Input/Output Packages
J. C. Cluley · 1983
Most parallel I/O packages provide two 8-bit external ports together with additional control and status lines. Typical of these is the Motorola M6821. This was originally produced as one of the M6800 packages under the type number M6820. The M6821 is almost identical as far as the user is concerned, but has slightly different internal logic and some functional improvements.