STM32 USART TX DMA没启动

USART tx DMA不工作
已经看到数据下入，寄存器也写入了；

代码如下：

void RS485_UartDMAConfig(void)
{
    DMA_InitTypeDef DMA_InitStructure;  
    NVIC_InitTypeDef NVIC_InitStructure;
    RCC_AHBPeriphClockCmd(RCC_AHBPeriph_DMA1,ENABLE);
    
    DMA_Cmd(DMA1_Channel5, DISABLE);     
    USART_DMACmd(USART3, USART_DMAReq_Tx, ENABLE);// 开启串口DMA发送
    
    DMA_DeInit(DMA1_Channel5);
    DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)&USART3->DR; //USART3_DR_Base
    DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t)byRS485TxBuffer;
    DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralDST;
    DMA_InitStructure.DMA_BufferSize = RS485TX_BUFFER_SIZE;
    DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
    DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
    DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
    DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
    DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;
    DMA_InitStructure.DMA_Priority = DMA_Priority_High;
    DMA_InitStructure.DMA_M2M = DMA_M2M_Disable;
    DMA_Init(DMA1_Channel5, &DMA_InitStructure);
    DMA_ClearFlag(DMA1_FLAG_GL2);
    DMA_ITConfig(DMA1_Channel5,DMA_IT_TC,ENABLE);
    USART_DMACmd(USART3, USART_DMAReq_Tx, ENABLE);// 开启串口DMA发送
    
    NVIC_PriorityGroupConfig(NVIC_PriorityGroup_0); //------------USART1 DMA1_Channel4    interrupt                                            
    NVIC_InitStructure.NVIC_IRQChannel = DMA1_Channel5_IRQn;     
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 1;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 6;    
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStructure);
}

void RS485_TransmitPacket(BYTE * buff,BYTE len)
{
    uint32_t i;

            GPIO_SetBits(RS485PORT,RS485_RE);
            for(i = 0; i < len; i++)
               byRS485TxBuffer[i] = buff[i];
            DMA_Cmd(DMA1_Channel5, DISABLE);
            DMA1_Channel5->CNDTR = len;
            DMA_Cmd(DMA1_Channel5, ENABLE);

            eRS485State = ERS485_TX;
      //  }
    //}
}

只能用CH2通道