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MOS Technology 6551
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<p>The 6551 Asynchronous Communications Interface Adapter (ACIA) is an <a href="/facts/Integrated_circuit/fjn6vg7C">integrated circuit</a> made by <a href="/facts/MOS_Technology/1eKsjBS1">MOS Technology</a>. It served as a companion <a href="/facts/Universal_asynchronous_receiver-transmitter/xMntJcsI">UART</a> chip for the widely popular <a href="/facts/MOS_Technology_6502/q9YLwkzm">6502</a> <a href="/facts/Microprocessor/Fx9CaQht">microprocessor</a>. Intended to implement <a href="/facts/RS-232/NCpvGuyq">RS-232</a>, its specifications called for a maximum speed of 19,200 <a href="/facts/Bits_per_second/Nv9VoT7T">bits per second</a> 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 <a href="/facts/Commodore_PET/08R12Xk4">Commodore PET</a> and <a href="/facts/Commodore_Plus%2f4/ubQLsy5o">Plus/4</a>. It was also used by <a href="/facts/Apple_Computer/oTJYta3k">Apple Computer</a> on the <a href="/facts/Apple_II_serial_cards/sDcPbAny">Apple II Super Serial Card</a>, and by <a href="/facts/RadioShack/EHzYMpkP">Radio Shack</a> on the Deluxe RS-232 Program Pak for their <a href="/facts/TRS-80_Color_Computer/KISXB65L">TRS-80 Color Computer</a>. </p><p>Several companies, including Dr. Evil Labs and <a href="/facts/Creative_Micro_Designs/eV4T7qhC">Creative Micro Designs</a>, marketed an add-on <a href="/facts/ROM_cartridge/mWLcNez7">cartridge</a> 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 <a href="/facts/U.S._Robotics/Nwaocofx">U.S. Robotics</a> and <a href="/facts/Hayes_Communications/bvAwftJx">Hayes Communications</a>. 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. </p>