Programming
Chapter 4
AT-MIO-16D User Manual
4-78
© National Instruments Corporation
interrupt. This interrupt is helpful when using the DACs to implement a waveform generator.
This interrupt can also be used to interrupt on an external signal connected to the OUT2 pin. If
OUT2 is connected to an external signal, make certain that Counter 2 is reset as described under
the Resetting the Hardware after a Data Acquisition Operation section earlier in this chapter. If
Counter 2 is not reset, it may drive the OUT2 line, preventing the external signal from
successfully driving the line high or low.
DIO-24 Circuitry Programming Considerations
The DIO-24 circuitry is designed around the 82C55A integrated circuit. The 82C55A is a
general-purpose peripheral interface containing 24 programmable I/O pins. These pins represent
the three 8-bit I/O ports (A, B, and C) of the 82C55A. These ports can be programmed as two
groups of 12 signals or as three individual 8-bit ports. This section includes programming
information for the DIO-24 circuitry, along with program examples written in C.
The three 8-bit ports of the 82C55A are divided into two groups–Group A and Group B (two
groups of 12 signals). One 8-bit configuration (or control) word determines the mode of
operation for each group. The Group A control bits configure Port A (A0 through A7) and the
upper 4 bits (nibble) of Port C (C4 through C7). The Group B control bits configure Port B (B0
through B7) and the lower nibble of Port C (C0 through C3). These configuration bits are
defined later in this chapter.
DIO-24 Circuitry Register Descriptions
Figure 4-2 shows the two control-word formats used to completely program the 82C55A. The
Control Word Flag determines which control-word format is being programmed. When the
Control Word Flag is 1, bits 0 through 6 determine the I/O characteristics of the 82C55A ports
and the mode in which they are operating (that is, Mode 0, Mode 1, or Mode 2). When the
Control Word Flag is 0, bits 3 through 0 determine the bit set/reset format of Port C.
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