DMX512 Line Tester

Its been over 28 years since I last had any real involvement with stage lighting control, but an occasion has arisen whereby I need to do some basic tests on a DMX512 'universe' - as a DMX controller/slave network is known.

Testing DMX protocol properly, that is, by decoding the data packets, requires an expensive DMX analyser. But, as with most electronics handled by 'amateurs' (as opposed to professional engineers and technicians - but certainly not ruling those out!) the commonest faults are likely to be due to wear and tear and bad handling of cables and connectors. So, a simple device that can prove the flow of data, and diagnose simple connection issues, is all I require.

Now, proper, compliant systems use XLR-5 connectors, and I will be making up a tester for this once the plug arrives. But, many cheaper or older DMX lighting controllers use the XLR-3 connector. The build shown here is for a 3-pin XLR-3 line tester.

Im not going to include a circuit diagram here, as these can be found on the net easily. In the XLR-3 the DMX512 signal is on pins 2 and 3, and ground pin 1. DMX uses the RS-485 hardware protocol, and is a differential pair signal. All that is needed for basic testing is an inverse-parallel pair of LEDs across the differential line, i.e. between pins 2 and 3, along with a suitable series resistor to prevent the tester taking much current from the line (which is rated 250mA max, but any current draw from the line can induce noise, so this tester should not be left connected when the system is in use). As the DMX512/RS-485 standard requires a line termination, a second resistor should also be used across the pair to match the cable impedance. This is about 120 ohm.

Rather than a separate pair of LEDs, a single bi-colour device is preferred. These have two pins but contain back to back chips, usually one red and one green. I didnt have any of these, but instead had tri-colour LEDs, which are the same chips but a common cathode, and hence three legs. So, I had to use a connection to ground on pin 1. A pair of 270 ohm 1/4w resistors act as Rseries and Rload. Im not actually sure this is the right approach, as these were specified for the 2-leg bi-colour LED, and I think Rload should be a 120 ohm unit, and Rseries whatever gives best compromise between current draw and brightness. I'll modify them for the XLR-5 build once I get the plug, and have chance to test the XLR-3 on a live system. It shouldnt matter much though as the hardware protocol is rather robust.

The photo below shows the tester wired up. Care was taken to ensure that the distance from the pins to the LED matched that as from the pins to the top of the cable strain relief when the plug is made up

With the pins, circuit and cable securing gland inserted into the barrel, and the cable relief screwed on to complete the device, the LED protudes nicely out of the end of the rubber!

A quick calculation shows that for this type of LED, with Vled of 2-2.2v, and an expected line voltage of 6v, this tester should draw about 14mA. Thats not too bad for a line with 250mA max, but personally I would prefer 10mA, which would be a 380 ohm series resistor.

For the XLR-5, I will use Rseries 380 ohm, and Rload 120ohm. 120ohm is a standard value in the E24 series, but 380 ohm is not. I will have to decide whether to use 390 ohm (dimmer) or 360 ohm (a touch more current)