Modbus RTU 开发示例

本文在 Linux 上基于 libmodbus 库进行 Modbus RTU 开发，在进行实验之前需要先安装 libmodbus 库，以及 socat 工具实现虚拟串口。

准备工作

安装 libmodbus

可以按 libmodbus 软件库 中「安装」一节的步骤进行手动编译安装。

安装 socat

sudo apt install socat

socat 是一个功能强大的网络工具，本实验将借助它来实现两个相互连接的虚拟串口，因此不需要准备真实的串口设备。关于 socat 的详细用法，可以参考 socat 命令。

代码实现

完整代码可从 https://github.com/getiot/linux-c/tree/main/library/libmodbus 获取。

Modbus RTU 从站

#include <stdio.h>
#ifndef _MSC_VER
#include <unistd.h>
#endif
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <stdbool.h>
#include "modbus.h" // 引用libmodbus库

#define Loop             1      // 循环次数
#define Server_ID       17      // 从端设备地址
#define ADDRESS_START    0      // 测试寄存器起始地址
#define ADDRESS_END     99      // 测试寄存器结束地址

int main(int argc, char *argv[])
{
    modbus_t *ctx = NULL;
    int nb = 0; // 需要测试的寄存器个数
    uint8_t* tab_rq_bits;       // 用于保存发送或接收的数据（下同）
    uint8_t* tab_rp_bits;
    uint16_t *tab_rq_registers;
    uint16_t *tab_rw_rq_registers;
    uint16_t *tab_rp_registers;
    char *port;
    modbus_mapping_t *mb_mapping;

    if (argc == 2) {
        port = argv[1];
    } else {
        port = "COM5";
    }

    // 创建一个RTU类型的容器
    ctx = modbus_new_rtu(port, 19200, 'N', 8, 1);
    // 设置从端地址
    modbus_set_slave(ctx, Server_ID);
    // 设置debug模式
    modbus_set_debug(ctx, true);
    // RTU 模式下表示打开串口
    if (modbus_connect(ctx) == -1)
    {
        fprintf(stderr, "Connection failed: %s \n", modbus_strerror(errno));
        modbus_free(ctx);
        return -1;
    }

    // 申请4块内存区用以存放寄存器数据，这里申请500个寄存器地址
    mb_mapping = modbus_mapping_new(500, 500, 500, 500);
    if (mb_mapping == NULL) {
        fprintf(stderr, "Error mapping: %s \n",modbus_strerror(errno));
        modbus_free(ctx);
        return -1;
    }

    // 循环接收查询帧并回复消息
    for (;;) {
        uint8_t query[MODBUS_TCP_MAX_ADU_LENGTH];
        int rc = 0;
        rc = modbus_receive(ctx, query);//获取查询报文

        if (rc >= 0) {
            // rc is the qury size
            modbus_reply(ctx, query, rc, mb_mapping); // 回复响应报文
        }
        else {
            // connection closed by the client or error
            printf("Connection Closed\n");
        }
    }
    printf("Quit the loop : %s \n", modbus_strerror(errno));

    // 释放内存
    modbus_mapping_free(mb_mapping);
    modbus_close(ctx);
    modbus_free(ctx);

    return 0;
}

Modbus RTU 主站

#include <stdio.h>
#ifndef _MSC_VER
#include <unistd.h>
#endif
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <stdbool.h>
#include "modbus.h" // 引用libmodbus库

#define Loop             1        // 循环次数
#define Server_ID       17        // 从端设备地址
#define ADDRESS_START    0        // 测试寄存器起始地址
#define ADDRESS_END     99        // 测试寄存器结束地址

int main(int argc, char *argv[])
{
    // printf("%c", 0b01000001); // A
    modbus_t *ctx = NULL;

    int nb = 0; // 需要测试的寄存器个数

    uint8_t* tab_rq_bits;       // 用于保存发送或接收的数据（下同）
    uint8_t* tab_rp_bits;

    uint16_t *tab_rq_registers;
    uint16_t *tab_rw_rq_registers;
    uint16_t *tab_rp_registers;
    char *port;

    if (argc == 2) {
        port = argv[1];
    } else {
        port = "COM4";
    }

    // 创建一个RTU类型的容器
    ctx = modbus_new_rtu(port, 19200, 'N', 8, 1);
    // 设置从端地址
    modbus_set_slave(ctx, Server_ID);
    // 设置debug模式
    modbus_set_debug(ctx, true);
    // RTU 模式下表示打开串口
    if (modbus_connect(ctx) == -1) {
        fprintf(stderr, "Connection failed: %s \n", modbus_strerror(errno));
        modbus_free(ctx);
        return -1;
    }
    printf("Connection successed\n");

    /*Allocate and initialize the different memory space */
    // 计算需要测试的寄存器个数
    nb = ADDRESS_END - ADDRESS_START;

    // 以下申请内存块，泳衣保存发送和接收各数据
    tab_rq_bits = (uint8_t*)malloc(nb * sizeof(uint8_t));
    memset(tab_rq_bits, 0, nb*sizeof(uint8_t));

    tab_rp_bits = (uint8_t*)malloc(nb * sizeof(uint8_t));
    memset(tab_rp_bits, 0, nb*sizeof(uint8_t));

    tab_rq_registers = (uint16_t*)malloc(nb * sizeof(uint16_t));
    memset(tab_rq_registers, 0, nb*sizeof(uint16_t));

    tab_rw_rq_registers = (uint16_t*)malloc(nb * sizeof(uint16_t));
    memset(tab_rw_rq_registers, 0, nb*sizeof(uint16_t));

    tab_rp_registers = (uint16_t*)malloc(nb * sizeof(uint16_t));
    memset(tab_rp_registers, 0, nb*sizeof(uint16_t));

    int nb_loop = 0, nb_fail = 0, addr = 0, rc = 0;

    while (nb_loop++ < Loop) {
        // 从起始地址开始顺序测试
        for (addr = ADDRESS_START; addr < ADDRESS_END; addr++) {
            sleep(1);
            int i = 0;

            // 生成随机数用于测试
            for (i = 0; i < nb; i++) {
                tab_rq_registers[i] = (uint16_t)(65535.0 * rand() / (RAND_MAX + 1.0));
                tab_rw_rq_registers[i] = ~tab_rq_registers[i];
                tab_rq_bits[i] = tab_rq_registers[i] % 2;
            }
            nb = ADDRESS_END - addr;

            // 测试线圈寄存器的单个读写
            printf("modbus_write_bit()...\n");
            rc = modbus_write_bit(ctx, addr, tab_rq_bits[0]); // 写线圈寄存器
            if (rc != 1) {
                printf("ERROR modbus_write_bit (%d)\n", rc);
                printf("Address = %d,value = %d \n", addr, tab_rq_bits[0]);
                nb_fail++;
            }
            else {
                // 写入之后，再读取并比较
                rc = modbus_read_bits(ctx, addr, 1, tab_rp_bits);
                if (rc != 1 || tab_rq_bits[0] != tab_rp_bits[0]) {
                    printf("ERROR modbus_read_bits single(%d)\n", rc);
                    printf("address = %d\n", addr);
                    nb_fail++;
                }
            }

            // 测试线圈寄存器的批量读写
            printf("modbus_write_bits()...\n");
            rc = modbus_write_bits(ctx, addr, nb, tab_rq_bits);
            if (rc != nb) {
                printf("ERROR modbus_write_bits (%d)\n", rc);
                printf("Address = %d,nb = %d\n", addr, nb);
                nb_fail++;
            }
            else {
                // 写入之后，再读取并比较
                rc = modbus_read_bits(ctx, addr, nb, tab_rp_bits);
                if (rc != nb) {
                    printf("ERROR modbus_read_bits \n");
                    printf("address = %d,nb = %d\n", addr, nb);
                    nb_fail++;
                }
                else {
                    // 进行比较
                    for (i = 0; i < nb; i++) {
                        if (tab_rp_bits[i] != tab_rq_bits[i]) {
                            printf("ERROR modbus_read_bits (%d)\n", rc);
                            printf("Address = %d, Val = %d(0x%x) != %d (0x%x)\n",
                                addr, tab_rq_bits[i], tab_rq_bits[i],
                                tab_rp_bits[i], tab_rp_bits[i]);
                            nb_fail++;
                        }
                    }
                }
            }

            // 测试保持寄存器的单个读写
            printf("modbus_write_register()...\n");
            rc = modbus_write_register(ctx, addr, tab_rq_registers[0]);
            if (rc != 1) {
                printf("ERROR modbus_read_bits (%d)\n", rc);
                printf("Address = %d, Val = %d(0x%x)\n",
                    addr, tab_rq_registers[0], tab_rq_registers[0]);
                nb_fail++;
            }
            else {
                // 写入之后进行读取
                rc = modbus_read_registers(ctx, addr, 1, tab_rp_registers);
                if (rc != 1) {
                    printf("ERROR modbus_read_registers (%d)\n", rc);
                    printf("Address = %d\n", addr);
                    nb_fail++;
                }
                else {
                    // 读取后进行比较
                    if (tab_rq_registers[0] != tab_rp_registers[0]) {
                        printf("ERROR modbus_read_registers (%d)\n", rc);
                        printf("Address = %d, Val = %d(0x%x) != %d (0x%x)\n",
                            addr, tab_rq_registers[0], tab_rq_registers[0],
                            tab_rp_registers[0], tab_rp_registers[0]);
                        nb_fail++;
                    }
                }
            }

            // 测试线圈寄存器的批量读写
            printf("modbus_write_registers()...\n");
            rc = modbus_write_registers(ctx, addr, nb, tab_rq_registers);
            if (rc != nb) {
                printf("ERROR modbus_write_bits (%d)\n", rc);
                printf("Address = %d, nb = %d \n", addr, nb);
                nb_fail++;
            }
            else {
                // 进行读取测试
                rc = modbus_read_registers(ctx, addr, nb, tab_rp_registers);
                if (rc != nb) {
                    printf("ERROR modbus_read_registers (%d)\n", rc);
                    printf("Address = %d\n", addr);
                    nb_fail++;
                }
                else {
                    for (i = 0; i < nb; i++) {
                        // 读取后进行比较
                        if (tab_rq_registers[i] != tab_rp_registers[i]) {
                            printf("ERROR modbus_read_registers (%d)\n", rc);
                            printf("Address = %d, Val = %d(0x%x) != %d (0x%x)\n",
                                addr, tab_rq_registers[0], tab_rq_registers[0],
                                tab_rp_registers[0], tab_rp_registers[0]);
                            nb_fail++;
                        }
                    }
                }
            }

            // 功能码 23 （0x17） 读写多个寄存器的测试
            printf("modbus_write_and_read_registers()...\n");
            rc = modbus_write_and_read_registers(ctx,
                addr, nb, tab_rw_rq_registers,
                addr, nb, tab_rp_registers);

            if (rc != nb) {
                printf("ERROR modbus_read_ad_write_registers (%d)\n", rc);
                printf("Address = %d,nb = %d\n", addr, nb);
                nb_fail++;
            }
            else {
                // 读取并比较
                for (i = 0; i < nb; i++) {
                    if (tab_rp_registers[i] < tab_rw_rq_registers[i]) {
                        printf("ERROR modbus_read_and_write_registers READ\n");
                        printf("Address = %d,value %d (0x%X) != %d (0x%X)\n",
                            addr, tab_rp_registers[i], tab_rw_rq_registers[i],
                            tab_rp_registers[i, tab_rw_rq_registers[i]]);
                        nb_fail++;
                    }
                }

                rc = modbus_read_registers(ctx, addr, nb, tab_rp_registers);
                if (rc != nb) {
                    printf("ERROR modbus_read_registers (%d) \n", rc);
                    printf("Address = %d,nb = %d\n", addr, nb);
                }
                else {
                    for (i = 0; i < nb; i++) {
                        if (tab_rw_rq_registers[i] != tab_rp_registers[i]) {
                            printf("ERROR modbus_read_and_write_registers \n");
                            printf("Address = %d,value %d (0x%X) != %d (0x%X)\n",
                                addr, tab_rw_rq_registers[i], tab_rw_rq_registers[i],
                                tab_rp_registers[i], tab_rp_registers[i]);
                            nb_fail++;
                        }
                    }
                }
            }
        }

        printf("Test: ");
        if (nb_fail) {
            printf("%d FAILS\n",nb_fail);
        } else {
            printf("SUCCESS\n");
        }
    }

    // FREE the memory
    free(tab_rq_bits);
    free(tab_rp_bits);
    free(tab_rq_registers);
    free(tab_rp_registers);
    free(tab_rw_rq_registers);

    // close the connection
    modbus_close(ctx);
    modbus_free(ctx);

    return 0;
}

Makefile

CC=gcc
CFLAGS=-I/usr/local/include/modbus/ -lmodbus -g

MASTER=modbus-rtu-master
SLAVE=modbus-rtu-slave

all:
        $(CC) -o $(MASTER) $(MASTER).c $(CFLAGS)
        $(CC) -o $(SLAVE) $(SLAVE).c $(CFLAGS)

clean:
        rm -rf $(MASTER) $(SLAVE)

测试

启动虚拟串口

$ socat  -d  -d  PTY  PTY
2021/07/16 14:36:43 socat[23932] N PTY is /dev/pts/7
2021/07/16 14:36:43 socat[23932] N PTY is /dev/pts/8
2021/07/16 14:36:43 socat[23932] N starting data transfer loop with FDs [5,5] and [7,7]

启动从站设备，等待主站指令

./modbus-rtu-slave /dev/pts/7

启动主站设备，依次给从站下发指令

./modbus-rtu-master /dev/pts/8

现在，你可以从终端看到 Modbus RTU 主站和从站的数据通信过程。