PXI-6551 connector

[ajog]

Hello,

I'm trying to build a custom connector with short breakout wires to reduce the capacitive load seen by the IO of the DUT.  It seems that the connector pin-out provided by National Instruments is encrypted because a straight through 68-pin VHDCI cable does not work and I must use the 3-foot cable provided by NI. 

That means the 6551 connector pin-out is correct at one end of a NI-supplied HSDIO cable but at the card end the channels may (must?) be scrambled.

Please advise regarding options. 

Sincerely

Anand

[Aaron_W.]

Anand,

If you look in the NI Digital Waveform Generator / Analyzer Help you should be able to find the pinout for the 655x cards. It is also on our website.

The other item you should look at is what type of VHDCI connectors we use and we have a KnowledgeBase document that gives the additional details that might help you.

Specifications and Manufacturers for Board Mating Connectors:

https://www.ni.com/en/support/documentation/cable-accessory-guide/daq-multifunction-i-o-cable-access...

Hope this helps. Thanks!

[Jesse_O]

Anand,

There is also a 0.5m cable that is available for the PXI-6551.  You can find the part under the product page for the PXI-6551:

https://www.ni.com/en-us/support/model.pxi-6551.html

It is listed under part number 781013-01

Hope this helps,

[ajog]

Thanks Jesse,

All cables (when you click to see picture) are shown to be 1m long.  It must be a typo. 

Question:

In the 5 fr flying lead cable, each channel has two small connectors.  One is the inner conductor of the coax cable.  Is the second one the shield?

Anand

[DJ_L.]

Hey Anand,

In the Single-Ended Digital Flying Lead Cable User Guide document, it says the following:

"The NI SHC68-H1X38 cable is separated into two bundles of 19 channels, and each channel is split into a pair of leads for connecting the signal and its associated ground."

The ground is shorted to the shield of the micro coax. There is other good information about the cable in the user guide linked above. Let us know if you have any other questions or concerns. Thanks, and have a great day.

Regards,

DJ L.

[ajog]

Thanks DJ,

I'm now also interested in the input attenuated acquisition.  In figure 4 of the flying lead cable user guide the peripheral device is shown to have a 50 Ohm resistor connected at the output.  If the input impedance of the HSDIO is set to 50 Ohms isn't the voltage divider now 50/(1050 + 50) ?  This is close to the value listed in the manual but I was trying to determine the logic behind the equation.

Also, the probe capacitance is listed at 100pF.  The flying lead cable is 5 ft long with 30pF per foot so we should see ~150pF.

Please clarify both points.

Thanks very much!

Anand

[ajog]

Hi DJ,

I think my question is moot.  Its basically a simple voltage divider whether or not a 50Ohm output resistor is employed by the DUT's driver.  So the 'original' value as referred to by the specifications could be one in which the DUT driver either had or did not have a 50 Ohm output termination.

Thanks.

Anand

[DJ_L.]

Hey Anand,

I just wanted to follow up on your previous posts, and to clarify the points that you made:

Q: If the input impedance of the HSDIO is set to 50 Ohms isn't the voltage divider now 50/(1050 + 50) ?  This is close to the value listed in the manual but I was trying to determine the logic behind the equation.

A: You are correct in your calculation, and the manual is 50 ohms off. Thank you for catching this. I will file a Corrective Action Report (CAR) on this, so it will be fixed in the next version of the document. But like you mentioned, it is a minor change, and overall the voltage divider is there and the percentage is still around 4.5% to 5%, like mentioned in the doc.

Q: Also, the probe capacitance is listed at 100pF.  The flying lead cable is 5 ft long with 30pF per foot so we should see ~150pF

A: Table 2 in the cable's user guide is intended to be a high level comparison, and to show the effective capacitive loading of the circuit that changes based on the probing option, but the actual spec is in the Specifications section of the doc, and as you stated, is listed as 150 pF.

Regards,

DJ L.