HSDIO - Hardware Configuration To Test Open-drain Bi-directional Signal?

[Steve1576]

Hello, We will be using a PXIe-6556 HSDIO card to test a DUT that contains open-drain bi-directional pins. The DUT bi-directional pins have about a 1K pull-up on them to Vdd (3.3V). What is the recommended and/or required hardware configuration to allow parametric (DC and timing) measurements testing on these pins. My main concern is with termination to maintain signal integrity and level measurements which may be affected negatively by the terminations. Another issue is that the 1K pull-up on the DUT end is going to be insufficient to drive the 50 ohm input of the 6556 to a valid logic level. Does anyone have any answers or links to other NI collateral that shows the best way to do this? Thanks, Steve

[QiongW]

Hi Steve,

For DC measurements, generally you can use PMU function to force/measure voltage/current.

For timing measurements, how fast is your signal? if you set 6556 input to high impedance, will that destroy your signal integrity?

If 50ohm input impedance is indeed needed for signal integrity, you will get (50/1050)*3.3V = 157mV on 6556 side. you have the option to set VIH/VIL on 6556 with 25mV accuracy, so it will probably work if you set your VIL to 75mV or something, however it still depends on how accurate the 3.3V and 1Kohm are

Also keep in mind that the 1m VHDCI cable adds 100pF capacitance, so with 1K source impedance,delay will be introduced. It would help if you can buffer your signal before sending to 6556.

I hope this helps. Please let me know if you have any other questions

Qiong

[hch3]

Hi,

I'm trying to execute a similar test, but my pull-up resistor is about 50 kohms.  I've been able to successly do HWC when setting VTT on the specific channel, but I was wondering if setting the channel to a different input impedance would help.

Do you mind answering the following?

• What are the different impedance values available for a 6556 channel in generate/acquisition mode?  Min?  Max?  Range?
• What what functions are necessary to set the impedance on a particular channel using the niHSDIO.dll (not LabView)?

I've read the manual but I haven't a clear answer for either.  I think I'm missing something.

Thanks 🙂

[kevins33]

Hi hch3,

The output impedance for the PXI 6556 is 50 Ohms. The input impedance can be selected between high impedance and a 50 Ohm impedance. Please reference the below document for information regarding the high impedance value.

What Does High Impedance mean in the PXIe 6555/6556 Specifications Manual?

http://digital.ni.com/public.nsf/allkb/07A48243A2A1AE2986257B7B0064AEC3

I am not familiar with what functions you will use to set the input impedance outside of LabVIEW. I do know that you will be able to use a property node in LabVIEW in order to change the impedance. What development enviornment are you using?

[Kyle_A.]

Hello,

From that same link above, in the Related Links section, is the KB for changing input impedance on the 6555/6556:

How Do I Set the Input Impedance On My PXIe-6556 to 50 Ohms?

To translate from LabVIEW to C (.dll), you will use NI-HSDIO Attributes NIHSDIO_ATTR_DATA_TERMINATION_MODE and NIHSDIO_ATTR_DATA_TERMINATION_VOLTAGE_LEVEL as indicated by the KB in LabVIEW. The C function to Get/Set Attributes should also be used with these attribute names to achieve the functionality you are looking for. We have our full C reference in our NI Digital Waveform Generator/Analyzer Help which can be found online in HTML format or on your computer as a .chm file.