Читайте документацию
Receiver Output Enable.
RO is enabled when RE is low;
RO ishigh impedance when RE is high.
Это просто пин включения приемника.
DE - Driver Output Enable. The driver outputs, Y and Z, are enabled by bringing DE high. They are high impedance when DE is low. If the driver outputs are enabled, the parts function as line drivers. While they are high impedance, they function as line receivers if RE is low.
Это пин включения передатчика. Зачем вам RTC/CTS ?