MOS Technology 6551

The MOS Technology 6551 is an Asynchronous Communications Interface Adapter , ACIA for short . He worked primarily as support for the 6502 - microprocessor . In order to implement an RS-232 interface, its top speed according to specification is 19,200 bits per second when using an external crystal and the internal divider. The design came largely from William (Bill) D. Mensch Jr., who had previously developed the very similar 6850 at Motorola. The MOS6551 was used in numerous computers including the Commodore PET , Commodore Plus / 4 and the Super Serial Card from Apple Computer .

Many companies, like Dr. Evil Labs or Creative Micro Designs (CMD), sold plug-in cards with the 6551 (SwiftLink-232, Datablast, CommPort) and thus enabled a standard RS-232 port for the C64 and C128. At the Dr. Evil Lab and the CMD card were possible up to 38,400 bits / s. Since a frequency of 1,843,200 Hz was intended for the external crystal and the divider could be switched off in the divider register so that only the 16: 1 prescaler remained active, a maximum baud rate of 115,200 bits per second was possible.

variants

Licensee ( Second Source ) Rockwell brought in addition to the R6551 the CMOS version R65C51 , a 2 MHz version R65C51x2 ( MOS6551A ) clocked with Phi2 and the dual ACIA R65C52 , which offered two 65C51 compatible interfaces in one module. Other licensees, such as Synertek and Harris, also offered versions with sometimes higher clock rates.

Web links

• Datasheet 6551 , PDF, English
• Component versions and pinout , English