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National Instruments Corporation
B-1
DAQCard-DIO-24 User Manual
Appendix
B
Register-Level
Programming
This appendix describes in detail the address and function of each of the
DAQCard-DIO-24 control and status registers.
Register Map and Descriptions
The DAQCard-DIO-24 is a parallel digital I/O card that contains
the 82C55A integrated circuit. The 82C55A is a general-purpose
peripheral interface containing 24 programmable I/O pins. These pins
represent three 8-bit I/O ports—port A, port B, and port C. These ports
can be programmed as two handshaking ports or as two 8-bit ports and
two 4-bit ports that perform simple I/O.
The three 8-bit ports of the 82C55A are divided into two groups
of 12 signals—group A and group B. One 8-bit configuration (or
control) word determines the mode of operation for each group.
Group A control bits configure PA<0..7> and the upper 4 bits (nibble)
of PC, PC<4..7>. Group B control bits configure PB<0..7> and the
lower nibble of PC, PC<0..3>. These configuration bits are defined later
in this appendix.
In addition to the registers on the 82C55A interface, the
DAQCard-DIO-24 provides registers that select which onboard signals
are capable of generating interrupts. The 82C55A interface uses two
interrupt signals, INTRA and INTRB. Individual enable bits select
which of these two signals can generate interrupts. Also, a master
enable signal determines whether the card can actually send an interrupt
request to the host computer. For more information about the
configuration bits for these registers, refer to the Interrupt Control
Registers section later in this appendix.
