Strange behaviour on a DMX chain

[timsabre]

I have felt all along that you have a stray voltage problem on the DMX - which wiring that green wire to mains earth could well be causing, in conjunction with another fault, probably a bad DMX GND connection or bad earth or something. Are all the fittings wired like that?

I would expect if you just cut off the green wires it'll work properly. They are maybe to power inline splitters or a remote controller or something?

[Stuart91]

Thanks, Tim. 

Yes, all the other fittings are wired the same way, and as far as I can see all of the other "clusters" of fittings have been done identically. 

There are at least half a dozen "troublesome" fixtures now so it's getting a bit fraught. I'll suggest dropping the green connection and see if that improves anything. 

[dbuckley]

OK, it seems the PI/PO lines are used to set the DMX address, with the intention that one device's PO is connected to the next device's PI in a daisy chain.  A Google or Ali search for DMX address writer will bring up the devices used to do the programming.  It seems that a bunch of ICs used in LED fixtures and controllers use one of a number of ICs, with Chinese data sheets(!), which allow address programming. 

Until about a minute ago, I was totally unaware of this whole PI/PO thing.

On the wider issue; I'm not fond of the DMX ground being the same as the mains ground, I'd rather the DMX ground was floating, but on a short run with no electrical problems disturbing the ground reference, there shouldn't be any real impact on DMX ground.

[Stuart91]

1 hour ago, dbuckley said:

Until about a minute ago, I was totally unaware of this whole PI/PO thing.

Me too!

Although I suppose it makes sense, as there's no other way of setting the address on the LED drivers. I was anticipating a hidden set of dip switches somewhere but I guess that wouldn't make sense on an IP rated unit. 

[sunray]

On 9/29/2023 at 2:48 PM, Stuart91 said:

 Brown and black cores are the DMX positive and negative respectively. There is a blue core, and the earth (green/yellow). 

It looks like the earth is connected to mains earth.

The blue core seems to be connected back to a "return" terminal on the casDMX. 

 

I deduced this is as fitted

On 10/2/2023 at 6:01 PM, sunray said:

When you can some pictures or a decent sketch would be helpful.

From your description I see this:

 

Now from this

 

There appears to be some discrepancy with the blue and black wires

 

I don't know what PI & PO are, however is there a possibility it stands for power in and out, which brings me back to

On 10/1/2023 at 12:56 AM, sunray said:

I've just looked up my notes from 2016, I didn't note the product, only wiring and colours. I show

Yellow = DMX+

Blue = DMX-

Black = 0V

Red = +24V

So straightaway there doesn't seem to be any similarity.

 

 

Edited November 1, 2023 by sunray

[sunray]

5 hours ago, dbuckley said:

OK, it seems the PI/PO lines are used to set the DMX address, with the intention that one device's PO is connected to the next device's PI in a daisy chain.  A Google or Ali search for DMX address writer will bring up the devices used to do the programming.  It seems that a bunch of ICs used in LED fixtures and controllers use one of a number of ICs, with Chinese data sheets(!), which allow address programming. 

Until about a minute ago, I was totally unaware of this whole PI/PO thing.

On the wider issue; I'm not fond of the DMX ground being the same as the mains ground, I'd rather the DMX ground was floating, but on a short run with no electrical problems disturbing the ground reference, there shouldn't be any real impact on DMX ground.

 

4 hours ago, Stuart91 said:

Me too!

Although I suppose it makes sense, as there's no other way of setting the address on the LED drivers. I was anticipating a hidden set of dip switches somewhere but I guess that wouldn't make sense on an IP rated unit. 

I didn't see these posts before posting my previous as I started writing at lunch time then granddaddy daycare kicked in.

I have done an extremely quick search and it seems you have hit this nail on the head, however I haven't yet found any detail except it requires a controller on the PI line, in the absence of such should we make an assumption it defaults to DMX1? or is it a programming line which sets the address once and it's saved in the fitting?

[dbuckley]

40 minutes ago, sunray said:

 

...should we make an assumption it defaults to DMX1? or is it a programming line which sets the address once and it's saved in the fitting?

I would guess absolutely that the thing will ordinarily be receiving DMX; the PI/PO is only needed if one wants to change the address of the thing. 

The pictures I found show PI/PO daisy-chaining, as I noted above, however, I didn't find anything (or, at least, anything I could understand!) about what to do with the first PI line when one doesn't want to fool with the programming.  I'd assume (yeah, we all know) that it can simply be left unconnected and insulated, as if it required to be pulled down or something it would be both damned inconvenient to the wirer-upper, and also prone to causing customer complaints when the pulldown wasn't connected.

Although this PI/PO thing is an interesting diversion, not to mention some education, I'd venture that it's not involved in the problem.

Other things that occur - the DMX line should be terminated; if adding termination causes problems, then the problem is not the termination, it's something else that is broken, and the presence or absence of the terminator is just exposing that problem.  Also, given that the PI/PO lines are directional (ie PI is in, and PO is out) I wonder if there is some directionality in the DMX flow too, implying DMX buffering per box, which would be, in a cheap Chinese box, highly unlikely, I grant you, but inbuilt buffering is not unheard of.

I'm still in the camp of corrupt signalling; is the CASbox adjacent to the chock blocks, or far, far away? I note the CASbox requires a class II power supply, so an ungrounded power supply. 

TimSabre earlier suggested getting a multimeter out, and checking resistances and voltages.  I support that suggestion.

 

[sunray]

19 hours ago, dbuckley said:

I would guess absolutely that the thing will ordinarily be receiving DMX; the PI/PO is only needed if one wants to change the address of the thing. 

The pictures I found show PI/PO daisy-chaining, as I noted above, however, I didn't find anything (or, at least, anything I could understand!) about what to do with the first PI line when one doesn't want to fool with the programming.  I'd assume (yeah, we all know) that it can simply be left unconnected and insulated, as if it required to be pulled down or something it would be both damned inconvenient to the wirer-upper, and also prone to causing customer complaints when the pulldown wasn't connected.

Although this PI/PO thing is an interesting diversion, not to mention some education, I'd venture that it's not involved in the problem.

I'm not so sure, OP doesn't show whether the blue and the green/yellow are daisy chained or teed, I for one assumed (yeah, we do all know) they'd been teed, however OP now states green/yellow is not continuous - possibly implying it's daisy chained and if so when in use it could be setting following addresses to something irrelevant.

 

19 hours ago, dbuckley said:

Other things that occur - the DMX line should be terminated; if adding termination causes problems, then the problem is not the termination, it's something else that is broken, and the presence or absence of the terminator is just exposing that problem.  Also, given that the PI/PO lines are directional (ie PI is in, and PO is out) I wonder if there is some directionality in the DMX flow too, implying DMX buffering per box, which would be, in a cheap Chinese box, highly unlikely, I grant you, but inbuilt buffering is not unheard of.

 

OP has stated 3 of the 4 wires have continuity, whether this is per unit or end to end I know not.

 

19 hours ago, dbuckley said:

I'm still in the camp of corrupt signalling; is the CASbox adjacent to the chock blocks, or far, far away? I note the CASbox requires a class II power supply, so an ungrounded power supply. 

TimSabre earlier suggested getting a multimeter out, and checking resistances and voltages.  I support that suggestion.

 

Now stated as fitting 2 being the faulty item and currently on the bench.

 

I've assumed (Yes) the casDMX is in the box with the choc bloc's and OP has stated as <10m of DMX line length so that tends to agree.

 

I'm still placing some credence on this discrepancy with blue and black wires:

On 9/29/2023 at 2:48 PM, Stuart91 said:

 Brown and black cores are the DMX positive and negative respectively. There is a blue core, and the earth (green/yellow). 

It looks like the earth is connected to mains earth.

The blue core seems to be connected back to a "return" terminal on the casDMX.

 

 

I previously agreed with Tims suggestion to test but I can't help thinking that this apparent wiring discrepency could potentially be something to do with it, particularly As I knew of a system running on single core screened cable with pins 1 & 3 connected together for a number of years until a change occurred and ...

[sunray]

A completely different situation but I worked on an all LED system consisting of fairly cheap 8"  (par64) 150 LED fittings. The thing about them being all DMX was buffered in each fitting and they could work master/slave but still controlled by DMX.

However (I hope I remember this right) the master fitting sent data to the slaves on DMX 1-4 then forwarded incoming data on addresses 5 and above.

Hopefully this may make some sense, starting at the desk on the left any fitting on the DMX line before the master would follow the channels on the desk. After the master (Column 'F') all of the addressess are increased by 4 with the slave taking the 1-4 position.

Adding another master fitting added its slave at DMX 1-4 and moves everthing beyond it up 4, it doesn't take too much to realise that with multiple masters in the system there could be fittings dotted around the system addressed 5, 9, 13, 17 all controlled by desk channels 5-8.

 

Giving that some comparison with this thread, I realise the DMX line is continuous (not buffered in each fitting) but if the PI line were somehow able to change addresses in the system (corrupted)...

 

 

 

 

[Stuart91]

On 11/2/2023 at 1:06 AM, dbuckley said:

Other things that occur - the DMX line should be terminated; if adding termination causes problems, then the problem is not the termination, it's something else that is broken, and the presence or absence of the terminator is just exposing that problem. 

Interestingly, there were some flickers from one lamp (which began literally as I was starting the engine on my van, ready to make my escape...)

Adding a terminator seemed to suppress them so I've advised them to add the resistors to the end of each chain. There's nothing there at present. 

[Stuart91]

On 11/2/2023 at 1:06 AM, dbuckley said:

I'm still in the camp of corrupt signalling; is the CASbox adjacent to the chock blocks, or far, far away? I note the CASbox requires a class II power supply, so an ungrounded power supply. 

The CASbox is in one enclosure, mounted on the back of the frame. The output from it runs into another enclosure further down which is where all the connection blocks reside. Roughly 1m between the two, and it's also done in 4 core HO7 0.5mm flex.

[Stuart91]

16 hours ago, sunray said:

OP has stated 3 of the 4 wires have continuity, whether this is per unit or end to end I know not.

This on the single unit that I have taken away with me and got on the workbench. So it seems to imply that the DMX signal is wired as I'd expect (with the fixture tee-d off the input and output connections) rather than being buffered. 

Meanwhile there is no continuity on the yellow/green core which is the PI / PO, suggesting that they're handled differently.

[sunray]

10 hours ago, Stuart91 said:

This on the single unit that I have taken away with me and got on the workbench. So it seems to imply that the DMX signal is wired as I'd expect (with the fixture tee-d off the input and output connections) rather than being buffered. 

Meanwhile there is no continuity on the yellow/green core which is the PI / PO, suggesting that they're handled differently.

Can you verify if it is wired to the top or bottom part of this sketch please.

[Stuart91]

On 11/3/2023 at 11:47 PM, sunray said:

Can you verify if it is wired to the top or bottom part of this sketch please.

Just checked the one photo I have of the innards of the enclosure. 

Brown is connected to DMX positive on the CasDMX (lining up with A on the drivers)

Blue is connected to DMX negative on the CasDMX (lining up with B on the drivers)

Black is connected to "Return" on the CasDMX (lining up with GND on the drivers)

 

The internal wiring inside the enclosure uses different colours (I'm guessing whatever fell to hand as they finished off the wiring...) so it's not as straightforward to trace as it should be. 

 

Meanwhile I've had a further play with the unit I have in my workshop at the moment. I hooked it up in a DMX chain with four other fixtures (just random LED battens from our hire stock, which are the same 4-channel RGB+White for control). No matter the order of fixtures in the chain, I can't get the same misbehaviour as I saw on site. 

However, I was able to connect the green wires (PO and PI on the driver) to my mains earth temporarily. The fixture started flickering immediately, although there wasn't any effect on the others in the chain. 

 

I've had radio silence from the fixture supplier regarding their opinion on what PI and PO are meant to be used for, so I'm inclined to agree with Tim and David's suggestions above. 

[timsabre]

David is right about the auto addressing thing. Ignore my comment about power.