DMX/RS-485 line capacity

[Keeper of the Keys]

So as we all know DMX-512(in all it's versions) uses RS/EIA-485 for it's physical layer and complies to it unless otherwise specified in the relevant documents.

 

I would like to get the following clarified, is the line-driver (in a non-RDM system case the desk) included in the 32 device (units of load) limit (ie. only 31 devices other than the desk unless you use a booster/splitter etc.) or is it line-driver + 32 units of load?

 

Different sources word it differently and have put me in my position of doubt and uncertainty that I'd like to resolve...

 

Also would you call splitting a line that has 30 devices connected into 2 as a percaution in case a failure causes a device to act as more than one unit of load or a cable not beng up to spec as paranoia or a healthy precaution?

[david.elsbury]

The desk is meant to count as 1 "unit" of load. And I would call the splitting that you've described healthy precaution, in most cases you'll probably get away with the single line, but if you have a splitter to hand, and if the toplology of the rig calls for it (I.e, not just a single truss), then why not split?

[Modge]

but if you have a splitter to hand, and if the topology of the rig calls for it (I.e, not just a single truss), then why not split?

Indeed I'd always rather split, it makes for easier cabling and fault finding as well to list just two of the more persuasive arguments. The only reason I see ever not to is lack of a split and that doesn't happen much. Whilst I'd admit they're not entirely inexpensive put next to the things they connect they're very reasonable.

 

Installs if anything I'm less inclined to bother on: it's one more thing to buy and the cabling \ fault finding is only done once. Not that I wouldn't rather have one even in the case of installs if there was money about.

[dbuckley]

And its best if the split is opto-isolated; then you cant get the possibility of bad things blowing up desks...

 

I'm also in the "always split" camp.

[boatman]

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I would like to get the following clarified, is the line-driver (in a non-RDM system case the desk) included in the 32 device (units of load) limit (ie. only 31 devices other than the desk unless you use a booster/splitter etc.) or is it line-driver + 32 units of load?

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Sorry to come to this a bit late, but the answer actually depends on the driver chip used in the source. The standard RS485 chip (SN75176) used in most systems has a receiver permanently connected to the transmitter inside the chip and thus counts as a unit load. Some RS485 driver chips do not have this and therefore can drive 32 external unit loads and some modern receiver chips only present a ½ unit or even a ¼ unit load to the line. However it is safest to assume each device, including the source, presents a unit load to the line and keep to the limit of 31 external loads. Personally, I would agree with the other posters and keep to a lower limit than the theoretical maximum and always use a splitter where needed.

 

Also don't forget to terminate all DMX lines which are loaded even if it's only a single fixture.

[timsabre]

Just to confuse the issue further, newer designs of RS-485 transciever chips present a much smaller load to the line than older ones. The 32 device limit is now up to 256 devices, if these receivers are used throughout.

 

Of course, you probably don't know what your lights have got in, so it's wise to stick to the 32 (or 31) devices. But I would expect to see the standard revised at some point in the (probably distant) future. And this lower loading gives a much better noise margin on the DMX cabling making it more reliable.

[collism]

I always use a opti-isolating splitter, just seems a sensible insurance measure, stops problems spreading to the desk and makes fault finding much easier. Also if something goes wrong mid-show at least its more likely to be some not all fixtures!

[Jivemaster]

If you know what your DMX line will be driving then you may be able to hang more that 32! Good modern kit will load the line less than standard kit or cheap disco kit. So far all my DMX failures have rotated about second class connectors. it really is worth changing the connectors on the cables for the best you can get (usually Neutrik ) and keeping them in perfect condition

 

Given a line of 30 I'd use a splitter. I would also have some really LONG DMX leads as 30m made of three 10m leads adds more connectors to check than a 30m lead.

[Keeper of the Keys]

Thanks for all the answers.

 

Is there any way of knowing other than trying (which I think is a waste of valuable time when you are trying to get a show up and running, and risky as you may have to end up troubleshooting) how much load a device presents?

I don't remember ever seeing a manual mentioning that the device only presents X load to the line.

[timsabre]

Thanks for all the answers.

 

Is there any way of knowing other than trying (which I think is a waste of valuable time when you are trying to get a show up and running, and risky as you may have to end up troubleshooting) how much load a device presents?

I don't remember ever seeing a manual mentioning that the device only presents X load to the line.

 

There's no way of knowing, I have never seen such information either. Safest to stick to the 32!

[boatman]

Thanks for all the answers.

 

Is there any way of knowing other than trying (which I think is a waste of valuable time when you are trying to get a show up and running, and risky as you may have to end up troubleshooting) how much load a device presents?

I don't remember ever seeing a manual mentioning that the device only presents X load to the line.

 

There's no way I know of measuring the load of a fixture, but you could open each one up, locate the RS485 interface chip and find its data sheet on Google. That's certainly not something you want to do during a get-in, but maybe sometime if you have an idle moment....

 

If you are doing a lot of temporary DMX installations then get one of these for your toolkit. I've only ever used mine in anger once and it saved several hours of fault finding.