National Instruments USB-6008 User Manual page 21

Figure 15. Example of Connecting an External User-Provided Resistor
External
Pull-Up
Re
Resistor
Rl
Load
The NI USB-6009 ports can also be configured as active drive using the NI-DAQmx API,
allowing 3.3 V operation with a source/sink current limit of ±8.5 mA. For more information
about how to set the DIO configuration, refer to the KnowledgeBase document, How Do I
Configure My NI Device to be Open-Drain (Open Collector) or Push-Pull (Active Drive)?. To
access this document, go to
Determining the Value of the User-Provided Pull-Up Resistor
Complete the following steps to determine the value of the user-provided pull-up resistor:
1. Place an ammeter in series with the load.
2. Place a variable resistor between the digital output line and the +5 V supply.
3. Set P0.0 to high.
4. Adjust the variable resistor until the ammeter current reads as the intended current. The
intended current must be less than 8.5 mA.
5. Remove the ammeter and variable resistor from your circuit.
6. Measure the resistance of the variable resistor. The measured resistance is the ideal value
of the pull-up resistor.
7. Select a static resistor value for your pull-up resistor that is greater than or equal to the
ideal resistance.
8. Reconnect the load circuit and the pull-up resistor.
20 | ni.com | NI USB-6008/6009 User Guide
+5 V
+5 V
P0.0
GND
A
and enter the Info Code
ni.com/info
NI USB-6008/6009
Rp
4.7 k Ω Onboard Resistor
Port Pad
.
ex52sp