Test equipment can be divided in two groups a simple where basics like cable connections and polarity are checked.
The other group is a bit more advanced where it is more an analyzer.
The advanced testers can typically the basic tests of cable pin-out.
Additional they have signal level and timing analyze eventually with a trigger output for oscilloscope.
But they also monitor channels, record and transmit DMX sequences.
Additional some has fixture predefined test pattern like dimmer, pan / tilt curves and so on.
Most error comes from cables that are faulty and a simple cable tester will reveal most errors.
It is my opinion that for people that not hardcore each day build-up large rigs with many lights or maintaining many fixtures a simple cable tester will be sufficient.
A tester like the picture bellow is relative cheap but very useful.
A simple cable tester can be a DIY project.
Here is a suggestion of some circuits that can be handy to have in a tester.
The first suggestion is a simple circuit that makes it possible, via a rotary switch, to see that all wires in the cable are ok and not broken or shorted.
Next a circuit that makes it possible to see if there are a DMX signal in the chain.
This sniffer circuit is just added so it is possible show the DMX signal here on the Wiki.
It is a circuit with two BNC connectors so it is possible to attach an oscilloscope to the lines.
Unfortunately is it too difficult to make a circuit that generates a trigger signal for a given DMX channel so this is not included under simple tester.
And it can be discussed if a serial resistor are to be added in two sniffer lines so an antenna effect are avoided.
See: Measurement on the DMX cable.
And bellow is some pictures of how it can be realized.