Digital I/O - National Instruments DAQPad-1200 User Manual

Chapter 4 Theory of Operation

Digital I/O

PA<0..7>
PB<0..7>
PC<0..7>
DAQCard-1200 User Manual
The DIO circuitry has an 82C55A integrated circuit. The 82C55A is a
general-purpose programmable peripheral interface (PPI) containing
24 programmable I/O pins. These pins represent the three 8-bit I/O ports
(A, B, and C) of the 82C55A, as well as PA<0..7>, PB<0..7>, and
PC<0..7> on the DAQCard-1200 I/O connector. Figure 4-6 shows the DIO
circuitry.
8
8
82C55A
8
Programmable
Peripheral
Interface
PC0
PC3
All three ports on the 82C55A are TTL-compatible. When enabled, the
digital output ports are capable of sinking 2.4 mA of current and sourcing
2.6 mA of current on each DIO line. When the ports are not enabled, the
DIO lines act as high-impedance inputs.
DATA<0..7>
Decode
Circuitry
DIO RD/WRT
2
To
Interrupt
Control
Figure 4-6. Digital I/O Circuitry
4-12
Data
8
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