National Instruments 653X - Manual (Page 20)

Chapter 2 Using Your 653X
653X User Manual 2-2 ni.com
Controlling and Monitoring Static Digital
Lines—Unstrobed I/O
This section explains how to control and monitor static digital lines through
software-timed reads and writes to and from the digital lines of your
653X device.
Configuring Digital Lines
For unstrobed I/O, the direction of each of the 32 data lines is individually
configurable. You can configure each data line to one of the following:
• Input
• Standard output
• Wired-OR output
Standard Output
A standard driver drives its output pin to approximately 0 V for logic low,
or +5 V for logic high. Advantages include:
• It does not require pull-up resistors.
• It is independent of the state of the DPULL line.
• It has high current drive for both its logic high and logic low states.
• It can drive high-speed transitions in both the high-to-low and
low-to-high directions.
Wired-OR Output
A wired-OR output driver drives its output pin to 0 V for logic low. For
logic high, the output driver assumes a high-impedance state and does not
drive a voltage. This is called tri-state. To pull the pin to +5 V for logic
high, a pull-up resistor is required.
To provide a pull-up resistor, connect the DPULL pin on the I/O connector
to the +5 V pin. This provides 100 k
Ω pull-up resistors on all data lines.
For more information about CPULL and DPULL, see the Power-On State
section in Appendix D, Hardware Considerations .