Hi Justin,
on the TX, RX pins of the SC16IS740 you will receive Differential signal.
You need to a transceiver, like the MAX485 to transfer it to unipolar signal referenced to GND. And wise versa, transfer unipolar signal to differential signal.
Yes, you need to either use a Crystal connected between XTAL1 and XTAL2, or you can use an external clock connected to XTAL1. XTAL2 can then be used as Clock output.
With Best Regards,
Jozef
