National Instruments PC-DIO-96/PnP, User Manual

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Chapter 4 Theory of Operation
© National Instruments Corporation 4-3 PC-DIO-96/PnP User Manual
The 82C53 device has two of its three counter output signals connected
to the interrupt circuitry. Any of these 10 signals can interrupt the host
computer if the interrupt circuitry is enabled and the corresponding
enable bit is set (see Appendix D, Register-Level Programming, for
more information). Normally, PC3 and/or PC0 of the 82C55A devices
are controlled by the handshaking circuitry; however, either of these
two lines can be configured for input and used as external interrupts. An
interrupt occurs on the low-to-high transition of the signal line. Refer to
Appendix D, Register-Level Programming, Appendix B, OKI 82C55A
Data Sheet, or Appendix C, OKI 82C53 Data Sheet, for more detailed
information.
82C55A Programmable Peripheral Interface
The four 82C55A PPI chips are the heart of the PC-DIO-96PnP. Each
of these chips has 24 programmable I/O pins that represent three 8-bit
ports: PA, PB, and PC. Each port can be programmed as an input or an
output port. The 82C55A has three modes of operation: simple I/O
(mode 0), strobed I/O (mode 1), and bidirectional I/O (mode 2). In
modes 1 and 2, the three ports are divided into two groups: group A and
group B. Each group has eight data bits and four control and status bits
from port C (PC). Modes 1 and 2 use handshaking signals from port C
to synchronize data transfers. Refer to Appendix D, Register-Level
Programming, or to Appendix B, OKI 82C55A Data Sheet, for more
detailed information.
82C53 Programmable Interval Timer
The 82C53 Programmable Interval Timer generates timed interrupt
requests to the host computer. The 82C53 has three 16-bit counters,
which can each be used in one of six different modes. The
PC-DIO-96PnP uses two of the counters to generate interrupt requests;
the third counter is not used and is not accessible to the user. Refer to
Appendix D, Register-Level Programming, or to Appendix C, OKI
82C53 Data Sheet, for more detailed information.