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RS-485 protection

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twam:
Hi everybody,

I'm still working on my flipdot project where I have several modules containing 16x14 flipdots running of 24 V. I want to daisy-chain several of those modules using RS-485 for the data connection. My question is now how much and what kind of protection I should put on the RS-485 lines. A few months back I watched all the awesome videos from Mike where he often mentions small hints on that topic, but is very hard to find all those tiny bits of informations again. So what kind of protection would you suggest? I guess the most common misuse would be mixing up power (24 V up to 8 A) and data lines.

Best regards
  Tobias

T3sl4co1l:
A transceiver that's tolerant of those voltages might be worth a look, e.g.
https://www.ti.com/lit/ds/symlink/sn65hvd1785.pdf
If you want to cook something yourself, you might consider:
- Series resistors (significantly worsens line drive, though)
- Depletion MOS current limiters
- TVS (e.g. SMAJ12CA) or SIDAC (e.g. TISP4015L1) with polyfuse, or somewhat less resistance (but, the resistor should be relatively high power)
- Uhh, anything else that comes to mind; could do worse than building your own transmitter from scratch (with adequate voltage ratings and current limiting), but that would kind of suck.

Tim

Brutte:
If it is not connected then you can kill a transceiver only with ESD but if you pick a version that has some ESD protection then it will be ok during handling.

Once you connect the transceivers with wires - I do not see the point to protect each device separately. Just a pair of TVS diodes somewhere near master would deal with EFT. If that is a super long bus then maybe at both ends is not a bad idea.

As for mis-wiring protection - that is a matter of personal preferences.

floobydust:
There are special TVS designed for RS-485, low capacitance 75pF and lop-sided clamping voltages -7V, +12V. See SM712.
Those are a tiny SOT-23, convenient and what I would consider the absolute minimum to use.
They protect against differential and common-mode overload.

But not 24V on the line. Nothing protects against that without smoking the transceiver or using a lot of parts.
Most RS-485 driver failures I see are due to line shorts, one leg of the transmitter fails, or an open ground.
Larger TVS are high capacitance so careful if you upsize to SMA or SMB sized parts.

How tough the protection is depends on your budget and the distances between nodes, and speed.

twam:
Thanks for your input.

I originally planned to use the SN65HVD7x as a transceiver, but the suggestion by T3sl4co1l pointed to me SN65HVD1780. This is a cheap upgrade and really seems to cover most of my fears.

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