Termination and Cabling Considerations for HSDIO Hardware: Digital I/O (DIO)
Problem: What considerations should I take into account when probing (also commonly known as "bus sniffing" or "snooping") the data bus with a HSDIO device? And where should I probe with respect to the transmitter and receiver? Solution: When working in high speed digital applications, the cable can no longer be ignored as a neglible part of the system. Depending on the cable's electrical characteristics, the cable introduces a signal delay of 1 - 2 ns per foot of cable. With digital testers extending into the GHz range nowadays, this means that multiple samples can be in a cable simultaneously (as shown in the figure below). Moreover, if the cable impedance doesn't match that of the HSDIO device (or vice versa), fractions of the digital data will be reflected back along the cable (also shown in the figure below). Because the cable's charateristic impedance must be factored in as part of the circuit, it is also important to consider where you probe along the cable. The following figure shows the acquired waveform based on where the probes are placed on the cable. Therefore, you should probe as close to the receiver as possible. Related Links: Attachments:  - HSDIOcable.JPG - HSDIOprobing.JPG - HSDIOla.JPG
Report Date: 03/28/2007 Last Updated: 03/29/2007 Document ID: 47RIBSJQ |
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