The 6551 Asynchronous Communications Interface Adapter (ACIA) was an integrated circuit

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

 made by MOS Technology

MOS Technology

MOS Technology, Inc., also known as CSG , was a semiconductor design and fabrication company based in Norristown, Pennsylvania, in the United States. It is most famous for its 6502 microprocessor, and various designs for Commodore International's range of home computers.-History:MOS Technology, Inc...

. It served as a companion UART chip for the widely popular 6502

MOS Technology 6502

The MOS Technology 6502 is an 8-bit microprocessor that was designed by Chuck Peddle and Bill Mensch for MOS Technology in 1975. When it was introduced, it was the least expensive full-featured microprocessor on the market by a considerable margin, costing less than one-sixth the price of...

 microprocessor

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

. Intended to implement RS-232

RS-232

In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...

, its specifications called for a maximum speed of 19,200 bits per second. It was used in the Commodore PET

Commodore PET

The Commodore PET was a home/personal computer produced from 1977 by Commodore International...

, Commodore Plus/4

Commodore Plus/4

The Commodore Plus/4 was a home computer released by Commodore International in 1984. The "Plus/4" name refers to the four-application ROM resident office suite ; it was billed as "the productivity computer with software built-in"...

, Apple Computer

Apple Computer

Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...

's Super Serial Card

Apple II serial cards

Apple II serial cards primarily used the serial RS-232 protocol. They most often were used for communicating with printers, Modems, and less often for computer to computer data transfer. They could be programmed to interface with any number of external devices which were RS-232 compatible...

 for the Apple II family, and possibly other lesser-known computers.

Commodore International

Commodore International

Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...

 omitted the 6551 from the popular VIC-20

Commodore VIC-20

The VIC-20 is an 8-bit home computer which was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the PET...

, C64

Commodore 64

The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

, and C128

Commodore 128

The Commodore 128 home/personal computer was the last 8-bit machine commercially released by Commodore Business Machines...

 home computer

Home computer

Home computers were a class of microcomputers entering the market in 1977, and becoming increasingly common during the 1980s. They were marketed to consumers as affordable and accessible computers that, for the first time, were intended for the use of a single nontechnical user...

s. Instead, these systems implemented a bit-banging

Bit-banging

Bit banging is a technique for serial communications using software instead of dedicated hardware. Software directly sets and samples the state of pins on the microcontroller, and is responsible for all parameters of the signal: timing, levels, synchronization, etc...

 UART via KERNAL

KERNAL

The KERNAL is Commodore's name for the ROM-resident operating system core in its 8-bit home computers; from the original PET of 1977, followed by the extended but strongly related versions used in its successors; the VIC-20, Commodore 64, Plus/4, C16, and C128...

 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 and Creative Micro Designs

Creative Micro Designs

Creative Micro Designs is a computer technologies company which today sells PCs and related equipment, but which started out in 1987 selling self-designed firmware updates and hardware for the Commodore 64 and C128 8-bit home/personal computers....

, 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

USRobotics Corporation is a company that makes computer modems and related products. It sold high-speed modems in the 1980s, and had a reputation for high quality and compatibility. With the reduced usage of voiceband modems in North America in the early 21st century, USR is now one of the few...

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