modbus作为工业领域的一个重要协议，那好就移植libmodbus库并测试。

一、源码下载&编译

ibmodbus官网是libmodbus，首页就是个很霸气的标语，copy过来“ The most popular Open Source library to communicate with Modbus devices. ”

最新版的是v3.1.10，考虑到Omni3576的SoC功能强大，直接把gz源码文件下载到开发板。

解压源码，可以顺便看看相关文件树：

luckfox@luckfox:~$ tar -xzvf libmodbus-3.1.10.tar.gz
libmodbus-3.1.10/configure.ac
libmodbus-3.1.10/libmodbus.pc.in
libmodbus-3.1.10/configure
libmodbus-3.1.10/AUTHORS
libmodbus-3.1.10/tests/
libmodbus-3.1.10/config.h.in
libmodbus-3.1.10/COPYING.LESSER
libmodbus-3.1.10/Makefile.am
libmodbus-3.1.10/README.md
libmodbus-3.1.10/NEWS
libmodbus-3.1.10/m4/
libmodbus-3.1.10/Makefile.in
libmodbus-3.1.10/aclocal.m4

执行./configure命令：

后面也没啥，直接make & make install即可。

luckfox@luckfox:~/libmodbus-3.1.10$ make
make --no-print-directory all-recursive
Making all in src
make[2]: Nothing to be done for 'all'.
Making all in tests
make  all-am
luckfox@luckfox:~/libmodbus-3.1.10$ sudo make install
Making install in src
 /usr/bin/mkdir -p '/usr/local/lib'
 /bin/bash ../libtool   --mode=install /usr/bin/install -c   libmodbus.la '/usr/local/lib'
libtool: install: /usr/bin/install -c .libs/libmodbus.so.5.1.0 /usr/local/lib/libmodbus.so.5.1.0
libtool: install: (cd /usr/local/lib && { ln -s -f libmodbus.so.5.1.0 libmodbus.so.5 || { rm -f libmodbus.so.5 && ln -s libmodbus.so.5.1.0 libmodbus.so.5; }; })
libtool: install: (cd /usr/local/lib && { ln -s -f libmodbus.so.5.1.0 libmodbus.so || { rm -f libmodbus.so && ln -s libmodbus.so.5.1.0 libmodbus.so; }; })
libtool: install: /usr/bin/install -c .libs/libmodbus.lai /usr/local/lib/libmodbus.la
libtool: finish: PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/sbin" ldconfig -n /usr/local/lib
----------------------------------------------------------------------
Libraries have been installed in:
   /usr/local/lib

If you ever happen to want to link against installed libraries
in a given directory, LIBDIR, you must either use libtool, and
specify the full pathname of the library, or use the '-LLIBDIR'
flag during linking and do at least one of the following:
   - add LIBDIR to the 'LD_LIBRARY_PATH' environment variable
     during execution
   - add LIBDIR to the 'LD_RUN_PATH' environment variable
     during linking
   - use the '-Wl,-rpath -Wl,LIBDIR' linker flag
   - have your system administrator add LIBDIR to '/etc/ld.so.conf'

See any operating system documentation about shared libraries for
more information, such as the ld(1) and ld.so(8) manual pages.
----------------------------------------------------------------------
 /usr/bin/mkdir -p '/usr/local/include/modbus'
 /usr/bin/install -c -m 644 modbus.h modbus-version.h modbus-rtu.h modbus-tcp.h '/usr/local/include/modbus'
Making install in tests
make[2]: Nothing to be done for 'install-exec-am'.
make[2]: Nothing to be done for 'install-data-am'.
make[2]: Nothing to be done for 'install-exec-am'.
 /usr/bin/mkdir -p '/usr/local/share/doc/libmodbus'
 /usr/bin/install -c -m 644 AUTHORS NEWS README.md '/usr/local/share/doc/libmodbus'
 /usr/bin/mkdir -p '/usr/local/lib/pkgconfig'
 /usr/bin/install -c -m 644 libmodbus.pc '/usr/local/lib/pkgconfig'
luckfox@luckfox:~/libmodbus-3.1.10$

Libraries have been installed in: /usr/local/lib
看看库的信息：

file /usr/local/lib/libmodbus.so.5.1.0
/usr/local/lib/libmodbus.so.5.1.0: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, BuildID[sha1]=20fb91e6292544afcd9a77e7f6d8494ecd6476e0, with debug_info, not stripped

二、测试准备

Omni3576默认配置了 2 路串口，UART4 和 UART5。

直接选择UART2吧，找到UART2的针脚即可。在Omni3576开发板的硬件手册中找到。

UART2默认波特率是9600，强迫症改成115200方便后续测试。

stty -F /dev/ttyS2 ispeed   115200   ospeed  115200

连上USB-TTL模块串口线，顺便测试下，在开发板上的终端执行以下命令，使用 echo 命令向终端设备文件写入字符串"Hello"：

echo Hello > /dev/ttyS2

PC端接收到了：

三、测试

在开发板上实现个MODBUS server，具体源码如下：

#include <errno.h>
#include <modbus.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

// clang-format off
#ifdef _WIN32
# include <winsock2.h>
#else
# include <sys/socket.h>
#endif

/* For MinGW */
#ifndef MSG_NOSIGNAL
# define MSG_NOSIGNAL 0
#endif
// clang-format on

#include "unit-test.h"

enum {
    TCP,
    TCP_PI,
    RTU
};

int main(int argc, char *argv[])
{
    int s = -1;
    modbus_t *ctx;
    modbus_mapping_t *mb_mapping;
    int rc;
    int i;
    int use_backend;
    uint8_t *query;
    int header_length;
    char *ip_or_device;

    if (argc > 1) {
        if (strcmp(argv[1], "tcp") == 0) {
            use_backend = TCP;
        } elseif (strcmp(argv[1], "tcppi") == 0) {
            use_backend = TCP_PI;
        } elseif (strcmp(argv[1], "rtu") == 0) {
            use_backend = RTU;
        } else {
            printf("Modbus server for unit testing.\n");
            printf("Usage:\n  %s [tcp|tcppi|rtu] [<ip or device>]\n", argv[0]);
            printf("Eg. tcp 127.0.0.1 or rtu /dev/ttyUSB0\n\n");
            return-1;
        }
    } else {
        /* By default */
        use_backend = TCP;
    }

    if (argc > 2) {
        ip_or_device = argv[2];
    } else {
        switch (use_backend) {
        case TCP:
            ip_or_device = "127.0.0.1";
            break;
        case TCP_PI:
            ip_or_device = "::1";
            break;
        case RTU:
            ip_or_device = "/dev/ttyUSB0";
            break;
        default:
            break;
        }
    }

    if (use_backend == TCP) {
        ctx = modbus_new_tcp(ip_or_device, 1502);
        query = malloc(MODBUS_TCP_MAX_ADU_LENGTH);
    } elseif (use_backend == TCP_PI) {
        ctx = modbus_new_tcp_pi(ip_or_device, "1502");
        query = malloc(MODBUS_TCP_MAX_ADU_LENGTH);
    } else {
        ctx = modbus_new_rtu(ip_or_device, 115200, 'N', 8, 1);
        modbus_set_slave(ctx, SERVER_ID);
        query = malloc(MODBUS_RTU_MAX_ADU_LENGTH);
    }

    header_length = modbus_get_header_length(ctx);

    modbus_set_debug(ctx, TRUE);

    mb_mapping = modbus_mapping_new_start_address(UT_BITS_ADDRESS,
                                                  UT_BITS_NB,
                                                  UT_INPUT_BITS_ADDRESS,
                                                  UT_INPUT_BITS_NB,
                                                  UT_REGISTERS_ADDRESS,
                                                  UT_REGISTERS_NB_MAX,
                                                  UT_INPUT_REGISTERS_ADDRESS,
                                                  UT_INPUT_REGISTERS_NB);
    if (mb_mapping == NULL) {
        fprintf(stderr, "Failed to allocate the mapping: %s\n", modbus_strerror(errno));
        modbus_free(ctx);
        return-1;
    }

    /* Examples from PI_MODBUS_300.pdf.
       Only the read-only input values are assigned. */

    /* Initialize input values that's can be only done server side. */
    modbus_set_bits_from_bytes(
        mb_mapping->tab_input_bits, 0, UT_INPUT_BITS_NB, UT_INPUT_BITS_TAB);

    /* Initialize values of INPUT REGISTERS */
    for (i = 0; i < UT_INPUT_REGISTERS_NB; i++) {
        mb_mapping->tab_input_registers[i] = UT_INPUT_REGISTERS_TAB[i];
    }

    if (use_backend == TCP) {
        s = modbus_tcp_listen(ctx, 1);
        modbus_tcp_accept(ctx, &s);
    } elseif (use_backend == TCP_PI) {
        s = modbus_tcp_pi_listen(ctx, 1);
        modbus_tcp_pi_accept(ctx, &s);
    } else {
        rc = modbus_connect(ctx);
        if (rc == -1) {
            fprintf(stderr, "Unable to connect %s\n", modbus_strerror(errno));
            modbus_free(ctx);
            return-1;
        }
    }

    for (;;) {
        do {
            rc = modbus_receive(ctx, query);
            /* Filtered queries return 0 */
        } while (rc == 0);

        /* The connection is not closed on errors which require on reply such as
           bad CRC in RTU. */
        if (rc == -1 && errno != EMBBADCRC) {
            /* Quit */
            break;
        }

        /* Special server behavior to test client */
        if (query[header_length] == 0x03) {
            /* Read holding registers */

            if (MODBUS_GET_INT16_FROM_INT8(query, header_length + 3) ==
                UT_REGISTERS_NB_SPECIAL) {
                printf("Set an incorrect number of values\n");
                MODBUS_SET_INT16_TO_INT8(
                    query, header_length + 3, UT_REGISTERS_NB_SPECIAL - 1);
            } elseif (MODBUS_GET_INT16_FROM_INT8(query, header_length + 1) ==
                       UT_REGISTERS_ADDRESS_SPECIAL) {
                printf("Reply to this special register address by an exception\n");
                modbus_reply_exception(ctx, query, MODBUS_EXCEPTION_SLAVE_OR_SERVER_BUSY);
                continue;
            } elseif (MODBUS_GET_INT16_FROM_INT8(query, header_length + 1) ==
                       UT_REGISTERS_ADDRESS_INVALID_TID_OR_SLAVE) {
                constint RAW_REQ_LENGTH = 5;
                uint8_t raw_req[] = {(use_backend == RTU) ? INVALID_SERVER_ID : 0xFF,
                                     0x03,
                                     0x02,
                                     0x00,
                                     0x00};

                printf("Reply with an invalid TID or slave\n");
                modbus_send_raw_request(ctx, raw_req, RAW_REQ_LENGTH * sizeof(uint8_t));
                continue;
            } elseif (MODBUS_GET_INT16_FROM_INT8(query, header_length + 1) ==
                       UT_REGISTERS_ADDRESS_SLEEP_500_MS) {
                printf("Sleep 0.5 s before replying\n");
                usleep(500000);
            } elseif (MODBUS_GET_INT16_FROM_INT8(query, header_length + 1) ==
                       UT_REGISTERS_ADDRESS_BYTE_SLEEP_5_MS) {
                /* Test low level only available in TCP mode */
                /* Catch the reply and send reply byte a byte */
                uint8_t req[] = "\x00\x1C\x00\x00\x00\x05\xFF\x03\x02\x00\x00";
                int req_length = 11;
                int w_s = modbus_get_socket(ctx);
                if (w_s == -1) {
                    fprintf(stderr, "Unable to get a valid socket in special test\n");
                    continue;
                }

                /* Copy TID */
                req[1] = query[1];
                for (i = 0; i < req_length; i++) {
                    printf("(%.2X)", req[i]);
                    usleep(5000);
                    rc = send(w_s, (constchar *) (req + i), 1, MSG_NOSIGNAL);
                    if (rc == -1) {
                        break;
                    }
                }
                continue;
            }
        }

        rc = modbus_reply(ctx, query, rc, mb_mapping);
        if (rc == -1) {
            break;
        }
    }

    printf("Quit the loop: %s\n", modbus_strerror(errno));

    if (use_backend == TCP) {
        if (s != -1) {
            close(s);
        }
    }
    modbus_mapping_free(mb_mapping);
    free(query);
    /* For RTU */
    modbus_close(ctx);
    modbus_free(ctx);

    return0;
}

直接在Omni3576开发板上编译好。然后PC运行modbus-poll。在开发板上打开测试程序。

luckfox@luckfox:~/libmodbus-3.1.10$ ./mod-server rtu /dev/ttyS2
Opening /dev/ttyS2 at 115200 bauds (N, 8, 1)
Waiting for an indication...

modbus-poll读寄存器，设备id是测试程序里面写死的。

开发板侧的log显示已经收到modbus信息，对比modbus-poll的信息：

OK，到此为止，可以接入MODBUS设备了。
最后来个测试视频。