FRDM-K64F is nice piece of development board. Unfortunately, it is “bonded” to MBED development platform, which is a nightmare! Adding RS485 “out of the box” to the FRDM-K64F in MBED is NoGo. But it is very easy.
Here is image from my scope sending single char via UART4 with enabled RTS as DE for RS485 in Half Duplex mode: