Question about behaviour of HAL_UART_Transmit_IT

Hello everyone,

I am having some trouble understanding the behaviour of the HAL_UART_Transmit_IT and HAL_UART_Receive_IT routines. I am using two NUCLEO-F767ZI boards that communicate via UART1 in a request/reply fashion - the 'client' sends a one-byte command and the 'server' immediately replies with the associated message. Both boards use a state machine which is programmed to ensure that at all times only one Receive_IT or Transmit_IT can be running. All the communication is carried out in interrupt mode. I have working IRQ handlers and Tx/Rx Completed Callbacks on both.

As I am running through a series of requests and replies, repeated every 10 ms by the client, the client board always starts to hang after a couple of cycles. But what is weird is that it seems to work fine only if I omit receiving the last reply of the server in a cycle, and skip directly to the next request without calling a HAL_UART_Receive_IT on the client. But from what I can see, the TxCpltCallback on the server side is still executed, although the client never called a Receive!

So I guess my question is: What is the behaviour of HAL_UART_Transmit_IT if nobody is answering on the other side?

Is there any other way a TxCpltCallback can be triggered? Do I have to clear any flags manually if I want to do what I described?