04-23-2012 09:50 AM
NI advertises the RDIO 68 cable for high speed digital interface to R series PXI boards such as the 7811R. The unpublished impedance of the RDIO was given to me
as ~90 ohms however the details of the termination network used (which is embedded in the RDIO cable end) are not available. My question is:
1. Why does NI suggest this cable for high speed interface but does not provide the required information to design a matched drive signal that will minimize ringing and signal reflection?
I have tried a variety of techniques using series terminations, HC and LS line drivers but cannot do any better than the attached scope trace which reveals a significant signal reflection.
05-02-2012 12:48 PM
The impedance of the cable is 90 ohms. Would it be possible for you to post a schematic of your set up? I have scene similar behavior when the cable's end of line termination was not properly used.
05-02-2012 01:31 PM
I am using a 75ALS191 line driver through a 90 ohm series termination resistor to drive the RDIO cable. I am simply transmitting a TTL (0-3V) data stream through the line driver which bumps it up to 0-5V. I think it would be a simple matter for NI to offer at least one TTL compatiable driver design that is optimized for the built in termination on the RDIO cable. I think you would agree that NI is not providing even the basic information to be able to use the RDIO cable for its intended purpose. Even the 90 ohm impedance spec is not included in the published RDIO spec sheet.
05-08-2012 10:18 AM
You are correct that our documentation is limited in this regards. I am going to file a documentation CAR about this issue. We appreciate you feedback concerning this issue.