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Chapter 2 Configuration and Installation
© National Instruments Corporation 2-11 Lab-NB User Manual
Figure 2-9 illustrates signal connections for three typical digital I/O applications.
14 PA0
22 PB0
30 PC0
13
DGND
Lab-NB Board
Switch
I/O Connector
+5 V
+5 V
LED
TTL Signal
Port B
PB<7..0>
Port A
PA<7..0>
Port C
PC<7..0>
Figure 2-9. Digital I/O Connections
In Figure 2-9, port A is configured for digital output, and ports B and C are configured for digital
input. Digital input applications include receiving TTL signals and sensing external device states
such as the switch in Figure 2-9. Digital output applications include sending TTL signals and
driving external devices such as the LED shown in Figure 2-9.
Port C Pin Connections
The signals assigned to port C depend on the mode in which the 82C55A is programmed. In
mode 0, port C is considered as two 4-bit I/O ports. In modes 1 and 2, port C is used for status
and handshaking signals with two or three I/O bits mixed in. The following table summarizes
the signal assignments of port C for each programmable mode. See Chapter 4, Register-Level
Programming, for programming information.
Warning: During programming, note that each time a port is configured, output ports A
and C are reset to 0, and output port B is undefined.