When the gating of transmitter and receiver lines are controlled through software (using RTS) in a multitasking operating system, it can sometimes take too long for a transmitter to become a receiver because the transmitter can not set RTS false quick enough. This means that when the transmitter finishes transmitting data it can potentially lose part of the response because it is unable to become a receiver in time.
To overcome this problem Brain Boxes RS422/485 devices have a hardware feature called autogating. This feature automatically detects that transmission is about to commence and gates the twisted pair to transmit. When the last stop bit has been transmitted the line is automatically gated to receive. This process happens very quickly at the hardware level (requires no software) and ensures that the transmitter is ready to receive a response as soon as it has finished transmitting.
In detail, the driver will write bytes of data into the transmit holding register (THR) on the UART. When the driver has finished, the UART will remain in the same state for 1.5 bits then the transmit holding register empty (THRE) bit will be set, this triggers the card to gate the outputs TX+ and TX- to become RX+ and RX-.