A twisted pair, or any long piece of wire coming out of the back of a PC, is first and foremost, a transmission line, and needs to be terminated properly. This has nothing to do with the DC bias. The impedance of such a line is generally 50R to 75R.
In the case of a printer port line, which is unbalanced, the line might have a 1k pullup, to satisfy DC conditions, but also needs a signal(AC) termination, usually a RC combination, and is often of low value, maybe a 100R and small capacitor. In the case of a proper balanced line, there is usually a single low value resistor (50R) across the line as it enters the differntial input of the amplifier, but thats not the case here. Trying to satisfy both AC and DC requirements with a single resistor, may either overload the port line if it is too low, or allow signal reflections if it too high. The line should be terminated with a pull-up according to the power output of the driver chip, then a scope used to select the correct RC combination. The RC connects the line to ground. Use a 100R resistor, and start with cap order of 50pF and increase it until you see a nice square wave. Roland 2009/10/9 Steve Blackmore <st...@pilotltd.net> > On Thu, 08 Oct 2009 21:34:12 -0400, you wrote: > > >If 120 ohms from the inputs to +5 is too much of a load, try 470 or 510 > ohms. > > Any pull up action should work against the noise. If that's where the > noise is attacking, > >some is better than none, and even 1K ohm might be enough to hold the > noise at bay. > >Of course it's easy for me to talk, when I have a parts bin with assorted > resistors handy. > > > >| Hi Jon - tried 120 Ohm across the inputs - it stopped the line receiver > >| working? The pull ups on the outputs helped marginally but it's much > >| better than it was and is quite usable. I'll order some better encoder > >| cable and will try and rewire. > > The 120 Ohm are not pull-ups, they are to balance the twisted pair. > > Steve Blackmore > -- > > > ------------------------------------------------------------------------------ > Come build with us! The BlackBerry(R) Developer Conference in SF, CA > is the only developer event you need to attend this year. Jumpstart your > developing skills, take BlackBerry mobile applications to market and stay > ahead of the curve. Join us from November 9 - 12, 2009. Register now! > http://p.sf.net/sfu/devconference > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > ------------------------------------------------------------------------------ Come build with us! The BlackBerry(R) Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now! http://p.sf.net/sfu/devconference _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
