DMX Isolation


There seems to be a lot of confusion about isolation in DMX systems. The following describes the differences between isolated and non-isolated DMX inputs.

RS485 comms (that DMX uses) is a 3-wire system. The data pair is differential, but they need to stay within the common mode voltage range for correct and safe operation.

Non Isolated

The the diagram below, you can see that the DMX data common is connected between the two DMX devices. Now, because each DMX device is also grounded (to mains earth), an "earth loop" is formed. If the mains earth of each DMX device is not at the same potential, excessive current will flow in the DMX data common wire, potentially causing data corruption or burnt PCB traces.

To avoid this earth loop, some people just leave the common reference wire disconnected at one end. This stops the "earth loop" and they think "that's that". But, then the common reference is via the (sometimes different) mains grounds. As a result, some devices may have their common reference swinging many Volts above and below the nominal data pair voltages. The ubiquitous MAX485 chips can handle a few Volts of "common mode voltage", but once you exceed that, it all goes bad. First the data gets corrupted , then the chips tend to go POOF!


With an isolated input, the common reference is still connected to all nodes, but the whole DMX "front end" is floating with respect to the rest of the device. So there can be no ground loop with isolated front ends. The isolation is usually achieved with the use of a small (isolated) DC-DC converter and a high speed opto-coupler such as the 6N137.


Adding isolation to DMX devices involves extra parts. The cost of the DC-DC converter and opto-coupler(s) can be significant, especially for low cost / DIY projects. The DIY'er would have to allow an extra $10 - $20 per board.


So, which way to go? It all depends on the application. If the DMX devices all share the same DC power supply, there is not a lot to be gained from isolating the DMX front end. If the devices are spread out over greater distances and powered from different (ground referenced) sources, then isolation would be prudent.


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