The 6551 Asynchronous Communications Interface Adapter (ACIA) was an integrated circuit made by MOS Technology. It served as a companion UART chip for the widely popular 6502

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...

. Intended to implement

RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' ('' data terminal equipment'') suc ...

, its specifications called for a maximum speed of 19,200 bits per second with its onboard baud-rate generator, or 125kbit/s using an external 16x clock. The 6551 was used in several computers of the 1970s and 1980s, including the

Commodore PET The Commodore PET is a line of personal computers produced starting in 1977 by Commodore International. A single all-in-one case combines a MOS Technology 6502 microprocessor, Commodore BASIC in read-only memory, keyboard, monochrome monitor, ...

and Commodore Plus/4. It was also used by

Apple Computer Apple Inc. is an American multinational technology company headquartered in Cupertino, California, United States. Apple is the largest technology company by revenue (totaling in 2021) and, as of June 2022, is the world's biggest company ...

on the Super Serial Card for their Apple II series, and by Radio Shack on the Deluxe RS-232 Program Pak for their Color Computer.

Commodore International Commodore International (other names include Commodore International Limited) was an American home computer and electronics manufacturer founded by Jack Tramiel. Commodore International (CI), along with its subsidiary Commodore Business Mac ...

omitted the 6551 from the popular VIC-20,

C64 The Commodore 64, also known as the C64, is an 8-bit home computer introduced in January 1982 by Commodore International (first shown at the Consumer Electronics Show, January 7–10, 1982, in Las Vegas). It has been listed in the Guinness Wo ...

, and C128 home computers. Instead, these systems implemented a

bit-banging In computer engineering and electrical engineering, bit banging is a "term of art" for any method of data transmission that employs software as a substitute for dedicated hardware to generate transmitted signals or process received signals. Soft ...

UART via KERNAL routines. This RS-232 implementation was not reliable over 1200 bit/s (see errata comments in the 6526 article), forcing some programmers of terminal programs to write carefully calibrated custom serial routines. The popular terminal program NovaTerm was able to achieve 4800 bit/s on the C64, and DesTerm achieved 9600 bit/s on the C128. Several other terminal programs achieved 2400 bit/s. Novaterm 9.6 on a Commodore 64 or 128 can achieve a maximum rate of 9600 bit/s on the user port, using an EZ-232 interface, designed by Jim Brain. Several companies, including

Dr. Evil Labs Doctor is an academic title that originates from the Latin word of the same spelling and meaning. The word is originally an agentive noun of the Latin verb 'to teach'. It has been used as an academic title in Europe since the 13th century, w ...

and Creative Micro Designs, marketed an add-on cartridge containing a 6551 and an industry-standard RS-232 port to allow the C64 and 128 to use high-speed modems from companies such as

U.S. Robotics U.S. Robotics Corporation, often called USR, is a company that produces USRobotics computer modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog ...

and Hayes Communications. The Dr. Evil and CMD cartridges pushed the 6551 to 38,400 baud and, with a faster-still clock crystal, some end users reported getting 115,200 bit/s from the 6551. The ADTPro file transfer program disables the baud rate generator in the 6551, allowing 115,200 bit/s transfers with an unmodified clock crystal.

Variants

The Rockwell 65C52 combines two CMOS 6551s on a chip.

Similar chips

The Motorola 6850 is a similar chip to the MOS Technology 6551. The 6850 is often used for

MIDI MIDI (; Musical Instrument Digital Interface) is a technical standard that describes a communications protocol, digital interface, and electrical connectors that connect a wide variety of electronic musical instruments, computers, an ...

. The Western Design Center WDC 65C51 is designed as a drop in replacement for the original MOS 6551, electrically, physically and programming- compatible with most 6551 and 6850 derivatives from most other suppliers.

External links

Datasheet
{{MOS Interface MOS Technology integrated circuits Input/output integrated circuits