National Instruments PC-DIO-24/PnP - Manual (Page 62)

Appendix C Register-Level Programming
PC-DIO-24/PnP User Manual C-2 © National Instruments Corporation
The PC-DIO-24 (non-PnP) does not have interrupt control registers.
Instead, it uses one of the port C lines to enable or disable interrupts.
See Appendix D, Using Your PC-DIO-24 (Non-PnP) Board for more
information.
The three 8-bit ports of the 82C55A are divided into two groups of
12 signals each: group A and group B. One 8-bit control word selects
the modes of operation for both groups. The group A control bits
configure port A (A7 through A0) and the upper 4 bits (nibble) of
port C (C7 through C4). The group B control bits configure port B
(B7 through B0) and the lower nibble of port C (C3 through C0). These
configuration bits are defined in the Register Description for the
82C55A section later in this appendix.
The 82C55A potentially requires up to 200 ns recovery time between
consecutive read or write cycles. Certain computers may provide
slightly less time than this between two back-to-back assembly-
language reads or writes. If you are programming in assembly language,
it is therefore recommended that you separate two 82C55A reads or
writes with at least one other instruction.