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Date:	Mon, 4 Mar 2013 03:52:23 +0000
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Subject:	Re: [geda-user] Unterminated logic signals
From:	Julia Longtin <julia DOT longtin AT gmail DOT com>
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I would hazard a guess that this depends on the voltage level of the logic
on the board, and the speed of logic gates on the board. extending a trace
like that could have effects that will cause undesired behavior if the
board runs 'too fast', or the difference between 'enough voltage to trigger
a gate also on the pin' and the supplied voltage to trigger are 'too close'
together.

you need some back of the envelope math, but you need the variables of bus
speeds, and logic voltage levels before you can do the math.

Julia Longtin -- Not an Expert. :)

On Mon, Mar 4, 2013 at 3:14 AM, Patrick Doyle <wpdster AT gmail DOT com> wrote:

> Hello Experts!
> I have a question for you...
>
> I have a PCB I purchased from a vendor that has a 15 pin connector on it,
> of which only 8 pins have defined functions.  (The functions of the other 7
> pins are undefined by the manufacturer of the board and may or may not be
> connected to logic on their board.  I have asked to what those pins are
> connected, but haven't heard anything back yet).  Of the 8 pins with
> defined functions, I use only 7.  I have a cable drawing that specifies
> which 7 pins should be connected at each end of the cable.
>
> The problem is, our manufacturing guys built (or allowed a 3rd party to
> build) cables with all 15  pins in the connector populated.  At the other
> end, they just cut the unused 8 wires and populated the connector with the
> 7 signals I use.
>
> I am trying to decide whether I need to tell our manufacturing guys to
> rework the cables or not.  How much do you think it matters that there
> might be digital logic on the PCB driving signals down those unused wires
> in my cable?  The cable itself is shielded, but I wonder what sort of cross
> talk the unterminated signals might cause.  (I also worry a little bit
> about the clipped ends shorting together, but for the purpose of this
> question, I'm going to assume (ha ha) that they insulated each end after
> they clipped it).
>
> What do you folks think?
>
> --wpd
>
>

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I would hazard a guess that this depends on the voltage level of the logic =
on the board, and the speed of logic gates on the board. extending a trace =
like that could have effects that will cause undesired behavior if the boar=
d runs &#39;too fast&#39;, or the difference between &#39;enough voltage to=
 trigger a gate also on the pin&#39; and the supplied voltage to trigger ar=
e &#39;too close&#39; together.<div>
<br></div><div>you need some back of the envelope math, but you need the va=
riables of bus speeds, and logic voltage levels before you can do the math.=
</div><div><br></div><div>Julia Longtin -- Not an Expert. :)<br><br><div cl=
ass=3D"gmail_quote">
On Mon, Mar 4, 2013 at 3:14 AM, Patrick Doyle <span dir=3D"ltr">&lt;<a href=
=3D"mailto:wpdster AT gmail DOT com" target=3D"_blank">wpdster AT gmail DOT com</a>&gt;</=
span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8e=
x;border-left:1px #ccc solid;padding-left:1ex">
Hello Experts!<div>I have a question for you...</div><div><br></div><div>I =
have a PCB I purchased from a vendor that has a 15 pin connector on it, of =
which only 8 pins have defined functions. =A0(The functions of the other 7 =
pins are undefined by the manufacturer of the board and may or may not be c=
onnected to logic on their board. =A0I have asked to what those pins are co=
nnected, but haven&#39;t heard anything back yet). =A0Of the 8 pins with de=
fined functions, I use only 7. =A0I have a cable drawing that specifies whi=
ch 7 pins should be connected at each end of the cable.</div>


<div><br></div><div>The problem is, our manufacturing guys built (or allowe=
d a 3rd party to build) cables with all 15 =A0pins in the connector populat=
ed. =A0At the other end, they just cut the unused 8 wires and populated the=
 connector with the 7 signals I use.</div>


<div><br></div><div>I am trying to decide whether I need to tell our manufa=
cturing guys to rework the cables or not. =A0How much do you think it matte=
rs that there might be digital logic on the PCB driving signals down those =
unused wires in my cable? =A0The cable itself is shielded, but I wonder wha=
t sort of cross talk the unterminated signals might cause. =A0(I also worry=
 a little bit about the clipped ends shorting together, but for the purpose=
 of this question, I&#39;m going to assume (ha ha) that they insulated each=
 end after they clipped it).</div>


<div><br></div><div>What do you folks think?</div><div><br></div><div>--wpd=
</div><div><br></div>
</blockquote></div><br></div>

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