The MOS 6551 Asynchronous Communications Interface Adapter was a simple UART chip produced by MOS Technology. A companion chip to the widely popular 6502 microprocessor, it was intended to be used to implement RS-232, and its specifications called for a maximum speed of 19,200 bits per second. It was used in the Commodore PET, Commodore Plus/4, Apple Computer's Super Serial Card for the Apple II family, the Atari 800 family, and possibly other lesser-known computers.

Commodore International infamously omited the 6551 from the popular VIC-20, C64, and C128 home computers, instead implementing RS-232 entirely in software. The default RS-232 implementation that was bundled with the operating system was not reliable over 1200 bit/s (1200 bit/s was already a bit of a stretch, 300 bit/s was barely within spec). This forced 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.

Several companies, including Dr. Evil Labs and Creative Micro Devices , 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 US Robotics 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.