HSDIO PXI-6552 DDC CLOCK OUT to STROBE ?

[engwei]

Hi,

I am confuse with this term "DDC CLOCK OUT to STROBE", as described in many available examples from NI sites.

Many examples ask user to physically connect the DDC clock out to Strobe cable (export clock through the cable to strobe as acquisition clock)

But from the HSDIO 6552 datasheet, 

Can anyone there explain this ? It really get me .. 

Meanwhile, I found that if I connect the DDC clock out to Strobe cable, the clock signal will strongly distorted (by default input impedance is 50kohm, for model E, PXI-6552.) 

Since my project need the clock to be exported to the application board, and the specification state that DDC cable cannot be routed to Strobe cable, I have configure generation and acquisition clock = onboard clock.

As I tried out error location - hardware compare example, If I don't connect these cable, the acquisition sample will be slowed by 1 cycle (at 20MHz).

(Setting: Generation and acquisition clock, start trigger been set to rising edge, data active event at active high)

And at 20MHz I cannot configure any delay at acquisition part. 

Any idea to solve this problem (delay)?

Thanks in advance, and Happy new year !! 

Message Edited by engwei on 12-30-2008 05:28 AM

Message Edited by engwei on 12-30-2008 05:29 AM

[engwei]

Maybe I am misunderstood with the term "routed". Maybe it means route internally by software. But it is mean PFI 1,2,3 could be routed by Strobe signal ? .. :S

[Keith.Shapiro]

Hi engwei,

The data sheet snippet you posted means that you cannot export a signal that is received on the Strobe terminal to DDC Clock Out.  It does not talk about making connections outside the device.  It is okay to connect DDC Clock Out and Strobe with a wire.

If you really need to use data delay on a 6552 device below 20MHz, we recommend oversampling.  For a 20MHz case, set your clock frequency to 40MHz and generate / acquire each sample twice.  Does this make sense?

Keith Shapiro

National Instruments R&D

[engwei]

Hi Keith,

Thanks for your recommendation. 

Since there is no problem with DDC Clock Out wire to Strobe, then I think I will not need oversampling to tackle this problem.

The problem only occur whenever I m trying to configure acquisition clock as OnboardClock.

There is another question here.. I need the DDC Clock to share clock source to another application board, but once I connect the Strobe cable to DDC Clock cable, the signal will be distorted, it is near to sinus waveform then clock waveform. 

Thanks for your reply on previous post. 

[Jesse_O]

Hello engwei,

If you are only requiring a frequency reference, I would recommend exporting your sample clock to the CLK OUT SMB front panel connector.  You can export the clock to both the CLK OUT front panel connector and the DDC Clock out on the cable, you just need to commit the session after each export signal command.  

If you are looking to synchronize two boards clocks, I would recommend using TClk.  You can find out more about TClk in the HSDIO manual located under Start -> Programs -> National Instruments -> NI-HSDIO -> NI Digital Waveform Generator-Analyzer Help.  There are shipping examples that demonstrate how to use TClk.  You can take a look in LabVIEW's Example Finder under Hardware Input and Output ->Modular Instruments -> NI-HSDIO -> Dynamic Generation ->Non-Scripted -> Multi-Device Dynamic Generation (TClk).vi

As for the signal quality.  I would not expect the DDC Clock out to appear sinusoidal, especially if you are only running at 20MHz.  The 50KOhm input impedance on the 6552 is not a problem.  The board can drive up to 50mA per channel.  I would be careful when trying to split a clock signal.  The 6552 is a 50 Ohm transmission line, and running two cables from flying lead cable (or terminal block) clock line will generate reflections.

[engwei]

Hi Jesse,

Thanks for your reply and recommendation. 

[engwei]

My situation required me to have all clock in synchronised.

And running at 20MHz, the signal distorted. 

Running at higher frequency, let say 50MHz or 66.67MHz or 100MHz, the DDC Clock Out signal behave like sinusoidal waveform. Unplug the Strobe cable will recover the waveform to square waveform. 

Just to add some explanation to my previous post.  

[Keith.Shapiro]

Hi engwei,

Have you resolved your problem, or are you still seeing data integrity issues?

When you measure DDC Clock Out with a scope, where are you taking the measurement?  The only way to get an accurate rendering of what the receiver is seeing is to measure the signal at its destination.  If you're measuring the signal in the middle of a transmission line, you will see distorted results.

I'm guessing that you're measuring in the middle of the transmission line, since you say that when you disconnect the wire to Strobe that the signal looks okay at the higher frequencies.

Does that make sense?

Keith Shapiro

National Instruments R&D

[engwei]

Hi Keith,

It really make sense to measure only at the destination. 

But, I m using flying lead cable to probe between DDC Clock Out and Strobe. And I consider the terminal at DDC Clock Out is the point I should measure. 

When I probe the Strobe cable to DDC Clock Out terminal, the signal at DDC Clock Out terminal will distorted, where this signal is the "destination" for my application board.

Anyway, if the device is designed to have this distortion, I will think about other way to isolate the loading of Strobe cable to the Clock source.

Thanks. 

[Jesse_O]

Hello engwei,

When you probe just the DDC Clock out at the end of the flying lead cable before you connect it back to Strobe, you are probing at the end of the transmission line.  This results in being able to measure a clean square wave.  As Keith mentioned above, when you connect the DDC Clock  to Strobe, and probe at the connection between them you are probing at the middle of the transmission line and this is not an indication of what you will see at the destination.

The destination of your signal is on the PXI-6552 where the STROBE signal ends.  Unfortunately this is not easy to probe, however this wrapback application you are talking about is typical and should work just fine for you.  If you were able to probe at the destination you should see a nice square wave when wrapping back DDC Clock to Strobe through the flying lead cable.  

Regards,