DMX Loop limitations

[ullrichr]

So I've been reading on the limitations of the number of fixtures on a single line, 32.  Yeah, I know it "could be more".

So if you stick to the 32 fixtures to a "DMX Loop", what constitutes a "loop".  From controller to the last light?  From a splitter to the last light?

How does adding a splitter figure into the max number of fixtures?  Does the splitter count in the max number?

Can you chain 15 lights and then drop in a splitter and do another 30? (Not that I would but could you?)

[indyld]

32 including the controller. Start counting again at each buffer point. But not from Thru. 

Edited May 5, 2023 by indyld

[sunray]

Oops... 168 and that was 13 rows of 13 LED PARs (except the middle)  wired as branches with choc block.

It was claimed to be trouble free until we installed a Zero88 dimmer and controller in the venue.

 

[Jivemaster]

AFAIK lots depends on the actual DMX chips and the true nature of the DMX cable. It's far better to spilt the desk output into 4/5/6 than to take a used line and try to boost it with any accumulated errors.

The concept of a loop is totally alien to DMX it should be (electrically) a straight line from source(desk or splitter) to the terminator at the end.

 

The problem always is that we all know a DMX rig that's functioned perfectly for years that actually doesn't comply with the tech spec. I knew a DJ who made his living from it who had a DMX controller and lights but at the top of a Tbar the DMX was all carried in twin flex. Fortunately the total length was about 5 metres and he got away with it.

Edited May 5, 2023 by Jivemaster

[Don Allen]

DMX512-A is a communications data protocol DMX 512-A Standard that uses RS485 as it transmission protocol , RS485 Quick Guide  so the limitation of 32 devices is an RS485 limitation. with development of newer RS485 drivers it is possible to have 64 devices on a single line. As Jivemaster points out it depends on the DMX chips and the only way to know that is to physically look inside the box. The cable definitely affects reliability of your RS485 Star Network. 

Always use terminators as it shows up cabling errors.
If using cheap Chinese origin lights, use a separate DMX feed from a splitter for each different model, so if one light goes faulty it does not affect the rest of the rig. I have seen cheap lights decide to become a Master when they go faulty which puts corrupt DMX data o the DMX network.
Stick to 32 devices on one line.

2 hours ago, Jivemaster said:

AFAIK lots depends on the actual DMX chips and the true nature of the DMX cable. It's far better to spilt the desk output into 4/5/6 than to take a used line and try to boost it with any accumulated errors.

The concept of a loop is totally alien to DMX it should be (electrically) a straight line from source(desk or splitter) to the terminator at the end.

The problem always is that we all know a DMX rig that's functioned perfectly for years that actually doesn't comply with the tech spec. I knew a DJ who made his living from it who had a DMX controller and lights but at the top of a Tbar the DMX was all carried in twin flex. Fortunately the total length was about 5 metres and he got away with it.

 

[kgallen]

7 hours ago, sunray said:

It was claimed to be trouble free until we installed a Zero88 dimmer and controller in the venue.

If the dimmer was a Betapack3 (or 4) then it will have had RDM, which probably didn’t like that arrangement!

Betapack1 and 2 just have the same RS485 receiver chip everything else has (plus input protection that ‘everything else’ might not have had!).

[sunray]

5 hours ago, kgallen said:

If the dimmer was a Betapack3 (or 4) then it will have had RDM, which probably didn’t like that arrangement!

Betapack1 and 2 just have the same RS485 receiver chip everything else has (plus input protection that ‘everything else’ might not have had!).

The additional kit we installed was a completely different system, the only commonality being mounted on the metal structure and the mains feed, I can't remember now what went in other than the make of controller.

 

Of course it was 'our fault' and I got the job to 'sort it out'. Luckily it was easy enough to get to the rig and all I had to do was make some m/m and f/f leads.

[graemeftv]

Some of the new ETC Source 4 led products are using 75lbc182 bus transceiver chips which the data sheet of the chip states "One-Fourth Unit Load Allows up to 128 Devices on a Bus"

We got fed up with 4 way buffers on our gigs, most of which everyone thinks because there are 8 XLRs it is an 8 way buffer. 

So we built the world largest 128 output DMX buffer!

https://ultralightsound.co.uk/product/ulsplit-DMX-buffer/

[david.elsbury]

45 minutes ago, graemeftv said:

 

We got fed up with 4 way buffers on our gigs, most of which everyone thinks because there are 8 XLRs it is an 8 way buffer. 

So we built the world largest 128 output DMX buffer!

https://ultralightsound.co.uk/product/ulsplit-DMX-buffer/

I was all excited for a minute because I thought “finally, a genuine post and not just marketing spam from you”, Graeme- basically every post of yours here is an advertisement for your company…

from your website : 

[quote]

8 DMX Universes 

8 isolated outputs per universe 

3pin and 5pin xlr for each output

Data led for each output

All the outputs are FULLY isolated. optically isolated and dc galvanic isolation. Meaning no matter what the fault on your DMX cabling or fixtures it is impossible for any of the other outputs to be effected 
[/quote]
 

so it appears you’re telling porkies; because 8 universes + 8 outputs per universe is 64; and there’s 128 sockets on this thing; so if you have a fault on output 1a (the 5 pin socket) it could actually affect output 1b (the 3 pin socket)?

of course if I’m wrong, I’d be more than happy to be corrected. Please, do tell ☺️

 

[ullrichr]

On 5/5/2023 at 3:11 PM, indyld said:

32 including the controller. Start counting again at each buffer point. But not from Thru. 

Just to be clear, in this crude example this would be 8 devices?  Not two lights in one run, three lights in the second run, and 3 lights in the third.  Or would it be 5 and 5?
If that's the case how could you ever control more than 32 devices with one board?

                                               |-Light----Light---Light
                                               |
      Board--Light--Light--Splitter
                                               |
                                               |-Light---Light---Light

 

[themadhippy]

Quote

If that's the case how could you ever control more than 32 devices with one board

when DMX was invented 32 units was considered big,I mean 186 dimmer channels wow,most of the    wiggly stuff either came with its own top secret protocol and desks,or was so expensive that  if you could afford 32 units the cost of a second operator and board was peanuts

[J Pearce]

32 devices per RS485 line. A proper active splitter generates a new RS485 line, so you can start counting again. So an 8 way splitter could happily feed over 200 devices.

[Tomo]

1 hour ago, ullrichr said:

Just to be clear, in this crude example this would be 8 devices?  Not two lights in one run, three lights in the second run, and 3 lights in the third.  Or would it be 5 and 5?
If that's the case how could you ever control more than 32 devices with one board?

                                               |-Light----Light---Light
                                               |
      Board--Light--Light--Splitter
                                               |
                                               |-Light---Light---Light

 

Assuming you're not using the "Thru" on the splitter, this would be 4 on the first run, then 4 on each of the other two runs.

Board + Light + Light + Splitter

Splitter + Light + Light + Light (twice)

That said, in reality you run into cable and reliability/redundancy issues long before you run into the line driver limits.

• You can only do "the maximums" if your cable is perfect. Nobody has perfect cable.
• Limit each segment to what you are prepared to lose simultaneously

The latter is generally more important.

If a single luminaire or cable breaks, what are you going to lose?
Are you going to have to stop the show, or can you afford to limp on without everyone on that cable segment?

Eg I'd never do the above, I'd always run directly Board > Splitter so a single failed light won't take out everything.

[J Pearce]

Also worth noting that often cable routing dictates splitter use long before fixture limits - i.e. doing a send and a return off a bar is more work (and points of failure) than just using 1 splitter output per bar. 

[ullrichr]

Thank you everyone for your input.  This has been helpful.