RS-485 half-duplex transmitter disabling time

[underflow]

Hello,

I'm trying to operate a UART in RS-485 half-duplex mode with tight timing. RTS controls the transceiver. When transmitting data, it takes about 2 us from activating RTS to the start of the first data bit. However after transmitting the last data bit there is a delay of about 40 us before RTS gets deactivated and data could be received again. In my case this is about an order of magnitude more time than the communication partner takes for its replies.

The UART driver interrupt handler is quite long and I attempted to move the code deactivating the transmitter at the very beginning whitout a significant change in timing. Any ideas what is causing the huge delay between the last data bit (, the UART_TX_DONE_INT,) and actually disabling RTS?


Best regards,

Christian