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The CMOS W65C51N Asynchronous Communications Interface Adapter (ACIA) provides an easily implemented, program controlled interface between microprocessor based systems and serial communication data sets and modems. It is produced by Western Design Center (WDC) and is a drop-in replacement for the MOS Technology 6551.
The ACIA has an internal baud rate generator, eliminating the need for multiple component support circuits. The transmitter rate can be selected under program control to be 1 of 15 different rates from 50 to 19,200 bits per second, or at 1/16 times an external clock rate. The receiver rate may be selected under program control to be either the Transmitter rate or at 1/16 times the external clock rate. The ACIA has programmable word lengths of 5, 6, 7 or 8 bits; even, odd or no parity 1, 1½ or 2 stop bits.
The ACIA is designed for maximum programmed control from the microprocessor (MPU) to simplify hardware implementation. Three separate registers permit an MPU to easily select the W65C51N operating modes, data checking parameters and determine operational status.
The command register controls parity, receiver echo mode, transmitter interrupt control, the state of the RTS line, receiver interrupt control and the state of the DTR line.
The control register controls the number of stop bits, the word length, receiver clock source and transmit/receive rate.
The status register indicates the status of the IRQ, DSR and DCD lines, transmitter and receiver data Registers, and overrun, framing and parity error conditions.
Transmitter and receiver data Registers are used for temporary data storage by the transmit and receiver circuits, each able to hold one byte.