Machine Vision

Hi Brad,

Thanks for posting! BNC connections are themselves differential connections.  So to answer your question, yes you can look at differential video with the A6822.  To do this, use a BNC cable and connect your camera to either Video 0, 1, 2, or 3 (since the 1409 has these 4 input channels).

Do you not have access to these channels? 

I hope this clears things up.  Let me know if you have more questions or if there are some details I need to be made aware of and I'll be happy to help. 

An ohmmeter shows that the outside shells of the video BNCs are connected to the outside shells of the sync inputs, etc.  They are definitely not isolated, so each video input can't be differential.  Who has a schematic diagram of that breakout box and the corresponding IMAQ-1409 cable?  Those are what we really need to see.  I couldn't find them online.  Is there any way you could post them?

Thanks,

Brad

Hi Brad,

I apologize, I believe you are correct.  Looks like the BNC's are in fact connected to common ground.  Unfortunately I do not have schematic diagrams for this breakout box.

What happens if you connect through the breakout box?  Does it result in unexpected results?  I haven't yet come across a situaton where the A6822 has caused unexpected results with the 1409.   If so, please let us know!

When we connect only one side of our differential video (the + side) to an A6822 video input, the video looks smeared and noisy.  However, as somewhat of a temporary, partial, Band-aid fix I've been connecting the negative video to another video input which cleans up the video being captured quite a bit.  I'm not sure if it's the additional 75-ohm load that is doing it, but it helps a lot for now.  I'm just going to have to get our SCB-68 interface box and use it in place of the A6822 while still using the 68-pin IMAQ cable.  We'll see how well that works. 

I'm suspecting that the A6822 is something that is not meant to be used with any differential signals at all.  So, there is no way to get schematics from whoever designed and/or built that breakout box?

- Brad

I couldn't find the IMAQ cable (used with the A6822) listed for sale on ni.com, so an email to a sales rep got a quote for it.  If anyone needs one, it's part #186823-01.

I connected a SCB-68 box to the A6822 with a cable, then used an ohm meter to check pinouts.

For the IMAQ cable pinout: corresponding pins on both ends are connected to each other (as you would expect).

The A6822 pinout is in the attached files.  The files correlate pin numbers, 1409 signal names and A4822 connectors.  The A4822 breakout box definitely cannot work with differential signals - with one odd exception.  Only the PCLK OUT connector has an isolated BNC shell (it's connected to the 1409 PCLKOUT(-) signal and not to other BNC shells).  However, the PCLK IN connector's BNC shell is tied to DGND and other BNC shells, so you can't loop a 1409-generated clock from the OUT to the IN directly.  I think the designers of the box had a few too many at the local pub as they drew it out on the back of a napkin.  It's too bad... it could have been a much better (more flexible) breakout box if it had been wired differently internally.

Wow... what a difference the SCB-68 box makes.  I have a beautiful, stable capture now using all differential signals through the IMAQ cable.  Amazing what a difference differential signals make along with the right hardware.

By the way... if anyone wants to know how I used a simple ($49) Avnet Spartan 3A FPGA card to generate an HSYNC signal for our very oddly-formatted video signal, I can go into that.  It allows the 1409 to capture our strangely-interlaced, 1020 by 240 video format.

- Brad