Determining The Pull-Up Resistor Value; Digital I/O Circuitry - National Instruments USB-6501 User Manual

Figure 6. Example of Connecting an External, User-Provided Resistor
Re
External
Pull-Up
Resistor
Load
Additionally, you can configure the DIO ports as active drive. When configured as active
drive, the total current sourced by all DO lines simultaneously should not exceed 65 mA.

Determining the Pull-Up Resistor Value

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.

Digital I/O Circuitry

The following figure shows some common examples of connections of DIO lines with
standard circuits. In the figure, P0.<0..7> is connected to example signals configured as digital
inputs and digital outputs. You can configure P1.<0..7> and P2.<0..7> similarly.
+5 V
P0.0
Rl
A
GND
Short Circuit Protection
Current Limiting Circuit
Rp
4.7 KΩ Onboard Resistor
NI USB-6501 User Guide | © National Instruments | 9
VBus
Port Pad