Hi,

That article is talking about REAL TTL circuits. PICs and HC-series logic
are CMOS and have a high input impedance (except for a small amount of
capacitance, a few pF, which may cause you problems if you are dealing with
fast signals), so you should be able to just add a termination resistor in
parallel. However, you DO have to bear in mind that whatever is on the
sending end must be able to drive the cable (with all it's capacitance) and
handle the extra current that will now be drawn by the termination
resistor, and also you usually want to use something which is robust to
voltage spikes as lightning and other things can cause these on long lines.

Since your signal is rather slow (200us pulses), you might want to consider
just using a low-pass filter on the receiving end. The propagation delay on
your 350 foot line is probably about 1 us, so several reflections take only
a tiny fraction of your pulse width, they would naturally die down quite
quickly. I would suspect that a LPF followed by a gate to restore the pulse
edges might help in your case.

Sean

At 08:29 AM 6/30/2002 -0500, cflat@ev1.net wrote:
>Thanks for the reply, it would seem this article disagrees with that.
>Thanks,
>Charles
>
>http://archives.e-
>insite.net/archives/ednmag/reg/1997/112097/24col.htm
>
> > One Word.... Termination impedance.
> >
> > "cflat@ev1.net" wrote:
> >
> > > Hi, I am out of my knowledge zone and thought I would pose this
> > > senario to the minds of this list for help.  I have a PIC16F877 that
> > > outputs a pulse of 200us duration every 2ms (500Hz pulse).  I need
> > > to send this pulse up to 350 feet down a cable.  This cable has 3
> > > conductors that aren't twisted (have no choice but to use this
> > > cable, more wires not an option) which are 18awg stranded.  Two of
> > > the wires are carrying 24V dc and the 3rd wire should carry the
> > > pulse.  The pulse is ground referenced to the positive 24V lead.  I
> > > have 5V on the receiving PCB also referenced to the 24V positive
> > > wire.  When looking at the pulse on the scope, it is a nice clean 5V
> > > square pulse coming off the board.  At the other end of the cable
> > > where it is received by another circuit board, it has a slight bit
> > > of ringing on the 0 to 5V transition and has a lot of ringing on the
> > > 5v to 0v transition.  I had a 74HC14 on hand and put it on the
> > > receiving PCB and connected the signal through 2 gates but it didn't
> > > help.  I think the ringing is too great a level.  If anyone knows a
> > > good way to get a clean signal or clean up the signal I'd be
> > > greatful for help. I've been searching the net for info and it would
> > > seem a differential method would be preferrable but I don't have an
> > > extra wire so its not an option.  Thanks for any help. Charles
> > >
> > > --
> > > http://www.piclist.com hint: To leave the PICList
> > > mailto:piclist-unsubscribe-request@mitvma.mit.edu
> >
> > --
> >   *
> >   |  __O    Thomas C. Sefranek   tcs@cmcorp.com
> >   |_-\<,_   Amateur Radio Operator: WA1RHP
> >   (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz
> >
> > http://hamradio.cmcorp.com/inventory/Inventory.html
> > http://www.harvardrepeater.org
> >
> >
> >
>
>--
>http://www.piclist.com hint: To leave the PICList
>mailto:piclist-unsubscribe-request@mitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu