Combined signal and power cables “Why are the lighting and sound cables different”

Well one thing to point out is that dmx uses rs485 and the physical layer of  RS 485 requires a  cable with a characteristic impedance of 120 ohms This prevents unwanted reflections of the signal and waveform distortion, For good reliable  high speed transmission dmx must have a cable with a characteristic impedance of 120 ohms. Audio frequency systems tend to be less sensitive to impedance matching of cables although it still does have an effect. There for using dmx 120 ohm cable should be fine for audio as well as dmx. Consequently the problem of needing two different specs of cable doesn’t exist, we can use 120 twisted pair screened cable for both.


Signal connectorsCombined signal and power down one cable

Now what about the signal connectors. In an audio system the output of a device is always male (has pins) and the input is female (has holes). In a dmx lighting system the signal connectors are reversed. The output is female and the input is male.

If we want a combined signal and power cable for a lighting system, they have the male xlr plug at the end of the cable which has the male iec  and the female xlr. at the other end of the cable with the female iec.

For an audio cable the signal cable is reversed. The female xlr plug is at the end of the combined signal and power cable which has the male iec and the male xlr. is at the other end of the cable with female iec.

Two different cables ?

At first sight then, you need two different cables, one for audio and one for lighting. Now have a look at all your DMX lights, all the ones intended for daisy chain operation have signal and power inputs and outputs (signal and power go in and out of each device). Therefore you can actually do daisy chain connections between lanterns using audio style cables (so long as they have a 120 ohm characteristic impedance).All you have to do is use the dmx out socket of each lantern as the dmx signal in. This is not a problem for most dmx units as the dmx in and out are actually connected in parallel. As a result, the daisy chain part is fine.

What’s the problem ?

The problem occurs when you look at the first unit in the chain whos’ input must be connect to the lighting desks’ output. This is a female dmx output socket and the “audio combined signal and power cable” has a female xlr plug at this end. This isn’t a great problem, all you need is a male to male xlr adapter to change the sex. So Now the whole daisy chain is connected using “audio style” “combined signal and power” cables.

The people who designed the spec for the lighting interconnect will be horrified, they will say “You cant stuff a dmx signal into the output socket of a lantern” , but because the dmx input and output sockets on most dmx lanterns are linked together you can! There is one proviso to this though, some lanterns have inputs and outputs which are isolated. As a result this solution won’t work and could possibly do damage, so check before trying it .

The other thing to point out with this solution is that, if you have lanterns which do not have a power through socket then all you need is s male to male adapter for the dmx source end as the lantern will always have a dmx in and out ,so we can still connect or dmx source to the dmx output of the lantern rather than its dmx in.

Using DMX “combined power and signal cables”

If you try this solution, but using lighting “combined power and signal cables” for both jobs, you are likely to run into more problems as the “through” arrangement for the signal and power are more rare on speakers. Also buffering of the audio in and out is  common. On units with buffered style of signal routing, the interchangeability of the signal “in” and “out” sockets  is not possible.

Safety warning

Always make sure you still maintain the power flow: power; output to power inputs. You must never have power coming out of a power in as this will leave the male pins in the input power connector exposed and life threatening.

About John Maggs

I have been in the industry and supported many events for many years. I pride myself in providing a personalised service and deliver a first class service.