The 6551 Asynchronous Communications Interface Adapter (ACIA) is an integrated circuit made by MOS Technology.
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 125 kbit/s using an external 16x clock.
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.
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 Motorola 6850 is a similar chip to the MOS Technology 6551, but without an onboard bit rate generator.