5 龙芯1C使用freemodbus主机模式测试例程，任何波特率下都无法读取到数据？

发布于 2021-08-30 19:20:35 浏览：1157 订阅该版

##前提如下：
- 龙芯1c
- freemodbus-latest及其测试例程
- PC端modbus slave 7.0.0
- 串口9600 8N1
- 另外，RT_TICK_PER_SECOND设置为1000

##过程
运行主机测试例程，可以观察到PC端modbus slave相应寄存器值的变化，于是在例程中加入读取从机功能（功能码03）
```c
extern USHORT   usMRegHoldBuf[MB_MASTER_TOTAL_SLAVE_NUM][M_REG_HOLDING_NREGS]; // modbus master 用全局二维数组来记录多个从机的数据

static void send_thread_entry(void *parameter)
{
    eMBMasterReqErrCode error_code = MB_MRE_NO_ERR;
    rt_uint16_t error_count = 0;
    USHORT data[2] = {0};

while (1)
    {
        /* Test Modbus Master */
        data[0] = (USHORT)(rt_tick_get() / 10);
        data[1] = (USHORT)(rt_tick_get() % 10);

error_code = eMBMasterReqWriteMultipleHoldingRegister(SLAVE_ADDR,          /* salve address */
                                                              MB_SEND_REG_START,   /* register start address */
                                                              MB_SEND_REG_NUM,     /* register total number */
                                                              data,                /* data to be written */
                                                              RT_WAITING_FOREVER); /* timeout */
 //测试读寄存器功能                                                             
        error_code = eMBMasterReqReadHoldingRegister(SLAVE_ADDR,          /* salve address */
                                        1,   /* register start address */
                                        1,     /* register total number */
                                        RT_WAITING_FOREVER);
 //读取解析值
		for(int i=0;i<10;i++){
			rt_kprintf("读取的值[%d]=%d \n",i,usMRegHoldBuf[0][i]);

}
        /* Record the number of errors */
        if (error_code != MB_MRE_NO_ERR)
        {
            error_count++;
        }
    }
}
```
##现象
- 读取到的值始终为0.无论波特率是115200，9600还是19200，读到的值均为0.

##分析过程
- 发现peMBMasterFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength )的返回值和参数值均不正确，导致状态无法到达EV_MASTER_EXECUTE。
```c
eMBErrorCode
eMBMasterPoll( void )
{
...省略...
    /* Check if there is a event available. If not return control to caller.
     * Otherwise we will handle the event. */
    if( xMBMasterPortEventGet( &eEvent ) == TRUE )
    {
        switch ( eEvent )
        {
        case EV_MASTER_READY:
            eMBState = STATE_ESTABLISHED;
            break;

case EV_MASTER_FRAME_RECEIVED:
            eStatus = peMBMasterFrameReceiveCur( &ucRcvAddress, &ucMBFrame, &usLength );

/* Check if the frame is for us. If not ,send an error process event. */
            if ( ( eStatus == MB_ENOERR ) && ( ucRcvAddress == ucMBMasterGetDestAddress() ) )
            {
                ( void ) xMBMasterPortEventPost( EV_MASTER_EXECUTE );
            }
            else
            {
                vMBMasterSetErrorType(EV_ERROR_RECEIVE_DATA);
                ( void ) xMBMasterPortEventPost( EV_MASTER_ERROR_PROCESS );
            }
            break;

case EV_MASTER_EXECUTE:
            .......
        case EV_MASTER_FRAME_SENT:
            .......
        case EV_MASTER_ERROR_PROCESS:
            ...省略...
}
```
- 同时，xMBMasterRTUReceiveFSM()函数中，xMBMasterPortSerialGetByte()可以完整的获取到串口接收的从机回复，但是switch判断却始终是STATE_M_RX_IDLE，并在该状态下触发超时vMBMasterPortTimersT35Enable()，始终无法进入接收状态STATE_M_RX_RCV，进而导致ucMasterRTURcvBuf[usMasterRcvBufferPos++]的数组只能保存最近的一个值。
```c
BOOL
xMBMasterRTUReceiveFSM( void )
{
    BOOL            xTaskNeedSwitch = FALSE;
    UCHAR           ucByte;

RT_ASSERT(( eSndState == STATE_M_TX_IDLE ) || ( eSndState == STATE_M_TX_XFWR ));

/* Always read the character. */
    ( void )xMBMasterPortSerialGetByte( ( CHAR * ) & ucByte );    //这里每次都可以完整的得到从机回复的数据帧

switch ( eRcvState )
    {
        /* If we have received a character in the init state we have to
         * wait until the frame is finished.
         */
    case STATE_M_RX_INIT:
        vMBMasterPortTimersT35Enable( );
        break;

/* In the error state we wait until all characters in the
         * damaged frame are transmitted.
         */
    case STATE_M_RX_ERROR:
        vMBMasterPortTimersT35Enable( );
        break;

/* In the idle state we wait for a new character. If a character
         * is received the t1.5 and t3.5 timers are started and the
         * receiver is in the state STATE_RX_RECEIVCE and disable early
         * the timer of respond timeout .
         */
    case STATE_M_RX_IDLE:
    	/* In time of respond timeout,the receiver receive a frame.
    	 * Disable timer of respond timeout and change the transmiter state to idle.
    	 */
    	vMBMasterPortTimersDisable( );
    	eSndState = STATE_M_TX_IDLE;

usMasterRcvBufferPos = 0;
        ucMasterRTURcvBuf[usMasterRcvBufferPos++] = ucByte;
        eRcvState = STATE_M_RX_RCV;

/* Enable t3.5 timers. */
        vMBMasterPortTimersT35Enable( );
        break;

/* We are currently receiving a frame. Reset the timer after
         * every character received. If more than the maximum possible
         * number of bytes in a modbus frame is received the frame is
         * ignored.
         */
    case STATE_M_RX_RCV:
        if( usMasterRcvBufferPos < MB_SER_PDU_SIZE_MAX )
        {
            ucMasterRTURcvBuf[usMasterRcvBufferPos++] = ucByte;
        }
        else
        {
            eRcvState = STATE_M_RX_ERROR;
        }
        vMBMasterPortTimersT35Enable();
        break;
    }

return xTaskNeedSwitch;
}
```
所以我现在应该怎么才能解决这个奇葩的问题>_<!!,希望了解的朋友可以帮忙看一下，非常非常感激