A brief tutorial on Logic Edges, wiring and cables.

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Len
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Joined: Sat Apr 28, 2018 6:49 pm

A brief tutorial on Logic Edges, wiring and cables.

Post by Len » Tue Jan 17, 2012 12:14 am

Modern logic has ever faster edge speeds. The fastest logic I know about can only  be run for 1 inch (25mm) before requiring controlled impedance tracks and terminations!  For a signal cable using twisted pairs with one side grounded is good for reducing crosstalk and matching impedances and for longer runs but the edge speed stays high.Without proper termination the faster the edge speed is the shorter the signal run length can be without getting excessive reflections,.Excessive reflections cause multiple clocking and often make the signal exceed the maximum signal limits, causing chip latch up and strange delay behavior.  There are series and parallel termination methods. They both have their place.Parallel termination at the beginning and end of a signal runs takes more power and more parts. Good for multi  drop TX/RX points.  For the SA a simple fix is to place a resistor in series with the CONTROL lines, as close as possible to the source, typically R = 47 - 150 Ohm. If it is a lightly loaded signal line (a few chips) place a small capacitor 47pF-120pF  to ground somewhere after the resistor.  If the signal is part of  a star wiring layout ( many ends) it may help to put the same R in each star leg.  The resistor and capacitor gives a SIGNAL RC time constant of  5 to 20 nsec.   Roughly 5-20 MHz clock rates.In practice this is slower because of the extra capacitance of the cable and IC inputs.  Be warned, make the edge too slow and the chips will not handle the clock signal properly either, some ICs have Schmitt  trigger clock inputs for this very reason. Strobed data lines, if the clock timing is phased properly with enough data settling delay, can usually be left alone.  Of course good grounding is important, even in a short cable loom (6” -150mm) should have at least 1 ground wire  for  every 1-2 signals. The more ground lines the merrier.Regards Len Spyker

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