© National Instruments Corporation 5 USB-6501 OEM User Guide
Figure 4. 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.
6. Select a static resistor value for your pull-up resistor that is greater than
or equal to the ideal resistance.
7. Reconnect the load circuit and the pull-up resistor.
Additionally, you can configure the USB-6501 OEM DIO ports as
push-pull.
Digital I/O Circuitry
Figure 5 shows P0.<0..7> connected to example signals configured as
digital inputs and digital outputs. You can configure P1.<0..7> and
P2.<0..7> similarly. Refer to Figure 4 for some common examples of
connections of DIO lines with standard circuits.
GND
P0.0
+5 V
Rp
Re
Rl
Port Pad
VBus
USB-6501 OEM DIO Box
4.7K Onboard Resistor
External
Pull-up
Resistor
Load
A
Short Circuit Protection
Current Limiting Circuit