DMX problems - am getting confused

[mumbles]

Hey BR peeps,

 

Have been asked to fix an intriguing problem that a local school is having. They are using a FatFrog, with 4 cheap heads and a load of Betapack 3s. 2 DMX runs (same universe being a FatFrog) come out of the desk, 1 goes to the venue dimmer room, and the second goes to the stage, through 3 Betapacks, then 2 heads, then runs to the other side of stage and into the other 2 heads.

 

Problem - the final 2 heads weren't responding to any desk commands at all. However they were showing as receiving DMX (and weren't entering their annoying STL feature which starts automatically if DMX is lost).

 

Having chased round the patch on stage and at the desk to try I find a problem I ended up re-running the DMX runs straight out the desk to the 2 heads to check they worked, and bam, immediately had control. Replace the run as it had been previously and they lose control again. In order to make the rig work for their show have resorted to one line of DMX coming straight out of the desk to the 2 heads SR, and the other line going to the venue dimmer room, then to the dimmer and heads SL. This cable run has used all of the pieces of cable used for the previous setup, plus a few more, and so I am discounting a signal cable problem.

 

However I am now rather confused. As far as I can see it, if the heads are receiving DMX (and hence aren't entering STL) but are not working, even though the patch is correct, then the only thing I can think is that they are always receiving (or thinking they are receiving) a constant refresh frame. As the heads have a direct bridge between input and output, it can't be that the second of the working pair is reading the DMX and then outputting corrupted data. And as I say, with the cables run directly from the desk the heads work, and so cannot be a patching error.

 

My only thought is that there is an intermittent cable fault, whether it could just be a break/high resistance in one of the signal cores, or possibly a short between cores or core and shield. As I don't know how DMX refresh frames are created I don't know whether any of these ideas could cause this problem?

 

Any thoughts, could a cable fault present these errors, or is there another explanation that I have missed?

 

Thanks

Martin

[lightsource]

Is there a Terminator on the last fixture?

[jodat2]

Sounds like it could be the DMX cable. Try a different DMX cable first and if it hasn't fixed the problem, it could be a fault in the DMX data link from the unit supplying the DMX signal to the final 2 heads.

 

If this is the situation, Change the DMX cable routing so that the unit supplying the DMX signal link to the final 2 heads is the last in the chain.

You could use a DMX splitter to send an individual DMX signal to this unit instead.

[david.elsbury]

It's not uncommon for some fixtures to appear to work on a faulty DMX cable, and others not to.

By trial and error (substitution) that's probably the best way, though time consuming, to find out which cable is bad.

 

It would, of course, only be a temporary fix to swap cables and signal routing around or to install a splitter, unless the wiring is not possible to replace. Much better to identify and resove the fault permanantly, IMO. :P

[pritch]

Is there a Terminator on the last fixture?

 

If there is one... does everything play nicely when you remove the terminator? If so, you may well be looking at a disconnected data line in one of your cables.

[mumbles]

Is there a Terminator on the last fixture?

 

If there is one... does everything play nicely when you remove the terminator? If so, you may well be looking at a disconnected data line in one of your cables.

Behaves same with and without terminator.

 

 

It's not uncommon for some fixtures to appear to work on a faulty DMX cable, and others not to.

By trial and error (substitution) that's probably the best way, though time consuming, to find out which cable is bad.

 

It would, of course, only be a temporary fix to swap cables and signal routing around or to install a splitter, unless the wiring is not possible to replace. Much better to identify and resove the fault permanantly, IMO. :P

Yeah, just called in between tour dates so hadn't been home to pick up extra cabling, or one of my splitters, or my other desk to read the DMX in to check it that way. Show has come and gone by the time I posted and the pile of cables is back in a pile on the workbench, waiting to be tested next time I'm home.

 

Oh well, will have to just put it down as one of the annoying little faults that will probably never present itself again, just wondered if anyone had any ideas as it doesn't seem to fit into the normal set of DMX faults you see all the time.

[Richard CSL]

I have recently had a similar problem. 14 moving heads all look the same externally, however one head is of a newer type.

 

So the problem was wherever in the DMX chain I put the new head. it would refuse to work. except at the beginning of the line. 60 feet from the desk. the head would show DMX signal and like yours did not go into built in programs, heads after it on the line worked fine.

 

When I opened it up and discovered the circuit board was different from the others, and hence I believe the baud rate or input resistance must be different, hence it would not work with the other heads. I ended up moving it to the front of the line so it works , but a mystery none the less.

 

These are the cheapest heads on the market so can't expect too much from them. Scanic Astute 150. only 200 pounds.

[mumbles]

Similarly these are cheapish heads, glad to see someone else has experienced a similar problem, will have a look inside them if I'm in the school again somepoint.

 

ta,

m