On Friday 01 April 2016 07:02:14 andy pugh wrote: > On 1 April 2016 at 11:46, Erik Christiansen <dva...@internode.on.net> wrote: > > Incidentally, is that cable in the last picture twisted pair? > > Shielded or no, is its characteristic impedance controlled? > > It is just what I cold find in Maplin that evening. > I will see how well it works, and if there is a problem I can buy some > of this: > http://www.rapidonline.com/cables-connectors/belden-9841-data-cable-fo >r-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, but should be on beldens own site. It looks quite similar to a black jacketed version used for balanced audio in a broadcast facility, with total usage often measured in miles. And while its great cable for short runs, its characteristic impedance is only 60 ohms, which when driven by the usual amplifier expecting to find a "ma bell" style 600 ohm load, the cables capacitance can and will cause an high frequency rolloff. So when I needed to re-design that amplifier rack because the existing shop made amps were power hungry and had a high failure rate, I turned them into op-amps which at unity or thereabouts gain, have an output impedance of almost zero ohms, then built it out to nominally 60 ohms with a 27 ohm resistor in series with each side of a driven line. The difference in audio quality on the air could very easily be heard. The HF losses at the far end of a 180 foot run to an editing booth in the news room was only a small, almost unmeasureable loss at 15 kilohertz. Perhaps .1 db. The old design had some .25% distortion, the new design was very close to .05%. And I suspect that was at least half accounted for in the ohmage losses of the cable conductors. With the amplifiers I could buy for broadcasting use at the time, that HF loss was almost 20 db in that same 180 feet of cable! An rs485 circuit should be treated similarly, particularly at the load end if the run is long enough that you can see the echo step coming back when you're looking at the source end of the circuit. With modern higher speed logic, that might be as little as 5 feet! If a 62 ohm resistor across the two wires at the load end reduces the errors, that would be somewhat crude way to check if a scope isn't handy. 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 Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
