On Friday 01 April 2016 11:33:18 andy pugh wrote: > On 1 April 2016 at 16:22, Gene Heskett <[email protected]> wrote: > >> http://www.rapidonline.com/cables-connectors/belden-9841-data-cable > >>-for-rs-485-51-6887 > > > > Twisted pair is a transmission line and subject to all the vagaries > > of load matching and such. > > > > That cables impedance is not listed there, > > Yes it is. 120 Ohms.
Resistance is NOT impedance, Andy. And I first read that as 1200 because my screen is high res and the ohm sign looks like another 0 at first glance, I mentaly threw that whole line out as being technically impractical, written by someone who obviously did not understand anything about transmission line theory. My bad for the miss read of 120 ohms as 1200 ohms, but still the use of the word resistance renders it null and void in my technical mind. If they meant impedance, they should have written impedance. If the loads J dactor is 0, then they would directly correspond, but that J=0 generally would be quite rare in the real world. However, my comments about the load termination, in this case with a 120 ohm resistor from wire to wire at the load end are still valid. Properly done, thats taken care of in the rs485 interface in the device, but you never know when some bean counter between the engineer and the production line will delete that seemingly useless part as a cost saver. I learned 25 years ago to never, ever, discount that possibility when something wasn't working right. The places where a well designed product was made into a wholesale failure make up quite a few of my "war stories". > It is also specifically described as RS-485 cable. > > But I also noticed that Van Damme list their DMX cable as suitable for > RS-485 and that that is available from eBay at fairly low cost, so > ordered some in on a "just in case" basis. > It is also a pretty purple colour. Chuckle. The "oh look, a pony" thing. ;) The electrons are color blind. Which is a good thing IMO, since if you see that color 5 years later, you should recall that the cable is an RS485 circuit, and thats a Good Thing(TM). Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Transform Data into Opportunity. Accelerate data analysis in your applications with Intel Data Analytics Acceleration Library. Click to learn more. http://pubads.g.doubleclick.net/gampad/clk?id=278785471&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
