (j = 0; j < 8; j++) {
            if (crc & 0x0001) {
                crc >>= 1;
                crc ^= 0xA001;
            } else {
                crc >>= 1;
            }
        }
    }

    return crc;
}

int main(int argc, char *argv[]) {
    int fd;
    struct termios options;

    if (argc < 2) {
        printf("Usage: %s /dev/ttyUSB0\n", argv[0]);
        return 1;
    }

    fd = open(argv[1], O_RDWR | O_NOCTTY | O_NDELAY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    tcgetattr(fd, &options);
    cfsetispeed(&options, B9600);
    cfsetospeed(&options, B9600);
    options.c_cflag |= (CLOCAL | CREAD);
    options.c_cflag &= ~PARENB;
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    options.c_cflag &= ~CRTSCTS;
    options.c_iflag &= ~(IXON | IXOFF | IXANY);
    options.c_iflag &= ~(ICANON | ECHO | ECHOE | ISIG);
    options.c_oflag &= ~OPOST;
    options.c_cc[VMIN] = 1;
    options.c_cc[VTIME] = 0;
    tcsetattr(fd, TCSANOW, &options);

    unsigned char request[8];
    request[0] = SLAVE_ADDRESS;
    request[1] = FUNCTION_READ_HOLDING_REGISTERS;
    request[2] = REGISTER_ADDRESS >> 8;
    request[3] = REGISTER_ADDRESS & 0xFF;
    request[4] = DATA_LENGTH >> 8;
    request[5] = DATA_LENGTH & 0xFF;
    unsigned short crc = crc16(request, 6);
    request[6] = crc & 0xFF;
    request[7] = crc >> 8;

    write(fd, request, 8);

    unsigned char response[5 + 2 * DATA_LENGTH];
    int n = read(fd, response, 5 + 2 * DATA_LENGTH);

    if (n < 5 + 2 * DATA_LENGTH) {
        printf("Error: Modbus response truncated\n");
        return 1;
    }

    crc = crc16(response, 3 + 2 * DATA_LENGTH);
    if (crc != (response[3 + 2 * DATA_LENGTH] + (response[4 + 2 * DATA_LENGTH] << 8))) {
        printf("Error: Modbus response CRC error\n");
        return 1;
    }

    if (response[1] & 0x80) {
        printf("Error: Modbus response exception code %02X\n", response[2]);
        return 1;
    }

    unsigned short result = (response[3] << 8) + response[4];
    printf("Result: %d\n", result);

    close(fd);

    return 0;
}

这个代码使用Linux系统提供的串口接口进行通信，需要按照实际情况修改串口设备文件名和波特率。在程序中，首先设置串口参数，然后构造一个读取保持寄存器的请求，发送到Modbus从机上，并等待响应。最后，检查响应是否有效，如果没有错误，则将结果打印出来。