This device is commonly called a UART (Universal Asynchronous Receiver/Transmitter). There is one for each COM port of the computer. They allow communication between computers directly or via telephone modems.

Asynchronous Communications Adapter

The Asynchronous Communications Adapter is a 4“H × 5”W card that plugs into a System Expansion Slot. All system control signals and voltage requirements are provided through a 2 × 31 position card edge tab. A jumper module is provided to select either RS-232-C or current loop operation.

The adapter is fully programmable and supports asynchronous communications only. It will add and remove start bits, stop bits, and parity bits. A programmable baud rate generator allows operation from 50 baud to 9600 baud. Five, six, seven or eight bit characters with 1, 1–1/2, or 2 stop bits are supported. A fully prioritized interrupt system controls transmit, receive, error, line status and data set interrupts. Diagnostic capabilities provide loopback functions of transmit/receive and input/output signals.

Figure (22) is a block diagram of the Asynchronous Communications Adapter.

The heart of the adapter is a INS8250 LSI chip or functional equivalent. The following is a summary of the 8250's key features:

• Adds or Delete Standard Asynchronous Communication Bits (Start, Stop, and Parity) to or from Serial Data Stream.

• Full Double Buffering Eliminates Need for Precise Synchronization.

• Independently Controlled Transmit, Receive, Line Status, and Data Set Interrupts.

• Programmable Baud Rate Generator Allows Division of Any Input Clock by 1 to (216–1) and Generates the Internal 16× Clock.

• Independent Receiver Clock Input.