Hi,
Unfortunately, we have only uart example in SDK package for LPC55S6x, but we have not RS485 example code. For RS485, you have to use a GPIO to control transmitter to enable/disable based on uart module.
Pls download SDK from the link:
MCUXpresso SDK | Software Development for Kinetis, LPC, and i.MX MCUs | NXP
BR
Xiangjun Rong