Scroller Cable

[dirkenstein]

Hi Everyone,

 

Our am-dram group has a collection of 12 Chroma Q Scrollers, two PSUs, as well as a large 'heap' of 4-pin scroller cable of various lengths, which I believe was all acquired together second-hand at the same time as the scrollers.

 

Over the past two years it we have been having various reliability problems with our DMX control set-up, which usually includes a Jands event 408, a few hired movers (VL5/VL6s or Mac500s) and two cheapie Botex 72 channel demuxes for the dimmers. The unreliable DMX problem has manifested itself as unresponsive or 'jittery' scrollers, randomly moving movers, or simply a loss of connectivity between the control desk and the scrollers/dimmer demux. In at least two cases we caught the problem during the get-in, but in one it manifested itself during the second evening of the show and made things rather 'sporty' (read- nightmare) for the crew.

 

We always terminate and don't try any peculiar DMX topologies, other than the bus split introduced by the scroller PSUs, which have their own built-in DMX repeater for the scroller loops. We do occasionally include a 4-way active DMX splitter/repeater, but we had the worst problems without this.

 

It always turned to to be a 'dodgy' length of cable causing the problem, with the frustrating characteristic that the cable would always appear to work initially, and degrade at some point during the get-in/performance.

 

Now it appears that the problem was caused by scroller cable in at least three cases. I can't remember if the nightmare performance involved a length of defective scroller cable or 'plain' DMX but we seem to have had to dispose of a lot more defective scroller cable over the last two-three years as of ordinary DMX cable- I think we only had one fault with that. I don't know what the branding of our ordinary DMX cable is- but it isn't mic cable, but I'm pretty certain it's data grade.

 

 

The scroller cable is one of the older brand-name types dedicated to that purpose, with extra-thick power wires. I suspect it's whatever the Chroma Q scrollers were officially provided with back in about 2000-2001.

 

The other characteristic of these cable faults has always been that they were not related to bad soldering or connections at the plugs, which are all Neutrik, and soldered with reasonable care judging by the few times I've ever taken one apart. The fault was always somewhere in the length of the cable. Only once was there any visible damage to the cable that might have indicated underlying problems, and even then it was only superficial chafing to the sheath that most people would overlook.

 

So based on my experience, I'm wondering if there's a known reliability problem with scroller cable in general. Is it relatively delicate? Does the stiffness, compared to other types of flexible cables (both power and flexible data/audio cables) make it more easily damageable? Is the insulation more susceptible to heat or mechanical damage? I get the impression that the stiffness and relative thickness of the cable and thinness of the outer sheath results in it getting 'mashed' by blunt mechanical forces that would leave other cable types undamaged.

 

We're only using the cable on 3-4 shows a year. It's not in weekly rental or touting use but it isn't statically installed.

 

As the scroller cable we're using is all second-hand, it could of course simply be that it suffered a great deal of use/abuse in its previous life and is now past its sell-by date. Unfortunately, we're not really in the financial position to replace all of it in one go.

 

Has anybody else had bad experiences with scroller cable? Is there more than one manufacturer? Is there a more flexible or robust variant of this cable available?

 

I'm wondering if anybody has any handling suggestions that would prevent the cable from degrading further. Right now we just coil it (neatly, without kinks or twists), tape it up with LX and lay it flat in trunks/boxes. I can't see anything obviously wrong with this.

 

Does anybody have suggestions for testing such cable (both ordinary DMX and scroller cable) quickly, especially a test that might reveal marginal but still functioning cables likely to 'lose it' in production?

 

Thanks,

 

Dirk

[David A]

Could I suggest that you need an active DMX tester like the Showtec or ADJ which tests the cable at its working frequency and reads the parameters of the signal, so that a borderline signal strength means the gear will work sometimes but not others.There is no other practical way to check these factors.These units will also output signal to test equipment.Having spent hours of eliminating various possible faults it is so easy to analyse the problem with this type of unit.

[pete LD]

The unreliable DMX problem has manifested itself as unresponsive or 'jittery' scrollers, randomly moving movers, or simply a loss of connectivity between the control desk and the scrollers/dimmer demux.

 

if you have a problem with scroller cable it could cause the scrollers to misbehave but that problem wont cause problems for the rest of your DMX system as each output from the psu should be isolated. where as if there is a problem with the DMX cable that links the psu and the movers, dimmers etc to the desk it can effect everything thats plugged in to it. so I would look at the DMX as opposed to the scroller cabling as a start.

hth

pete

[MarcT]

In my experience, "jittery" scrollers have usually been caused by an issue (faulty cable, no termination) in the DMX network. I think I've only ever come across one faulty bit of scroller cable (and that was obvious - no power mid chain).

 

The first thing I always try is to disconnect the DMX lines from the scroller PSU and see if the jitter stops. If it does, try running a "clean" DMX line from the desk straight in to the PSU & fit a terminator its DMX out. If the problem goes away then the scroller cabling is almost certainly ok.

 

A few other thoughts:

 

1) Always use a return cable from the last unit. This provides the DMX termination and a second path for power (like a domestic "ring main").

Make sure the return goes to the corresponding out (ie don't cross the circuits over).

 

2) Make sure the cable is wired correctly. It may sound daft, but different scroller vendors use different pins in the XLR4 for power & data. Its possible that the data is going down the "thick" power conductors, and the power down the thin DMX lines... Check the actual wiring against the ChromaQ manual:

ChromaQ-CQ1D

 

3) Don't exceed the maximum distance or number of scrollers per PSU (in the docs above).

 

4) There was a load of second hand scroller cable on the market a couple of years ago (I bought some). It was made from screened quad cable with one pair shorted together for Power +, the screen used for Power -, and the remaining pair for DMX +&-. However the XLR4 wiring (in my lot) was incorrect for ChromaQ. The solution is to chop the XLRs off and resolder the ends. Tedious, but I've had no problems with this stuff at all...

 

4) Try and use a DMX splitter, and terminate each line. In theory you can have 32 DMX devices on a single line. In practice...

 

5) There used to be a tech notice on the AC Lighting website about some early 18 way PSUs loading up the DMX line too much. Could be worth looking into this if yours is an early 18 way PSU.

 

EDIT: Look here (courtesy of the Internet Archive):

Technical Bulletins

 

6) The venue's permanent DMX wiring could be causing you problems. I've certainly had this in the past and couldn't get to the bottom of it. Jittery scrollers when connected to their DMX out. In the end I took another universe off the desk, fed it down the sound multicore via 5-3 & 3-5 changers, and into the scroller PSU. Problem solved.

 

All the best with your troubleshooting!

Kind Regards,

Marc

[dirkenstein]

Could I suggest that you need an active DMX tester like the Showtec or ADJ which tests the cable at its working frequency and reads the parameters of the signal, so that a borderline signal strength means the gear will work sometimes but not others.There is no other practical way to check these factors.These units will also output signal to test equipment.Having spent hours of eliminating various possible faults it is so easy to analyse the problem with this type of unit.

 

Hmm- anybody hire these out? They're really really useful but the more well-known brands seem to be about £300 (which would buy us a shedload of new cable). Is the Showtec one the same as the Botex Dr. DMX you can get from Thomann at £118? That would fall into my personal 'avoiding ulcers at get-ins' budget. Has anybody actually used that one? Any good at all?

 

I was thinking a serious stress test for short cables would be making a physical 10BaseT-to-DMX adaptor and ramming packets down a cable at 10mbit/s. Just loop the cable back at itself (Ethernet TX pair to go to DMX female, Ethernet RX to DMX male) and measure packet error rates. Probably won't work under Windows but I'll bet I can get Linux to play with some fiddling. For really long cables you might even be able to use the TDR feature in some Ethernet chipsets to find shorts/opens. Silly idea, really.

 

The unreliable DMX problem has manifested itself as unresponsive or 'jittery' scrollers, randomly moving movers, or simply a loss of connectivity between the control desk and the scrollers/dimmer demux.

 

if you have a problem with scroller cable it could cause the scrollers to misbehave but that problem wont cause problems for the rest of your DMX system as each output from the psu should be isolated. where as if there is a problem with the DMX cable that links the psu and the movers, dimmers etc to the desk it can effect everything thats plugged in to it. so I would look at the DMX as opposed to the scroller cabling as a start.

hth

pete

 

We've unfortunately had both problems. In three cases we lost control of scrollers due to faulty scroller cables.

Luckily only one was during a live run, and we could swap cables during the interval.

 

The more confusing case was when we thought we had the same phenomenon (jittery, uncontrollable scrollers) but a section of DMX cable downstream from the scroller PSU on the FOH bar was faulty (intermittent shorts between data + and -). We figured this out only after in desperation removing the whole FOH scroller ring during a show, freezing the scrollers on the FOH bar, only to discover the problem reappearing with unresponsive dimmers and movers on-stage. The problem was particularly confusing as it first manifested _upstream_ of the actual fault, which is not at all unlikely on an RS485 bus-based system, just somewhat counterintuitive. The fact that the DMX splitter we were using at the time was also suspect, as we built it ourselves, didn't help. As the final straw, we were working around a bug on the Jands involving scroller-equipped generics failing to dim completely in black-outs, which made diagnosing any failure that seemed to involve DMX or scrollers a game of 'point finger at random equipment' or 'guess where and wiggle'. An object lesson in the dangers of confirmation bias in fault-finding.

[David A]

Yes the Boetex is the same the euros price is Thormann 177 Highlite !!!.

[dougmorgan743]

I have had this problem in the past. Remove the return on the circuit that is giving you the grief. Until you have time to find the problem cable.