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Appendix E Register-Level Programming
© National Instruments Corporation E-25 Lab-PC+ User Manual
Modes of Operation for the 8255A
The three basic modes of operation for the 8255A are as follows:
• Mode 0 – Basic I/O
• Mode 1 – Strobed I/O
• Mode 2 – Bidirectional bus
The 8255A also has a single bit set/reset feature for Port C. The 8-bit control word also
programs this function. For additional information, refer to Appendix C, OKI 82C55A Data
Sheet.
Mode 0
This mode is for simple I/O 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 nibble of Port C).
• Any port can be input or output.
• Outputs are latched, but inputs are not latched.
Control Words
Mode 0 provides simple I/O functions for each of the three ports with no handshaking. Each port
can be assigned as an input port or as an output port. The 16 possible I/O configurations are
shown in Table E-5. Notice that bit 7 of the control word is set when programming the mode of
operation for each port.