DMX Cable Impedance Tester

[chrisprescott]

Hi does anyone know of a good reliable DMX tester that can test a cables impedance as well as continuity?

 

We recently had a problem on a job with DMX doing some crazy things I have never experienced before and even a number of opto isolators could not solve the problem. (It was terminated and the not mike cable before anyone says it :)

 

I am convinced we have some cables which are problematic and want to identify them but all test ok for continuity.

 

Any input appreciated.

 

Thanks

[Jivemaster]

My experience has shown that the connectors are more likely to be the problem.

[Brian]

...a good reliable DMX tester that can test a cables impedance as well as continuity?

 

There isn't one. And if there was a good one about it'd cost around £2k. The only way to test a cable fully for impedance is with a TDR (google it).

 

You can do a lot with a multimeter, especially if it has a continuity bleep function.

 

1) Visually, and by running it through your hand, inspect the cable for nicks, scrapes and lumps

2) Check the connectors are wired correctly. Particularly make sure that the screen is not connected to any connector bodies.

3) Check each conductor for continuity while flexing the cable especially where they go into the connector body.

4) Check each conductor for shorts to other conductors while flexing the cable especially where they go into the connector body.

 

If you still have suspicions that something is wrong then you might look at getting a multimeter with a capacitance range. The capacitance between the two signal conductors will ideally be around 40pF/m and between a signal conductor and the shield around 75pF/m. Note that one of the reasons microphone cables are not always a good idea for DMX is that they often have a much higher capacitance which will reduce the maximum length of cable run before things start to misbehave.

 

If you want to get really serious you'll want to investigate eye-pattern testing.

[chrisprescott]

Hi

 

Thanks I will investigate these techniques.

 

Thanks

[timsabre]

As jive says, if the cables test ok for continuity, it will be dodgy connectors or wiring within one or more fixtures. This causes loss of one side of the DMX which makes it unreliable.

You can make a test plug with a red led + 220 ohm resistor from pin 2 to 1 and a green led + resistor from pin 3 to 1. Then go round plugging it in and check you have both red and green lights. When one is missing, you can track down the fault.

[Ken Coker]

I've recently bought and used an AL NanoScope to try and track down a DMX problem - http://goo.gl/l7ZqkC.

 

It doesn't do exactly what you want, but it might help.

 

KC

[GR1]

I use an Artistic Licence MicroScope. The DMX network test is very useful to find which part of the system is faulty. It does need the parts of the system to be connected together with proper 5 pin XLR connectors so that you can test parts of the system and find the fault by a process of elimination.

[david.elsbury]

"Proper 5 pin connectors" ?

 

Surely if the DMX chain is on 3pin connectors you could just use converters to "proper" connectors? :)

[Sirbrian]

Hello All,I know I'm waaay to late for this subject but I thought I might help someone now with a similar problem.I have Hantek 2D72 handheld oscilloscope with a built-in signal generator and it makes a decent TDR (Time Domain Reflectometer).The signal generator has about a 5nS edge speed and a 50 Ohm output impedance.I put a 62 Ohm resistor in series with it and have a 112 Ohm signal source.You can connect the signal to a DMX pair.Connect channel 2 before the resistor to generate a trigger and channel 1 to monitor the tested cable.I get fairly decent results with it.As a regular scope it's decent for looking at signals for fidelity.

It's also a DVM which makes it a dual purpose tool.Try to use a non-inductive resistor.

Edited November 26, 2019 by Sirbrian

[bigclive]

At the risk of spamming the forum with one of my own videos, I find a tester like this incredibly useful on large DMX installations, and much simpler and cheaper than TDR testing.

 

 

 

Moderation: link fixed

[alistermorton]

Can you think of a reason why the two designs couldn't be combined? If you had nothing on the two red LEDs, that could either mean no signal being received at all, or lost ground, but the bi-colour LED would show the presence of the differential signal.

[Sirbrian]

Yep the two LED tester is a good thing to have. I was glad to see someone posted how to make the tester. We have several and they are good first look tools that solve most problems. The TDR/SCOPE approach would be used for more esoteric issues like marginal signal quality. We have had situations where one signal had a resistive connection to ground which degraded the signal and caused light flickering. I had to look a the signals + and - and figure where it was happening. The Oscilloscope showed me the fidelity of the signals and the TDR gave me a distance approximation so I didn't have to open every luminaire. Much more work than looking at a light-bulb but needed. Also RDM communication to LED drivers needs good signals so having the pipes nice and clean to communicate is a good thing. Thanks all Edited November 26, 2019 by Sirbrian

[bigclive]

Can you think of a reason why the two designs couldn't be combined? If you had nothing on the two red LEDs, that could either mean no signal being received at all, or lost ground, but the bi-colour LED would show the presence of the differential signal.

 

In the event of an open data line the bi-colour LED would potentially couple enough current across from the good data line to the open circuit line to make its LED look active.

[alistermorton]

Good enough for me :-)

[Sirbrian]

Bigclive is correct in that you might get a false reading with another LED added. You could add the extra LED with a series switch to disconnect it to get around the leakage issue. Edited November 27, 2019 by Sirbrian