It is not recommended or feasible to pull up the digital output from 3.3V to 5V on the PXIe 782xR, PXIe-784xR, PXIe-785xR, PXIe-786xR, and USB R Series due to the protection circuitry used in those devices.
The output impedance for the R Series devices is non-linear due to the circuitry used to provide 5V tolerance. The output impedance is low (<50 ohms) for output voltages near 0V. As the output voltage increases to around 3.3V, the impedance rises to approximately 100 ohms. As the voltage rises beyond 3.3V, the impedance rises very quickly to megohms.
Here's an approximation for how to calculate the value of pull-up resistor that you need:
- Assume that the R Series device outputs will drive the digital output line quickly between 0V and 3.3V. The pull-up resistor will have to pull the output up from 3.3V to 5V. For a rough approximation, assume that this is a simple RC circuit where Rp is the value of your pull-up resistor, and Ctot is the total capacitance* on the digital output line. The R Series device output will be high-impedance beyond 3.3V, so it can be ignored for this calculation.
- Solve for the value of Rp required to pull-up the output based on the timing requirements of your application. Lower values for Rp will pull-up the output faster.
- Make sure that the value of Rp that you have chosen will not result in too much current when the R Series card is trying to drive the output low.