rs485 automatic driver enable
Some recent UARTs have an option to generate RS485 TE automatically, e.g. EXAR ST16C650, but standard 16550 doesn't. The common solution is a a timer, that enables the transmitter for defined period after a TX bit. There are basically two options:
- Using a long TE intervall (> 1 character frame). Involves a risk of collisions, if the peer responds too fast.
- A short TE intervall, just sufficient to drive the cable capacitance to the opposite level. Operation depends on meaningful dimensioned bias resistors in the termination network and isn't realiable for long cables.
An exact, frame accurate transmitter control can be achieved from an UART generated TE signal (as with said ST16C550), with a driver software generated TE (at least at low and medium speed) and by an external hardware, that is decoding the protocol (as with some RS485 repeaters).