MOS Technology 6551


The 6551 Asynchronous Communications Interface Adapter was an integrated circuit made by MOS Technology. It served as a companion UART chip for the widely popular 6502 microprocessor. Intended to implement RS-232, 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 and Commodore Plus/4. It was also used by Apple Computer 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 omitted the 6551 from the popular VIC-20, C64, and C128 home computers. Instead, these systems implemented a bit-banging UART via KERNAL routines. This RS-232 implementation was not reliable over 1200 bit/s, 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, 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. 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.
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.