MOS Technology 6551

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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. Intended to implement RS-232, 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, Atari ST (where it is used for keyboard, mouse and MIDI support) and possibly other lesser-known computers.

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 (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.

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 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.

[edit] Variants

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

Interface chips from MOS Technology and second source/clone vendors

6520 PIA ● 6522 VIA ● 6523 TIA ● 6525 TPI ● 6526 CIA ● 6529 SPI ● 6530 RRIOT ● 6532 RIOT ● 6551 ACIA