DMX problem

[indyld]

To answer the original question from an in-the-field point of view, yes this kind of unfathomable symptom is pretty common. In my experience, trying to either divide and decide or follow a DMX problem logically along a chain can often throw up weirdness. Not least because it's not really a chain but a large parallel connection. 

One thing that helps with this is termination. It can make certain faults more obvious to track down, including stopping dodgy connections from communicating at all. 

I've also found that additional line drivers such as a buffer can change how the fault shows up and often remove them where possible as part of the fault finding process.

Standard lampie practice on diagnosing a run is to disconnect everything, run an FX from the desk, and reconnect fixtures and cables one at a time. However, the last thing you connected that seemingly introduced the fault isn't always the problem. 

 

[sunray]

9 hours ago, Jivemaster said:

We all know of a DMX network that works but doesn't comply (I knew a well paid DJ who fanned out DMX in twin flex and it worked for years) BUT the first fault finding needs to be the cable and the connectors. Because DMX is effectively RF reflections can occur at damage or breaks in cables these can be reflected back down the line and confuse or conceal the sent DMX -hence the frequent finding that the fault shows up somewhere else. 

Not only that but I have repaired many a commercial T-bar system where mains and DMX are fanned out from a central connexion box, admittedly the leads were only 800mm or so maximum but the cable used was akin to twisted bellwire and far too flimsy for roadshow use.

I knew a band who used multi core mains flex to provide mains and DMX to their stands using bulgin 8 pin connectors which were simply paralleled and fanned from a 'stagebox'.

And a nightclub with a matrix of 13x13 LED PARS where the DMX ran along one end of the rows and fed to each row using choc bloc off the feed line. We found the bad method when we added some incandescent lights and dimmer packe, completely unrelated to them and caused interference.

 

4 hours ago, indyld said:

Standard lampie practice on diagnosing a run is to disconnect everything, run an FX from the desk, and reconnect fixtures and cables one at a time. However, the last thing you connected that seemingly introduced the fault isn't always the problem. 

 

Initially another reckoned it was 'the faulty fitting' so my first action was to exchange it for a spare, then add a terminator, then swap with the following fitting but the fault stayed in the same place. Then unplugging DMX pointed to further on and at the next fitting wiggling the faulty cable was obvious.

 

I'm far from a lighting expert but have dabbled for 50 years starting in school and from mid 90's with Martin then DMX but none of my own kit till about 5 years back. Generally I've found a faulty cable affects some/all fitting following it. I carry a terminator in all sorts of bits of kit but It's quite rare for me to remember to fit one on my tiny systems as I think this is the first fault I've had on my stuff.

Edited July 6, 2022 by sunray

[david.elsbury]

What kind of wire is that? It looks like Musiflex? Or is it proper DMX cable?

[mackerr]

41 minutes ago, david.elsbury said:

What kind of wire is that? It looks like Musiflex? Or is it proper DMX cable?

Best response so far...

Mac

[sunray]

I have no difficulty in admitting I've not purchaced any specific cables for my own DMX use.

I'm fully aware of the insistence that using audio cable is wrong and I don't disagree, however my DMX systems are tiny, that particular universe was 5 devices and well under 20m of cable, possibly only half of that. To date I'd not had any problems with my kit and replacing the broken cable resumed full service.

Whilst I recognise the comment and I'm grateful for it,  it was far from the reason for the failure.

[Tomo]

Due to the nature of DMX, a lot of broken systems "appear to work fine" right until the moment they ... don't.

A lot of the time, more than half the packets could get rejected due to framing errors and you'd never know - 20 fps looks very similar to 44 fps.

If the fixtures respond relatively slowly (like tungsten or motorised parameters), single-packet wrong values are often invisible as the device simply can't respond quickly enough for you to see them.

A system with broken wires, wrong impedance cable or missing/wrong termination does have reflections, but reflections don't actually matter until a receiver is sat on a node. Changing the cable length moves the nodes, so adding or removing cable might move a node onto a fixture - or off it.

Changing the DMX levels can make it more or less likely that a fixture is going to miscount slots - the most classic flicker. eg it misses the actual start bit due to a noise spike, but the levels for the next few slots look like proper framing. (This is why checking both stop bits matters, but many devices don't)

A broken common/shield raises the noise floor and also means the common-mode voltage will be set entirely by capacitive coupling to "something". Which depends on what's near the cables, the specific details of the fixtures beyond the break and the condition of the liver in the chicken you sacrificed before the show. Once you exceed the common mode limit the transceiver chip could start to do almost anything. Eventually it might enter thermal shutdown or even burn out, but that'll take a while. Until it actually fails there may not be any visible symptoms at all.

LED fixtures (and strobes) can respond far quicker, so it's fairly common for a system to be "fine" for years, then they get some new LED fixtures that don't have any smoothing and they flicker.

TL;DR: Use the right cable, always fit a terminator.

[david.elsbury]

9 hours ago, sunray said:

I have no difficulty in admitting I've not purchaced any specific cables for my own DMX use.

I'm fully aware of the insistence that using audio cable is wrong and I don't disagree, however my DMX systems are tiny, that particular universe was 5 devices and well under 20m of cable, possibly only half of that. To date I'd not had any problems with my kit and replacing the broken cable resumed full service.

Whilst I recognise the comment and I'm grateful for it,  it was far from the reason for the failure.

Is it Musiflex? http://www.connectronics.uk.com/musiflex.htm
 

it has a “conductive thermoplastic shield”- which I imagine does not perform well for DMX data… a quick Google suggests Musiflex has almost double the capacitance between conductors as “proper” RS485/DMX cable. I can’t find an impedance spec for it.

did the problem go away when you stopped using the cable? If so you can hardly  say that it’s “far from the reason for the failure”

 

I have used audio cable for DMX before too- but Musiflex is different and I’d never attempt to use it for non-audio uses. ☺️

 

Edited July 8, 2022 by david.elsbury

[Brian]

6 hours ago, david.elsbury said:

I can’t find an impedance spec for it.

Interestingly the actual impedance doesn't really matter. Any two-core plus overall screen cable is going to be in the 90-130 Ohm range and any of them will work. Likewise, for a termination resistor, anything in the range 100-150 Ohms is way superior to having nothing. And, for DMX, it's not that important to match the resistor to the cable.

The reason for the latter is that there are so many discontinuities the overall impedance is rarely a single figure.

What is importance is, as David says above, the cable's capacitance.

[sunray]

On 7/8/2022 at 2:51 AM, david.elsbury said:

Is it Musiflex? http://www.connectronics.uk.com/musiflex.htm
 

it has a “conductive thermoplastic shield”- which I imagine does not perform well for DMX data… a quick Google suggests Musiflex has almost double the capacitance between conductors as “proper” RS485/DMX cable. I can’t find an impedance spec for it.

did the problem go away when you stopped using the cable? If so you can hardly  say that it’s “far from the reason for the failure”

 

I have used audio cable for DMX before too- but Musiflex is different and I’d never attempt to use it for non-audio uses. ☺️

 

I hadn't considered than aspect, I was  not going to bother repairing the lead as I have far in excess to what I ever use, but I'm curious now and will repair/reinstate it into place to see.

In the meantime I've tested cable with a chaep multimeter and get 160-172pf twix conductors and 270-290pf to screen the variation is moving cable (2.8m long) and meter leads, in contrast a 6m cable marked 'DMX - DIGITAL AUDIO CABLE' (which I think is same as between desk and 1st fitting -dimmer pack) shows 300-360pf & 490-530. 9501 is listed as 40 & 74pf/ft, both leads come in under that.

[david.elsbury]

FYI the cable I quoted above was for a 1m length 

[sunray]

As a follow up to this I've done a little bit of playing. I hadn't intended to repair the broken lead but the thread pointed to different things, I soldered the broken screen ie I didn't reterminate the whole end, just removed the airgap. putting it back into the original location it performed correctly. To verify, while the system was running through a simple colour changing scene, I applied the soldering iron the the repair and straight away the flickering started and equally resoldering corrected it. At that I moved the lead (open circuit) to all of the other parts of the small system, (meticulously retaining the position of all other cables) it was the only position it caused problems. I also tried another cable (Canford DFT) about 200mm shorter and the flickering started as soon as I cut the screen.

I checked earth to metalwork is sound, also QTX dimmer pack(fitting 2) has earth continuity to pin 1 of XLR, the other dimmer packs (different universe) don't have this so I swapped 2 over with no apparent effect.

 

As much as I'd like to have a different answer I think this has concluded the problem was the broken screen.

[Ynot]

I didn't think that the DMX spec includes tying pin 1 to screen/earth??

I certainly don't do that when making up cables

[IRW]

26 minutes ago, Ynot said:

I didn't think that the DMX spec includes tying pin 1 to screen/earth??

I certainly don't do that when making up cables

Indeed not. This is a Very Bad Thing...a mains short to earth in the dimmer pack will result in potential serious damage of kit down/upstream!

[sunray]

14 hours ago, IRW said:

Indeed not. This is a Very Bad Thing...a mains short to earth in the dimmer pack will result in potential serious damage of kit down/upstream!

FWIW the Martin products used to be tied to earth and yes we did have a fault (on the moving rig motors) that took out all of the control boards in the system.

[timsabre]

You definitely shouldn't connect pin 1 to earth/casing in the cabling but some products (incorrectly) have it connected inside the fixture. In this case the missing cable screen as in Sunray's fault would not cause a loss of DMX as the ground continuity would be kept via mains earth.

Other products I have seen use a 100R resistor between pin 1 / system ground and earth which is slightly more tolerant to mains faults.