有关CAN总线通信的几个问题

发布于 2022-02-03 16:56:58 浏览：1389 订阅该版

最近在研究CAN总线，有几个问题不是很明白
1.CANH和CANL必须要并联一个120Ω电阻吗![Inked1_LI.jpg](https://oss-club.rt-thread.org/uploads/20220203/a128214ccfd6a25a5ab4c9f95495121b.jpg.webp)
2.为什么总线模式必须要设置为RT_CAN_MODE_LOOPBACK，rt_device_write才不会报错![3.jpg](https://oss-club.rt-thread.org/uploads/20220203/354e2a78f29c58a7b1bf22e6dbd09c81.jpg.webp)
3.为什么rt_device_write执行之后，在PC上的USB-CAN Tool软件中接收不到刚发出来的信息![4.jpg](https://oss-club.rt-thread.org/uploads/20220203/2743a2a91598941efecc4218ca4c65d9.jpg.webp)

我使用的代码如下```
    /*CAN设备操作*/
    rt_err_t can_ret = 0;
    rt_size_t size = 0;
    struct rt_can_msg msg = { 0 };
    //can1使用PA11 PA12引脚
    can_dev = rt_device_find(CAN_DEV_NAME);
    if (can_dev == RT_NULL)
    {
        LOG_E("rt_device_find [can1] failed...\n");
        return -EINVAL;
    }
    //以中断接收及发送模式打开 CAN设备
    can_ret = rt_device_open(can_dev, RT_DEVICE_FLAG_INT_TX | RT_DEVICE_FLAG_INT_RX);
    if (can_ret < 0)
    {
        LOG_E("rt_device_open [can1] failed...\n");
        return can_ret;
    }
    //设置 CAN 通信的波特率为 500kbit/s
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_SET_BAUD, (void *) CAN500kBaud);
    //获取 CAN 总线设备的状态
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_GET_STATUS, &status);
    can_ret = rt_device_control(can_dev, RT_CAN_CMD_SET_MODE, (void *)RT_CAN_MODE_LOOPBACK);
    LOG_D("can bus current status:%d", can_ret);
    //初始化CAN接收信号量
    rt_sem_init(&can_sem, "can_sem", 0, RT_IPC_FLAG_FIFO);
    //创建CAN接收线程，并开启
    can_th = rt_thread_create("can_rx", can_rx_thread, RT_NULL, 1024, 25, 10);
    if (can_th != RT_NULL)
    {
        rt_thread_startup(can_th);
    }
    else
    {
        rt_kprintf("create can_rx thread failed!\n");
    }
    msg.id = 0x78; /* ID 为 0x78 */
    msg.ide = RT_CAN_STDID; /* 标准格式 */
    msg.rtr = RT_CAN_DTR; /* 数据帧 */
    msg.len = 8; /* 数据长度为 8 */
    /* 待发送的 8 字节数据 */
    msg.data[0] = 0x00;
    msg.data[1] = 0x11;
    msg.data[2] = 0x22;
    msg.data[3] = 0x33;
    msg.data[4] = 0x44;
    msg.data[5] = 0x55;
    msg.data[6] = 0x66;
    msg.data[7] = 0x77;
    /* 发送一帧 CAN 数据 */
    size = rt_device_write(can_dev, 0, &msg, sizeof(msg));
    if (size == 0)
    {
        rt_kprintf("can_dev write data failed!\n");
    }
```