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Chapter 3 Theory of Operation
© National Instruments Corporation 3-9 Lab-NB User Manual
corresponding to an LSB change in the digital code word. For both unipolar and bipolar output,
one LSB corresponds to the following formula:
10 V
4,096
Digital I/O Circuitry
The digital I/O circuitry is designed around an 82C55A integrated circuit. The 82C55A is a
general-purpose 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
Lab-NB I/O connector. The 82C55A also has a control register to configure each of the three
I/O ports on the chip. These ports can be programmed as two groups of 12 signals or as three
individual 8-bit ports. In addition, the board can be programmed in one of the three modes of
operation: basic I/O, strobed I/O, or bidirectional bus. The programming of the digital I/O
circuitry is covered in Chapter 4, Register-Level Programming.
82C55A
Programmable
Peripheral
Interface
To
Interrupt
Control
DIO RD/WR
DATA<0..7>
PC0
PC3
PC<0..7>
PB<0..7>
PA<0..7>
I/O Connector
NuBus
2
8
8
8
Figure 3-5. Digital I/O Circuitry Block Diagram