Confusion comes from lack of understanding. Good on you for investigating! As usual, these things are more complex than first appears.
DMX doesn't need a
ground! Sending or receiving devices usually do, but mixing that with the signal reference is not required, and as noted troublesome. A member of the
DMX standard committee has said that +90% of the noise rejection is due to the twists of the data pair. That would make shielding almost unnecessary. Even Mr Fleenor has described situations where the signal reference isn't needed or used.
That said, I will follow the published standard and not put my professional reputation at risk.
As said above, there is confusion between
ground, shielding and signal reference.
DMX512 is a 3
conductor system.
The third
conductor may or may not be a
shield, and it can be shown as stated above that the
shield provides minimal protection from noise. The signal reference is the third
conductor and should be connected
thru on Pin 1. The other major problem is
ground loops. If the signal reference is connected to
ground at more than 1 location,
ground loops are possible. This is a negligible problem over 10 feet, but when you start to talk about 80m it is problematic. By connecting the
shield at multiple locations, you have now strung a piece of 22awg
wire in parallel with the earth
ground conductor and general
current flow within the building's steel. The only sure method is to use a single
point ground, ideally at the transmitter, and the receivers are isolated from
ground such that the resistance between Pin 1 and
safety earth is on the order of meg-ohms. Generically the standard calls out for 22M which is probably unreasonably high. To do this costs money. It means the
DMX transceiver is likely to need its own isolating
power supply internally. It's one of the differentiating factors between cheap
LED fixtures and good ones.
It's also one of the justifications for using opto-splitters. Install the
opto-splitter at the
dimmer end of the 80m run and split off to your fixtures from there.
Bob Goddard/Goddard Design Co.