National Instruments 6008 - Manual (Page 18)

NI USB-6008/6009 User Guide and Specifications 18 ni.com
Caution Exceeding the maximum input voltage ratings or maximum output ratings, which
are listed in the Specifications section, can damage the DAQ device and the computer.
National Instruments is not liable for any damage resulting from such signal connections.
Source/Sink Information
The default configuration of the NI USB-6008/6009 DIO ports is open
collector, allowing 5 V operation, with an onboard 4.7 k
Ω pull-up resistor.
An external, user-provided, pull-up resistor can be added to increase the
source current drive up to a 8.5 mA limit per line as shown in Figure 14.
The NI USB-6009 ports can also be configured as active drive using the
DAQmx API, allowing 3.3 V operation with a source/sink current limit of
±8.5 mA. Refer to the NI-DAQmx Help for more information about how to
set the DIO configuration.
Figure 14. Example of Connecting External User-Provided 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.
3. Adjust the variable resistor until the ammeter current reads as the
intended current. The intended current must be less than 8.5 mA.
4. Remove the ammeter and variable resistor from your circuit.
5. Measure the resistance of the variable resistor. The measured
resistance is the ideal value of the pull-up resistor.
GND
P0.0
+5 V
Rp
Re
Rl
NI USB-6008/6009
Load
A
+5 V
Port Pad
4.7 K Ω Onboard Resistor
External
Pull-up
Resistor