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Nothing to solve, just sharing what I found: Manufactured fault in DMX splitter
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raphaeltube:
Hi
I just thought I'd share what I found in this DMX splitter. -I thought it was interesting.
For those not familiar with DMX, it's basically RS585 with a few extra specifications, used in stage lighting to control lights.
The specification says to use 5-pin XLR connectors, but most cheaper gear uses 3-pin XLRs with basically the same pin arrangement as pro audio so microphone cables can be used.
Pin1 = Shield, Pin2 = Data+, Pin3 = Data-
Typically light units get daisy chained in parallel from the main controller, with a termination resistor (120 ohm) at the end of the chain.
Obviously there is a limit to how long these chains can be, so that's where boosters & splitters come in.
Anyway, so I bought this Beamz 8-way splitter for a show I was doing.
https://www.beamzlighting.com/product/dmx8-splitter-booster-8-way/
It has (or is supposed to have) a handy switchable termination resistor built in on it's input.
I noticed that the termination switch wasn't working as it should, but it's an easy thing to work around by using an external terminator (like you would with any other piece of gear). But I was curious what the problem was, so after the show, I opened it up to investigate...
-see pics
What I found was that functionally, the switch & termination resistor were across power & ground, instead of across the DMX differential pair.
raphaeltube:
I guess, the most obvious explanation is that first, the silk screen on the front panel was printed with the 'PWR' & 'DMX' LED labels swapped. Then during assembly, that could be corrected by reversing the order of the leads between the main board & the little board with the LEDs & switch. -that fixes which LED is which, but unfortunately also puts the termination on the power instead of the DMX lines.
So an easy functional fix for me would have been to just reverse the order of the leads again, but the labels on the front panel would then be swapped again (Red LED would be DMX & Green LED would be PWR). So instead I opted to change the routing on the small board, as per the following picture, which fixed the problem in a way that I didn't have to re-label the front panel.
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