Re: Signal interference in a long cable

On Wed, Dec 21, 2016 at 7:51 PM, Michał Pleban <lists@michau.name> wrote:
> Hello!
>
> I am fighting with signal interference in a long cable. I have clock and
> data signals, plus some others too (they come from the C65 to the 1565
> drive).
>
> The signals interfere with each other, causing the clock signal to look
> like this:
>
> http://i68.tinypic.com/nqqth.jpg
>
> The longer the cable, the worse the signal distortion. The distorted
> clock signal confuses the shift register, making it shift some unwanted
> bits and causing corruption of the received data.
>
> I tried some simple stuff like termination but it didn't really change
> anything. Passing the clock signal via some receiver circuit (for

what kind of termination did you use?
What gate drives the signal?
Is the power to the driver and receiver chip very well bypassed?

> example, 7414 Schmitt trigger gate, or a 75182 line receiver) helps
> *somewhat*, but with a long enough cable (over 1 meter) the distortions
> become too big to filter them out this way.

if it's really an "interference" (the exact term here is crosstalk)
between two signals, you have to run a GND wire between each two data
(or clock or whatever else) wires. That's how original parallel SCSI
and HPIB bus cables were made. Of course these busses are also *very*
well terminated.
It might be a termination issue or a bypassing issue likely.
You should first try to add a 47 ohm resistor just before the cable
beginning at the driver side, on every signal (but VCC if any and
GND).
The take a picture of the scope with AC coupling, gnd clip on gnd pin
of the driver and probe tip on the VCC pin of the driver. The do the
same on receiver chip. This will show you if there's any ground or
power bounce (bad decoupling in this case).

Also, whenever you suspect a crosstalk issue, take a picture of both
signals, not just one :)

F

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