UART4 RX Signal level

I use the UART4 like the following initialization.

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void uart4_configuration(void)
{
 GPIO_InitTypeDef GPIO_InitStructure;
 USART_InitTypeDef USART_InitStructure;
 /* enable usart clock */
 RCC_APB1PeriphClockCmd(RCC_APB1Periph_UART4, ENABLE);
 
 
 RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOC, ENABLE);
 
 
 // Configure USART Tx as alternate function
 GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
 GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP; //GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
 GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
 GPIO_InitStructure.GPIO_Pin = RS485AXM_TX_PIN;
 GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
 GPIO_Init(RS485AXM_COM_PORT, &GPIO_InitStructure);
 
 // Configure USART Rx as alternate function
 GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
 GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_DOWN; //GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
 GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
 GPIO_InitStructure.GPIO_Pin = RS485AXM_RX_PIN;
 GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
 GPIO_Init(RS485AXM_COM_PORT, &GPIO_InitStructure);
 
 GPIO_PinAFConfig(GPIOC, GPIO_PinSource10, GPIO_AF_UART4);
 GPIO_PinAFConfig(GPIOC, GPIO_PinSource11, GPIO_AF_UART4);
 
 
 USART_StructInit(&USART_InitStructure);
 USART_InitStructure.USART_BaudRate = 9600;
 USART_Init(UART4, &USART_InitStructure);
 
 USART_Cmd(UART4, ENABLE);
 
 NVIC_EnableIRQ(UART4_IRQn);
}

The Rx signal level is abnormal like the following.

The low level is not the zero(ground) level.

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But the Tx level is ok like the followings.

Although I detach the RX Pin from the PCB Board, and tie with the TX Pin,

RX/TX Pin level is like the abnormal level(the first image)

Is the initialization is wrong?

Thanks

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