Termination for PCI-6534

[blarivee]

I was wondering what type of termination I should be using for my PCI-6534 board to prevent overshoots, undershoots, and ringing. I read all the specifications and I understand the need for the Schottky diodes for preventing undershooting and overshooting, but do I also need resistive termination to prevent the ringing? Do I need the capacitor for stabilization if I am connecting the diodes to the +5V node on my external device, coming from the cable from the IO board? Also, if I am using the shielded twisted-pair cable, do I need to bother with these termination ideas at all, or do they provide a good enough medium if it is only 1 meter long? These points do not seem clear to me from the specs and I do not want to put a whole bunch of re
sistors and diodes on my external board if I don't have to. Thank you.

[rgharrison]

Hi,

It is good to consider all the possible problems when dealing with a high-speed digital product. I haven't seen significant noise when dealing with my PCI-6534 and my D1 cable. The D1 cable includes components to reduce the noise during transmission (twisted pair etc.). However, dealing with noise depends on your circuit that you are connecting to. I would try using the standard setup (no Schottky diodes) and see if you have problems with your circuit. This usually only happens with very sensitive equipment. If this is the case then you would then need to employ the diodes to remove the under/overshoots. The diagram D-5 of the user manual gives a good connection diagram for this type of situation.

You shouldn't need to use termination resisto
rs since receiving output from the 6534 or driving input is correctly terminated at the 6534's side.

The bottom line is that you probably don't need to use Schottky diodes and you will more-than-likely not need to use termination resistors. Test your signals with your device or an oscilloscope to determine the need to clean the signals up.

Hope that helps.

Ron
Applications Engineer
National Instruments

[mlloyd]

For anyone who is still interested in the topic, we did not terminate our interface because we expected the source-termination scheme to take care of ringing and overshoot.  However, we found that this was NOT the case: we had 9V spikes at the input buffers to our board!  We added Schottky diodes with limited results...  it was difficult to get the diodes close enough to the input because the board had alread been built without the diodes on it.  I would recommend putting the Schottky diodes on from the very beginning.

I'm currently working with the PCI-6541 and they suggest a 10 k-ohm parallel terminating resistor.  I'm wondering if this would be a good idea for the PCI-6534 as well...

Regards,

mlloyd

[mlloyd]

At what point does the peripheral interfacing with the 6534 need a capacitor near the diode?  Does long in the documentation refer to meters or inches?

Thanks for your help,

mlloyd