MOS Technology 6551

The MOS Technology 6551 was an Asynchronous Communications Interface Adapter ACIA shortly. He worked primarily as a support for the 6502 microprocessor. Conceived to implement a RS- 232 interface, was his top speed according to specification 19,200 bits per second when using an external crystal and internal divider. The shoe was designed largely by William (Bill) D. man, Jr., who had been the very similar 6850 previously developed at Motorola. The MOS6551 been used in many computers, including the Commodore PET, Commodore Plus / 4 and in the Super Serial Card by Apple Computer. In the Atari ST, he was responsible for keyboard, mouse, and MIDI.

Commodore International in 6551 avoided the known VC 20, C64 and C128. There is a substantially for 6551 compatible UART is implemented as software. However, this software RS- 232 interface was only useful up to 1200 bit / s (see also errata for 6526 ). Skillful programmers succeeded yet to realize even greater speeds.

Many companies, like Dr. Evil Labs or Creative Micro Designs (CMD ), sold plug with the 6551 ( Swift Link -232, Data Blast, CommPort ) and allowed as a standard RS -232 port for the C64 and C128. When Dr. Evil Lab and the CMD Card to 38,400 bits / s were possible. As for the external crystal frequency of 1.8432 million Hz was provided and you could turn off the splitter in the divider registers, so that only the 16:1 prescaler remained active, a maximum baud rate of 115,200 bits per second was possible.


The R6551 was a replica of Rockwell, the R65C51 the CMOS version of the block, the R65C51x2 (and the MOS6551A ) accepted a system clock ( Phi 2 ) of 2 MHz, the R65C52 combined two 65C51 on a chip. Other replicas with partly higher clock rates, there was also Synertek and Harris.

  • I / O- chip
  • Commodore
  • MOS Technology