I have a doubt, when I have a circuit that has a driver for a serial interface, such as a TTL-to-RS485 or TTL-to-422 buffer. The typical situation I'm in is that this chip is connected on one side to an MCU and on the other side to a connector on the edge of the board.
The question is, is it better to have the buffer as close to the MCU as possible or as close to the connector as possible?
Reasoning in the first case (as close as possible to the MCU) I have the shortest possible 3.3V TTL line, so I minimize the noise on the MCU-BUFFER side. While in the second case (as close as possible to the connector) I favor the signals coming out of the connector.
Which situation is the most advisable.
PS: the board is general purpose, so the environment is not defined a priori.
If you follow good design practices (and under "normal" conditions), it does not make big difference where you place the transceiver.
Good design practices would be to have impedance controlled signal lines from the transceiver to the connector and to have good signal integrity on the CMOS/TTL lines from MCU to transmitter. And of course you would need to have little cross-coupling from/to other signals on the board, etc.