The 6551 Asynchronous Communications Interface Adapter (ACIA) is 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 125 kbit/s using an external 16x clock. The 6551 was used in several computers of the 1970s and 1980s, including the Commodore PET and Plus/4. It was also used by Apple Computer on the Apple II Super Serial Card, and by Radio Shack on the Deluxe RS-232 Program Pak for their TRS-80 Color Computer.
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.
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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, but without an onboard bit rate generator. 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. The WDC 65C51 has errata, in which the transmitter “ready” bit is “stuck” in the ready state.
External links
This page was last edited on 23 April 2024, at 15:42