We have already dealt with one type of UART, the 8250 and compatibles found in your PC. These devices have the configuration of the registers available via address bus and data bus which help initialize the device before its usage. It's not a problem if your device is controlled by a microprocessor, but there are cases when this device is free-running. In this case there may be problems initializing the device. 

Most  microprocessors and microcontrollers have built-in Serial Communication Interfaces (SCI). This has narrowed the scope of application for 40 pin type 16550 connections (e.g. 68HC11).If you still prefer Z-80 or 8086, then application of 16550 is optional.

Pin Diagrams for 16550, 16450 & 8250 UARTs