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The6551Asynchronous Communications Interface Adapter (ACIA) is anintegrated circuit made byMOS Technology.[1] It served as a companionUART chip for the widely popular6502microprocessor. Intended to implementRS-232, its specifications called for a maximum speed of 19,200bits per second 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 theCommodore PET andPlus/4. It was also used byApple Computer on theApple II Super Serial Card, and byRadio Shack on the Deluxe RS-232 Program Pak for theirTRS-80 Color Computer.
Several companies, including Dr. Evil Labs andCreative Micro Designs, marketed an add-oncartridge containing a 6551 and an industry-standard RS-232 port to allow the C64 and 128 to use high-speed modems from companies such asU.S. Robotics andHayes 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.
TheRockwell 65C52 combines twoCMOS 6551s on a chip.
TheMotorola 6850 is a similar chip to the MOS Technology 6551, but without an onboard bit rate generator. The 6850 is often used forMIDI.
TheWestern 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.