The 6551 Asynchronous Communications Interface Adapter (ACIA) is an

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

made by

MOS Technology MOS Technology, Inc. ("MOS" being short for Metal Oxide Semiconductor), later known as CSG (Commodore Semiconductor Group) and GMT Microelectronics, was a semiconductor design and fabrication company based in Audubon, Pennsylvania. It is ...

. It served as a companion

UART A universal asynchronous receiver-transmitter (UART ) is a peripheral device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to ...

chip for the widely popular

6502 The MOS Technology 6502 (typically pronounced "sixty-five-oh-two" or "six-five-oh-two") William Mensch and the moderator both pronounce the 6502 microprocessor as ''"sixty-five-oh-two"''. is an 8-bit microprocessor that was designed by a small ...

microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

. Intended to implement

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

, its specifications called for a maximum speed of 19,200

bits per second In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction ...

with its onboard baud-rate generator, or 125 kbit/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

Plus/4 The Commodore Plus/4 is a home computer released by Commodore International in 1984. It was part of the Commodore 264 series, which also included the Commodore 16 and Commodore 116 models. The Plus/4 was marketed as "the productivity computer wit ...

. It was also used by

Apple Computer Apple Inc. is an American multinational corporation and technology company headquartered in Cupertino, California, in Silicon Valley. It is best known for its consumer electronics, software, and services. Founded in 1976 as Apple Computer Co ...

on the Apple II Super Serial Card, and by

Radio Shack RadioShack (formerly written as Radio Shack) is an American electronics retailer that was established in 1921 as an amateur radio mail-order business. Its parent company was purchased by Tandy Corporation in 1962, which shifted its focus from ma ...

on the Deluxe RS-232 Program Pak for their

TRS-80 Color Computer The RadioShack TRS-80 Color Computer, later marketed as the Tandy Color Computer, is a series of home computers developed and sold by Tandy Corporation. Despite sharing a name with the earlier TRS-80, the Color Computer is a completely different ...

. Several companies, including Dr. Evil Labs and

Creative Micro Designs Creative Micro Designs, Inc. (CMD) was founded in 1987 by Doug Cotton and Mark Fellows. It is a computer technology company which originally developed and sold products for the Commodore 64 and C128 8-bit personal computers. After 2001 it sold P ...

, 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 and

Hayes Communications Hayes Microcomputer Products was a US-based manufacturer of modems. The company is known for the Smartmodem, which introduced a control language for operating the functions of the modem via the serial interface, in contrast to manual operation wi ...

. 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 Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss ", , ) is a type of MOSFET, metal–oxide–semiconductor field-effect transistor (MOSFET) semiconductor device fabrication, fabrication process that uses complementary an ...

6551s on a chip.

Similar chips

The Motorola 6850 is a similar chip to the MOS Technology 6551, but without an onboard bit rate generator. The 6850 is often used for

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

. 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. The WDC 65C51 has errata, in which the transmitter “ready” bit is “stuck” in the ready state.

References

External links

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