@ard_newbie, thanks - I saw post You mentioned before I wrote here.
Issue here is I don't want to poll - I want to be notified after someting cames to receiver - to lock transmission.
I believe code You pasted is to manage RTS pin for RS485 transmission, right ?
USART1->US_CR |= US_CR_RSTRX | US_CR_RSTTX | US_CR_RXDIS | US_CR_TXDIS;
USART1->US_MR |= US_MR_USART_MODE_RS485 // USART1 IN RS485 mode
| US_MR_PAR_NO // No parity
| US_MR_OVER; // 8 X oversampling
USART1->US_CR |= US_CR_TXEN | US_CR_RXEN; // Enable Transmitter and Receiver