On Sat, 20 Oct 2007, Kirk Wallace wrote: > Date: Sat, 20 Oct 2007 09:05:39 -0700 > From: Kirk Wallace <[EMAIL PROTECTED]> > Reply-To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> > Subject: Re: [Emc-users] Threading Hickups > > On Sat, 2007-10-20 at 07:36 -0700, Peter C. Wallace wrote: >> SNIP------------------------------------------------------------------> >>> Using an oscilloscope, when I probed the power supplies with the spindle >>> running, I got about .5 Volts of ripple that had a three stair step up >>> and down appearance. Probing the +5 Volt differential signals I got a >>> very short 10 Volt spike on the rising edge of each pulse, but otherwise >>> they looked well formed. Without the differential boards, the +5 Volt >>> encoder pulses had a more drawn out spike on the rising edge and the >>> tops varied about a Volt above +5. >>> >>> The whole idea of the differential boards where to reduce the >>> susceptibility of the system to noise, but the boards themselves seem to >>> make the effect of the noise worse. >>> >> >> Are your differential inputs terminated? If not, that would account for the >> spike... >> >> CAT5 has 100 Ohm charateristic impedance, so you need a 100 Ohm resistor >> across each A /A, B /B, X /X pair. >> >> >> Peter Wallace > > Now that you mention it, I recall that the app note in the data-sheet > for the RS-422 pair shows an Rt. To learn more, I found: > > http://www.microesys.com/pdf/TN-422_Signal_Termination.pdf > > http://www.bb-elec.com/tech_articles/rs422_485_app_note/system_configuration.asp > > Apparently, there is a fair amount of information available on this, if > one is inclined to look for it. I was trouble shooting with the CAT-5 > because it was handy, and I guessed that it has noise attenuation > properties. The installed cable is about three feet of stranded 22 gauge > (that's funny, spell check shows gauge as miss-spelled, are you Brits at > it again?) four twisted pair. Each pair has a foil wrap and stranded > drain. I don't know its impedance, but I suppose I could start with 200 > Ohms and work my way up. The data-sheet indicates that anything lower > that 90 Ohms should not be used. I'll give it a try today.
If you start at 200 Ohms, you need to work your way down... Shielded twisted pair is usually less than 100 Ohms impedance.. Termination need not be perfect, the overshoot is very roughly 1/2 of the impedance mis-match (20% mismatch = 10% overshoot) Open ended cables (no termination) will have 100% overshoot (likely the cause of your 10V spike) Peter Wallace ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
