I'm currently working on an RS485 DMX circuit and am thinking about proper protective measures.
I've oriented myself on this question. I understand that galvanic isolation does not equal ESD protection, so I need some TVS diodes on the incoming signal lines which clamp to chassis ground, not isolated ground (because the isolation devices only work up to 600V/1000V.)
This raises some further questions:
- Do I need to filter the input of the DCDC as well (because of switching noise?) If yes, how much? I don't think I need a full CM-filter though, as the signal is relatively slow at 250kbps.
- Although the circuits are isolated, they are not allowed to drift apart too much if I use TVS diodes, otherwise they'd trigger randomly. As I use ESD-protected drivers, I don't have to choose tightly specified diodes. I only need to protect the isolation devices, not the driver. But I still need to keep chassis ground and iso ground close enough that the TVS doesn't trigger. Should I just put 10Mohm between the grounds to couple them, or am I overthinking this? (see Edit)
- The former point adresses data line ESD events. What about ESD on the ground plane? Usually you don't protect GND from ESD events, but if I don't then the ground has +8kV in respect to the signal lines, which can't be good either - and I'll violate the 600V isolation to the main circuit. So can I just put a TVS on isolated ground to chassis ground? How does this mesh with the ground coupling?
Maybe it's an XY question. What I want to do: Protect my IO from ESD and misconfigured devices down the line. Is this appropriate or am I overthinking stuff?
Edit: I've read ANSI E1.11 (the DMX Standard), which says that galvanic Isolation for transmitters is not necessary, but can be done (4.2 & Annex A1). I'll go with isolation, because the ports can also act as an receiver which should be isolated. Also, I'll need to connect iso gnd to chassis gnd with >22MOhms.
So question 2 has answered itself. Question 1 & 3 still remain; does the R between iso gnd and chassis gnd pose problems to ESD events? I haven't found a reference implementation for ESD protection.