National Instruments DAQCard-1200 - Manual (Page 66)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-13 DAQCard-1200 User Manual
Digital I/O
The digital I/O 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
digital I/O circuitry.
Figure 4-6. Digital I/O Circuitry
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 digital I/O line. When the ports are not enabled,
the digital I/O lines act as high-impedance inputs.
8
Dat
PCMCIAI /O Channel
82C55A
Programmable
Peripheral
Interface
To
Interrupt
Control
DIO RD/WRT
DATA<0..7>
PC0
PC3
PC<0..7>
PB<0..7>
PA<0..7>
I/O Connector
2
8
8
8
PCMCIA P
o
rt
Decode
Circuitry