用STM32F103ZET6做了一块板子，专门实现通讯功能，目前状况是COM1和COM2是RS232口，都以调通；讲COM3口作为RS485，目前还没有调通。
COM3口硬件上的接线是PB10->COM3_TX，PB11->COM3->RX，PB12->RS485Control，PB12是用于控制485芯片上的接收和发送引脚，这种硬件接法应该没什么问题吧？
在软件上是如下处理：
void uart3_init(u32 bound){
//GPIO端口设置
USART_InitTypeDef USART_InitStructure;
GPIO_InitTypeDef GPIO_InitStructure;
NVIC_InitTypeDef NVIC_InitStructure;
/* Enable USARTx clocks */
RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART3, ENABLE);
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB | RCC_APB2Periph_AFIO, ENABLE);
/* Configure USART2 Tx (PB.10) as alternate function push-pull */
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_10;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIOB, &GPIO_InitStructure);
/* Configure USART2 Rx (PB.11) as input floating */
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_11;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_IN_FLOATING;
GPIO_Init(GPIOB, &GPIO_InitStructure);
USART_InitStructure.USART_BaudRate = bound;
USART_InitStructure.USART_WordLength = USART_WordLength_8b;
USART_InitStructure.USART_StopBits = USART_StopBits_1;
USART_InitStructure.USART_Parity = USART_Parity_No;
USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
USART_Init(USART3, &USART_InitStructure);
//Usart3 NVIC 配置
NVIC_InitStructure.NVIC_IRQChannel = USART3_IRQn;
NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority=3 ;//抢占优先级3
NVIC_InitStructure.NVIC_IRQChannelSubPriority = 3; //子优先级3
NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE; //IRQ通道使能
NVIC_Init(&NVIC_InitStructure); //根据指定的参数初始化VIC寄存器
USART_ITConfig(USART3, USART_IT_RXNE, ENABLE);
USART_Cmd(USART3, ENABLE);
}
void USART1SendByte(u8 ch)
{
USART_SendData(USART1, ch);
while(USART_GetFlagStatus(USART1, USART_FLAG_TXE) == RESET);
}
在main函数中调用这两函数：
{
......
uart3_init(9600);
USART1SendByte(0x55);
......
}
试验结果发现，串口只能发送出0x00，请高手指点迷津，如果硬件可以那样设计的话，那么软件会有什么问题呢？