IRW Posted July 24, 2013 Share Posted July 24, 2013 Hi All,I'm just about to start sorting out a big pile of 'DMX' cable, which, in reality, is a mixture of microphone cable, with odd lengths of proper control cable thrown into the mix. A lot of it actually has the old 'Professional Microphone cable' printed on it, so that's a no-brainer, however, there are a number of lengths that have nothing printed on it, and by the feel, could go either way. Can anyone advise on if I can work out with a multimeter if it is 'proper' control cable or not by measuring the impedence, or something along those lines? Link to comment Share on other sites More sharing options...
timsabre Posted July 24, 2013 Share Posted July 24, 2013 Can anyone advise on if I can work out with a multimeter if it is 'proper' control cable or not by measuring the impedence, or something along those lines? Not with a multimeter. You might be able to tell by measuring the capacitance if you had some known samples of cable to compare it with. Link to comment Share on other sites More sharing options...
ddproduction Posted July 24, 2013 Share Posted July 24, 2013 Can you open up a plug from each cable and guess from the appearance of the cores? Link to comment Share on other sites More sharing options...
jpk Posted July 25, 2013 Share Posted July 25, 2013 The DMX cables have 5pin connectors :unsure: Okay so they're probably all 3pin. Measure the capacitance between the 2 data conductors (pins 2 & 3), measure the cable length and divide capacitance by length to calculate the capacitance per meter. Compare this figure with:Microphone cable around 100pF/mRS485 cable (varies depending on insulation type used) around 20pF/m You should get 2 distinct reading groups, high and low, which will be microphone and DMX respectively. Link to comment Share on other sites More sharing options...
Brian Posted July 25, 2013 Share Posted July 25, 2013 You should get 2 distinct reading groups, high and low, which will be microphone and DMX respectively. So where does one of the most popular touring grade DMX cables sit at 75pF/m? ;) Link to comment Share on other sites More sharing options...
timsabre Posted July 25, 2013 Share Posted July 25, 2013 You should get 2 distinct reading groups, high and low, which will be microphone and DMX respectively. So where does one of the most popular touring grade DMX cables sit at 75pF/m? ;) Indeed, there's some mic cable below 50pf/m as well. I think your 2 groups of readings may be rather indistinct. Link to comment Share on other sites More sharing options...
revbobuk Posted July 25, 2013 Share Posted July 25, 2013 I think I might suggest a more empirical test. Have a square wave generator set up, and a scope with a t-piece and appropriate terminator. Plug the cable between the two, and if the output waveform looks OK, then it's OK to use as DMX. If it rolls off the edges too much, then it isn't. Pretty much irrespective of what it says on the side! And at least once you've got the scope and sig. gen. set up the whole test is really quick. Link to comment Share on other sites More sharing options...
timsabre Posted July 25, 2013 Share Posted July 25, 2013 I think I might suggest a more empirical test. Have a square wave generator set up, and a scope with a t-piece and appropriate terminator. Plug the cable between the two, and if the output waveform looks OK, then it's OK to use as DMX. If it rolls off the edges too much, then it isn't. Pretty much irrespective of what it says on the side! And at least once you've got the scope and sig. gen. set up the whole test is really quick. Um, both mic and data cables would look OK with that test unless they are massively long cables, you'd need 100's of metres of cable connected to see any difference. DMX is only 250kHz which is pretty slow by today's comms standards. (This may possibly suggest that everyone worries too much about using mic cables for DMX, which I believe to be the case, but that's another argument) Link to comment Share on other sites More sharing options...
revbobuk Posted July 25, 2013 Share Posted July 25, 2013 Then if you can't see any difference, does it really matter? Link to comment Share on other sites More sharing options...
timsabre Posted July 25, 2013 Share Posted July 25, 2013 Then if you can't see any difference, does it really matter? Not in my opinion, but it worries some people.In my experience mic cable works absolutely fine for DMX, and whenever a problem occurs it is due to a cable fault rather than the fact that it's mic cable. Link to comment Share on other sites More sharing options...
AHYoung Posted July 25, 2013 Share Posted July 25, 2013 The only difference between mic cable and 3pin DMX cable, is that the DMX cable you bought whereas the mic cable you stole from the pa guys Link to comment Share on other sites More sharing options...
jonathanhill Posted July 26, 2013 Share Posted July 26, 2013 The only difference between mic cable and 3pin DMX cable, is that the DMX cable you bought whereas the mic cable you stole from the pa guysMore likely you picked it up off the floor after the noise had finished their 'get-out'... Link to comment Share on other sites More sharing options...
Kevin Robertson Posted July 26, 2013 Share Posted July 26, 2013 The only difference between mic cable and 3pin DMX cable, is that the DMX cable you bought whereas the mic cable you stole from the pa guysMore likely you picked it up off the floor after the noise had finished their 'get-out'.../like/ Link to comment Share on other sites More sharing options...
Oldradiohand Posted July 26, 2013 Share Posted July 26, 2013 The simple way is to ask the sound guys to reclaim their mic cables! Link to comment Share on other sites More sharing options...
ianknight Posted July 31, 2013 Share Posted July 31, 2013 This is of course why those of us with Mark 1 Linebackers love them and cherish them - the high speed cable test is ideal for this situation :) Link to comment Share on other sites More sharing options...
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