BigYinUK Posted September 21, 2017 Posted September 21, 2017 Hi all I've just bought some DMX cables from Studio Spares and I find that Pin1 is connected to the XLR shield at both ends of the cable. This surprised me as I always thought you shouldn't do that. Could any of the DMX gurus here please clarify? Regards
timsabre Posted September 21, 2017 Posted September 21, 2017 I've just bought some DMX cables from Studio Spares and I find that Pin1 is connected to the XLR shield at both ends of the cable. This surprised me as I always thought you shouldn't do that. It's not a great idea as if you plug into a metal XLR on a metal cased device the 0V/reference of the DMX gets connected to mains earth. However it'll mostly work just fine as nearly everything has plastic XLRs on it now. The fun can start if there is a wiring fault, if the DMX is linked to mains earth you can get large voltages on the line which start blowing stuff up. But this is pretty unlikely.
BigYinUK Posted September 21, 2017 Author Posted September 21, 2017 I've just bought some DMX cables from Studio Spares and I find that Pin1 is connected to the XLR shield at both ends of the cable. This surprised me as I always thought you shouldn't do that.It's not a great ...... The fun can start if there is a wiring fault, if the DMX is linked to mains earth you can get large voltages on the line which start blowing stuff up. But this is pretty unlikely. Hi Timsabre Yep, I realised that was why you shouldn't. Just surprised to find ready made cables with the shield connected.
timsabre Posted September 21, 2017 Posted September 21, 2017 Maybe ask them why... I can't think of a good reason to link them. It's an even worse idea on audio cables.
Stuart91 Posted September 21, 2017 Posted September 21, 2017 There's definitely no upside to having pin 1 connected to the XLR shell, so you're probably better off snipping the link. It's a reasonably quick job. Bulk pre-made cables can be a bit of a lottery. The pin 1 -shield thing is quite common, and some of the cheaper ones that I've come across have had terrible termination, with far too much bare wire exposed and almost shorting. A few years ago a local charity had bought a whole rig's worth of mic cable (connecting both the input side and powered speakers etc.) and every single cable had a pin 1/2 swap. They only discovered this when they set it up for the first time at a festival, and couldn't stop the buzzing.
david.elsbury Posted September 21, 2017 Posted September 21, 2017 Do you mean shield, or SHELL The cable shield should be connected to pin 1, and the data pair to pins 2&3
BigYinUK Posted September 21, 2017 Author Posted September 21, 2017 Do you mean shield, or SHELL The cable shield should be connected to pin 1, and the data pair to pins 2&3 Shell (oops)
sunray Posted September 22, 2017 Posted September 22, 2017 every single cable had a pin 1/2 swap. They only discovered this when they set it up for the first time at a festival, and couldn't stop the buzzing. Been there done that.Fortunately I had enough of mine there to do the job.
BigYinUK Posted September 23, 2017 Author Posted September 23, 2017 Interesting... Plugged all the lights up and everything worked - first time - which is a first for us. So, OK, maybe using proper DMX cables does make a difference. :** laughs out loud **:
Brian Posted September 23, 2017 Posted September 23, 2017 The Recommended Practice issued by PLASA and USITT is quite clear on the matter...don't do it.
Stuart91 Posted September 26, 2017 Posted September 26, 2017 every single cable had a pin 1/2 swap. They only discovered this when they set it up for the first time at a festival, and couldn't stop the buzzing. Been there done that.Fortunately I had enough of mine there to do the job. We had to scrounge an entire stage's worth of cable from the other venues on site, since it was the only option given the time available. Then someone had to make an overnight trip back to our warehouse to collect more and get things up and running properly. All because someone thought they had found some "bargain priced" XLR in the local music shop... The Recommended Practice issued by PLASA and USITT is quite clear on the matter...don't do it. It's great that there's a clear industry consensus on this. It would be nice if suppliers / distributors could pay attention to it. Maybe with a bit of promotional work it could become a selling point. If punters were looking out for cables that were built to "PLASA/USITT Standard" then there might be a lot less problems.
timsabre Posted September 26, 2017 Posted September 26, 2017 The Recommended Practice issued by PLASA and USITT is quite clear on the matter...don't do it.It's great that there's a clear industry consensus on this. It would be nice if suppliers / distributors could pay attention to it. Maybe with a bit of promotional work it could become a selling point. If punters were looking out for cables that were built to "PLASA/USITT Standard" then there might be a lot less problems. Trouble is most sensibly priced XLR cables are imported by the containerload from China. And the recommendations are only recommendations, they are not a standard.
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