6551

6551 can refer to the decimal number itself or to a hardware part number used in vintage computing. In the context of computer hardware, the MOS Technology 6551 ACIA (Asynchronous Communications Interface Adapter) is a single-chip universal asynchronous receiver-transmitter designed for 8-bit microprocessor systems. It provides a serial interface for data communication by combining a UART with a small amount of control logic and a baud-rate generator.

The 6551 ACIA presents an 8-bit data path to the host system and interfaces with a serial port through external line drivers and level shifters. It supports asynchronous serial communication, with a programmable baud rate and a set of status and control registers that allow the host processor to configure transmission and reception, monitor the status of the serial lines, and manage interrupts. Typical use cases include connecting 8-bit microcomputers to modems, printers, or other serial peripherals of the era. The device generates interrupts for events such as received data, transmit buffer status changes, and error conditions, enabling responsive serial I/O within the constraints of the CPU’s timing.

The 6551 ACIA was one of several serial interface options available during the 1980s for 6502- and compatible-bus systems. Over time, newer interfaces and higher-integration peripherals largely supplanted it, but the 6551 remains of interest to vintage-computing enthusiasts and retro computing projects. See also related serial interface devices such as the Intel 8251 and other ACIA/USART implementations.