Collision Detection on RS485 on STM32F103CB

Hi

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I'm doing a topic to detect the collision on RS485. Since I wish to stop transmitting when collision occurred, I need to know how's the bus look like, however while transmitting, the transmit enable pin pull high on SP485EEN, lead the receiving pin get nothing, so how can I notice the collision on the bus by the transmitting device itself?

By the way I'm playing on J1708 / J1587, if there are any examples please let me know.

Thanks