Change NI PXI 7851 R TTL Voltage logic Levels

[RRI_user]

Hi all,

Wish you Merry Christmas and a Happy New Year 2018!

I am using NI PXIe-1062Q Chasis (connected to a Dell workstation Precision 1700) with the following modules :

1. NI PXIe-8381

2. NI PXI 7851 R

3. NI PXIe-6738

4. NI PXIe-6535

In my workstation, I have plugged in the PCIe-8381 module for connecting NI PXIe-8381 to the computer into a PCI-E Gen 3 x16 slot. Now here is my question :

Q.1. My NI PXI 7851 R card is working when in use it to generate TTL high and low signal. I have made an FPGA VI which has a flat sequence with three frames: to generate a TTL high signal on one DIO channel, then I wait for some time using wait function, and finally I generate a TTL low signal on that DIO channel. All this I can nicely see on an oscilloscope as a TTL rising up and then falling after the wait time.

However, the voltage rise is only 1 volt in contradiction with the datasheet specifications of the 3.3 V which NI PXI 7851 R card should produce. I am using a coupling impedance of 50 Ω in the oscilloscope channel.

Also I was wondering if it is possible to configure the logic levels of NI PXI 7851 R using LabVIEW or MAX. I found some methods online but I am unable to implement them as those options are not shown in my LabVIEW.

[crossrulz · Accepted solution]

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RRI_user wrote:

However, the voltage rise is only 1 volt in contradiction with the datasheet specifications of the 3.3 V which NI PXI 7851 R card should produce. I am using a coupling impedance of 50 Ω in the oscilloscope channel.

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Read the data sheet again.  There should be a Max current specification for the DIO.  It is typically 4mA.  4mA*50 Ohms is 200mV.  So change your oscilloscope to the 1M input resistance.

[RRI_user]

Hi crossrulz,

Thanks a lot. It worked. I see a similar requirement of 1 MΩ coupling resistance for AO channels also.

Just out of curiosity, I was wondering how one would explain the 1-volt signal I had seen with 50 Ω. Your calculation says it should be 200 mV.

Thanks anyway!