Chapter 3
Signal Connections
©
National Instruments Corporation
3-9
Digital I/O State Selection
You can power up the digital I/O lines for the DAQPad-6507/6508 devices
in a user-defined state. The DAQPad-6507/6508 devices facilitate
user-configurable pull-up or pull-down tasks. Each DIO channel is
connected to a 100 k
Ω
resistor and can be pulled high or low using the front
panel switch. You can use this switch to pull all 96 DIO lines high or low,
or you can allow the lines to float. However, if all lines are high, you might
want to pull some lines low. To do this properly, you must understand the
nature of the drive current on those lines and adhere to TTL logic levels, or
you can let the lines float and tie them all high or low as necessary.
High DIO State
If you select the pulled-high mode, each DIO line will be pulled to V
CC
(+5 VDC) with a 100 k
Ω
resistor. To pull a specific line low, connect
between that line and ground and use a pull-down resistor (R
L
) whose value
will give you a maximum of 0.4 VDC. The DIO lines provide a maximum
of 2.5 mA at 3.7 V in the high state. Use the largest possible resistor so that
you do not use more current than necessary to perform the pull-down task.
Also, make sure the resistor value is not so large that leakage current from
the DIO line along with the current from the 100 k
Ω
pull-up resistor drives
the voltage at the resistor above a TTL low level of 0.4 VDC.
Figure 3-6. DIO Channel Configured for High DIO State with External Load
DAQPad-6507/6508
Digital I/O Line
100 k
Ω
GND
R
L
+5 V
82C55A
