User manual

Appendix D Register-Level Programming
© National Instruments Corporation D-11 PC-DIO-96/PnP User Manual
Programming Considerations for the 82C55A
Modes of Operation for the 82C55A
The three basic modes of operation for the 82C55A are as follows:
• Mode 0—Basic I/O
• Mode 1—Strobed I/O
• Mode 2—Bidirectional bus
The 82C55A also has a single bit set/reset feature for port C, which is
programmed by the 8-bit control word. For additional information, refer
to Appendix B, OKI 82C55A Data Sheet.
Mode 0
This mode can be used for simple input and output operations for each
of the ports. No handshaking is required; data is simply written to or
read from a specified port.
Mode 0 has the following features:
• Two 8-bit ports (A and B) and two 4-bit ports (upper and lower
nibbles of port C).
• Any port can be input or output.
• Outputs are latched, but inputs are not latched.
Mode 1
This mode transfers data that is synchronized by handshaking signals.
Ports A and B use the eight lines of port C to generate or receive the
handshake signals. This mode divides the ports into two groups
(group A and group B) and includes the following features:
• Each group contains one 8-bit data port (port A or port B) and one
4-bit control/data port (upper or lower nibble of port C).
• The 8-bit data ports can be either input or output, both of which are
latched.
• The 4-bit ports are used for control and status of the 8-bit data ports.
• Interrupt generation and enable/disable functions are available.