Passively splitting DMX.

[ianknight]

1 hour ago, kgallen said:

Would that be a slightly portly gentleman with a beard? 🤣

Possibly... :P

[indyld]

5 hours ago, kgallen said:

I’m a bit bemused. Why wouldn’t one just put in a terminator as part of the install in the first place?

"Did they send any terminators?"

"Nope."

"Do we own any terminators?"

"We had some. Maybe two. They are somewhere."

"Anyone got time to look for them?"

"Nope"

 

 

 

[kgallen]

Isn’t that a bit like:

’Did we get any Sharpies?’

’Nope’

’Did we order any?’

’Nope’

 

[DrV]

I think the differing attitudes towards this are emblematic of the difference between technicians and engineers.

Installing a terminator at the outset is a particularly effective test of the cabling - if there are any dodgy connections (especially high resistance) that may change during a show, the terminator may well cause them to show up immediately. Far better to pick up such a thing before it happens than to have to work around it later.

Another argument in favour of a terminator is that it allows you to check the integrity of the whole chain quickly with an ohmmeter. Measure between the Data+ and Data- connections at the desk end and you'll see the resistance of the terminator. If not you have a break in one leg which is the classic fault which "works until it doesn't".

[indyld]

55 minutes ago, DrV said:

Another argument in favour of a terminator is that it allows you to check the integrity of the whole chain quickly with an ohmmeter. Measure between the Data+ and Data- connections at the desk end and you'll see the resistance of the terminator.

This is the theory. One might say the 'engineers' view, or at least one from the comfort of the bench. 😉

In practice, with road-wrecked cable stock that turns out to be corroded by water ingress, has been yanked on by meatheads, jammed in a flightcase lid, and then run over by a Telehandler, the reading you might manage to get at the desk could literally be anything. 

This is why DMX is great. It (appears that it) works correctly - despite all the above. Showbusiness is about creating an illusion that everything is perfect and effortless, and the tech is often no exception. Luckily, we don't take 'scopes on site even if there was time to use them. In fact, getting out any diagnosis tool on a load-in is a sign that the day is as good as lost. 🙂

To clarify, I'm all for termination. I've just spent decades with other people putting in rigs without them. Some companies can't even scrabble together a couple from stock if asked. I've sometimes resorted to stuffing a resistor into the last XLR.

Edited February 18, 2022 by indyld

[alistermorton]

Kind of like you don't need a safety wire, until you do.

[dje]

On 2/17/2022 at 3:09 PM, kgallen said:

The resulting stub length on the transmission line within the fixture is a couple of centimetres, not long lengths of cable. Electrically, at DMX frequencies, this is a very different situation to making a passive split then hanging tens-of-meters of cable off each leg. Electrically, a DMX network is a transmission line, which has very specific electrical characteristic and behaviour. The tolerance to stubs and impedance mismatches causing reflections is a function of frequency. At DMX frequencies, short unterminated stubs like those inside the fixture can be tolerated whereas such stubs would be completely unacceptable at RF frequencies.

Absolutely valid, was just making the point that it's not the splitting that causes the harm.

[sunray]

3 hours ago, dje said:

Absolutely valid, was just making the point that it's not the splitting that causes the harm.

Not unless there are a number of them and all terminated...

Been there too.

[Tomo]

Quite a lot of fixtures have a built-in self-terminating circuit on the "Thru" XLR-5 - it's just a PCB-mount female XLR-5 with a built-in microswitch, and a resistor.

So you may well find that many of those runs were terminated anyway, even if you didn't do anything at all.

The real trouble with DMX is that you've no idea how close you are to disaster.
It's very difficult to tell the difference between a DMX run that's right on the edge of a massive flicker storm, and one that's got plenty of headroom.

I recall one site that had been running 'just fine' for years, then suddenly it started having flicker storms  lasting several minutes.
The problem there turned out to be an XLR-5 that had not been soldered at all, they'd just stuffed the cores into the buckets.

RDM tends to detect these borderline installs - if RDM doesn't work over a passive run, chances are  that the DMX signal is very poor and you've been living on the edge.

Edited February 21, 2022 by Tomo

[Stuart91]

1 hour ago, Tomo said:

The real trouble with DMX is that you've no idea how close you are to disaster.

Similar to HDMI...

[sunray]

34 minutes ago, Stuart91 said:

Similar to HDMI...

Except DMX isn't limited to 2ft

[timsabre]

Well any digital comms protocol is the same, they either work perfectly or very very badly. Because DMX has no error correction it is probably easier to spot approaching trouble - protocols with error correction tend to mask any minor problems so you go from 100% to nothing, with DMX you do start getting dodginess that indicates something is going wrong.