The WDC W65C21 (W65C21N and W65C21S) is a very flexible Peripheral Interface Adapter (PIA) for use with WDC’s 65xx, 68xx, and other 8-bit microprocessor families. The W65C21 provides programmed microprocessor control of up to two peripheral devices (Port A and Port B). Peripheral device control is accomplished through two 8-bit bidirectional I/O Ports, with individually designed Data Direction Registers. The Data Direction Registers provide selection of data flow direction (input or output) at each respective I/O Port. Data flow direction may be selected on a line-by-line basis with intermixed input and output lines within the same port. The “handshake” interrupt control feature is provided by four peripheral control lines. This capability provides enhances control over data transfer functions between the microprocessor and peripheral devices, as well as bidirectional data transfer between W65C21 Peripheral Interface Adapters in multiprocessor systems.

• 2 x 8−bit bidirectional I/O ports with individual data direction control
• Automatic "Handshake" control of data transfers
• 2 Interrupt outputs with program control
• Static to 14MHz operation, with high speed Port A, CA2 outputs
• 40 Pin Plastic Dip and 44 Pin Plastic PLCC versions
• 5V Power Supply
• Bus compatible with 14 MHz W65C02S and W65C816S
• Compatible with the 65xx and 68xx family of microprocessors
• Replacement for Motorola/Freescale / Rockwell / AMI / MOS Technology / MOSTEK / HITACHI / ST Microelectronics / GTE / CMD, 6520, 6521, 6820, 6821, PIA′s
  

• The W65C21S is lower power, faster and direct drive outputs with no current limiting resistors on outputs ports and TTL input thresholds.
• The W65C21N is plug replacement of NMOS 6521 and 6821 devices with current limiting resistors on outputs ports and TTL input thresholds.
• The W65C21N Port A input buffers supply 200uA pull-up current at 2.4V in the input mode, while the W65C21S Port A input buffers supply 50uA pull-up current at 2.4V in the input mode.