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  8250 and Compatible UARTs

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 of UARTs - 16550, 16450 & 8250
Pin Diagrams for 16550, 16450 & 8250 UARTs

Pin No.Name Notes
Pin 1:8D0:D7 Data Bus
Pin 9RCLK Receiver Clock Input. Its frequency should equal the receiver's baud rate x 16
Pin 10RD Receive Data
Pin 11TD Transmit Data
Pin 12CS0 Chip Select 0 - Active High
Pin 13CS1 Chip Select 1 - Active High
Pin 14nCS2 Chip Select 2 - Active Low
Pin 15nBAUDOUT Baud Output - Output from Programmable Baud Rate Generator. Frequency = (Baud Rate x 16)
Pin 16XIN External Crystal Input - Used for Baud Rate Generator Oscillator
Pin 17XOUT Crystal Output (external)
Pin 18nWR Inverted Write Line
Pin 19WR Write Line-Not Inverted
Pin 20VSS Connected to Common Ground
Pin 21RD Inverted Read Line
Pin 22nRD Read Line - Not Inverted
Pin 23DDIS Driver Disable. It goes low when CPU is reading from UART. Can be connected to Bus Transceiver if data bus has high capacity .
Pin 24nTXRDY Transmit Ready
Pin 25nADS Address Strobe. Used if signals are unstable during read or write cycle
Pin 26A2 Address Bit 2
Pin 27A1 Address Bit 1
Pin 28A0 Address Bit 0
Pin 29nRXRDY Receive Ready
Pin 30INTR Interrupt Output
Pin 31nOUT2 User Output 2
Pin 32nRTS Request to Send
Pin 33nDTR Data Terminal Ready
Pin 34nOUT1 User Output 1
Pin 35MR Master Reset
Pin 36nCTS Clear To Send
Pin 37nDSR Data Set Ready
Pin 38nDCD Data Carrier Detect
Pin 39nRI Ring Indicator
Pin 40VDD + 5 Volts
Pin Assignments for 16550A UART
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