The Philips Semiconductors SCN2681 Dual Universal Asynchronous Receiver/Transmitter (DUART) is a single-chip MOS-LSI communications device that provides two independent full-duplex asynchronous receiver/transmitter channels in a single package.  It interfaces directly with microprocessors  and may be used in a polled or interrupt driven system.
The operating mode and data format of each channel can be programmed independently.  Additionally, each receiver and transmitter can select its operating speed as one of eighteen fixed baud rates, a 16X clock derived from a programmable counter/timer, or an external 1X or 16X clock.  The baud rate generator and counter/timer can operate directly from a crystal or from external clock inputs.  The ability to independently program the operating speed of the receiver and transmitter make the DUART particularly attractive for dual-speed channel applications such as clustered terminal systems.
Each receiver is quadruply buffered to minimize the potential of receiver over-run or to reduce interrupt overhead in interrupt driven systems.  In addition, a flow control capability is provided to disable a remote DUART transmitter when the buffer of the receiving device is full.
Also provided on the SCN2681 are a multipurpose 7-bit input port and a multipurpose 8-bit output port. These can be used as general purpose I/O ports or can be assigned specific functions (such as clock inputs or status/interrupt outputs) under program control.
The SCN2681 is available in three package versions: 40-pin and 28–pin, both 0.6” wide DIPs; a compact 24-pin 0.4” wide DIP; and a 44-pin PLCC.

FEATURES

• Dual full-duplex asynchronous receiver/transmitter

• Quadruple buffered receiver data registers

• Programmable data format – 5 to 8 data bits plus parity – Odd, even, no parity or force parity – 1, 1.5 or 2 stop bits programmable in 1/16-bit increments

• Programmable baud rate for each receiver and transmitter selectable from: – 22 fixed rates: 50 to 115.2k baud

• 16-bit programmable Counter/Timer – Non-standard rates to 115.2Kb – One user-defined rate derived from programmable timer/counter – External 1X or 16X clock

• Parity, framing, and overrun error detection • False start bit detection • Line break detection and generation • Programmable channel mode – Normal (full-duplex) – Automatic echo – Local  loopback – Remote loopback • Multi-function programmable 16-bit counter/timer

• Multi-function 7-bit input port – Can  serve as clock or control inputs – Change of state detection on four inputs – 100kΩ typical pull-up resistor

• Multi-function 8-bit output port – Individual bit set/reset capability – Outputs can be programmed to be status/interrupt signals

• Versatile interrupt system – Single interrupt output with eight maskable interrupting conditions – Output port can be configured to provide a total of up to six separate wire-ORable interrupt outputs

• Maximum data transfer: 1X – 1MB/sec, 16X – 125kB/sec

• Automatic wake-up mode for multidrop applications • Start-end break interrupt/status

• Detects break which originates in the middle of  a character • On-chip crystal oscillator

• Single +5V power supply

• Commercial and industrial temperature ranges available

• DIP and PLCC packages