The Rockwell R6551 Asynchronous Communications lnterface Adapter (ACIA) provides an easily implemented, program controlled interface between 8-bit microprocessor-based systems and serial communication data sets and modems.
The ACIA has an internal baud rate generator. This feature eliminates the need for multiple component support circuits, a crystal being the only other part required. The Transmitter baud rate can be selected under program contro! to be eithec 1 of 15 different rates trom 50 to 19,200 baud, or at 1'16 times an external clock rate. The Receiver baud rate may be selected under program control to be either the Transmitter rate, or at 1/16 times an extemal clock rate. The ACIA has programmable word lengths of 5, 6, 7, or 8 bits; even, odd, or no parity; 1, 11h, or 2 stop bits.
The ACIA is designed for maximum programmed control from the microprocessor (MPU), to simplify hardware implementation. TIYee separate registers permit the MPU to easily select the R6551's operating modes and data checking parameters and determine operational status.
The Command Register controls parity, receiver echo mode, transmitter interrupt contro!, the state of the RTS line, receiver interrupt contrai, and the state of the DTR line.
The Controi Register controls the number of stop bits, word length, receiver clock source, and baud rate.
The Status Register indrcates the states of the IRQ, OSA, and DCD hnes, Transmitter and Receiver Data Registers, and Overrun, Framing, and Parity Error conditions.
The Transmitter and Receiver Data Registers are used for temporary data storage by the ACIA Transmit and Receiver circuits.
Features
•Compatible with 8-bit microprocessors
•Full duplex operatlon with buffered receiver and transmitter
•Data set/modem contro! functions
•Internal baud rate generator with 15 programmable baud rates (50 to 19,200)
•Program-selectable internally or externally controlled rece1ver rate
•Programmable word lengths, number of stop b1ts, and parrty bit generat1on and detection
•Programmable interrupt contro!
•Program reset
•Program-selectable serial echo mode
•Two chip selects
•2 or 1 MHz operation
•5.0 Vdc :: 5% supply requirements
•28-pin plastic or ceramic DIP
•Full TTL compatibility
•Compatible with A6500, R6500/' and R65COO m1cro processors
R6551AP 
Asynchronous Communications lnterface Adapter
