C++ modbus TCP 协议跟PLC通信_c++ freemodbus qplc

modbus 用的是git上的开源代码，地址：https://github.com/stephane/libmodbus

讲一下基本使用(给指定地址发送 on/off命令以及读取)：

modbus_t *ctx = NULL;
ctx = modbus_new_tcp("127.0.0.1", 502);//填写的是plc的ip、port
if (ctx == NULL)
{
    cout << "Unable to allocate libmodbus context" << endl;
    return -1;
}

//modbus_set_debug(ctx, TRUE);

if (modbus_connect(ctx) == -1)
{
    cout << "Connection failed: " << modbus_strerror(errno) << endl;
    modbus_free(ctx);
    return -1;
}

/* WRITE BIT */

int rc = modbus_write_bit(ctx, addr, value); //addr: 0x6000 value:true

if (rc != 1)

{

    cout <<"ERROR modbus_write_bit  "<< rc << endl;

}

uint8_t tab_rp_bits[4] = { 0 };
rc = modbus_read_bits(ctx, addr, 1, tab_rp_bits);

if (rc != 1)

{

    cout <<"ERROR modbus_read_bits  "<< rc << endl;

}

/* Close the connection */
modbus_close(ctx);
modbus_free(ctx);