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I have a reuirement to protect RS485 transmission lines from up to 500v which may be prolonged. Basically I have 485 data within the same cable as 500v, and the cable is under water, so sometimes if a fault occurs, not only do i need to rectify the fault, but I always need to replace the RS485 transceivers at either or both ends. space is limited so I can not fit in adams units with this built in, I am down to chips and components, but I can't opto isolate like I can with 232 because 485 is a loop as opposed to an individual line for tX and RX. Has anyone got a good idea for me, ideally a chip that can be plug in would be best for speed and ease of replacement if they fail.

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    \$\begingroup\$ RS485 is not a loop. \$\endgroup\$
    – brhans
    Commented Dec 14, 2017 at 12:34
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    \$\begingroup\$ You might want to show a bit of your schematics so that one could suggest a solution that ties into the already existing protection mechanisms you have in place. \$\endgroup\$
    – PlasmaHH
    Commented Dec 14, 2017 at 12:41

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I use a scheme that combines gas discharge tubes, inductors, TBU protection devices, TVS diodes, and pi filters to protect my 485 buses. There are two basic schemes I use: One for the supply lines and one for the data lines. Both are shown below:

Supply line protection:

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Data line protection:

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These are fairly common methods to protect RS-485 buses and you will find them in datasheets and app notes for the RS-485 transceivers.

You don't say what sort of space you do have. The above generally takes about 1 square inch per scheme on a PCB. You could put one set of protection on the top layer of a PCB and the other on the bottom side to only take up that ~1 sq. in. of space.

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    \$\begingroup\$ I hadn't known about the TBU before -- very interesting. But I'm concerned about the possible continuous application of 500VDC to a 230V gas discharge tube -- won't that destroy it? \$\endgroup\$
    – Dave Tweed
    Commented Dec 14, 2017 at 13:06
  • \$\begingroup\$ @DaveTweed The 230V rating is the nominal voltage at which the tube conducts for a given transient rate of rise. These devices are generally used to protect against high voltages that appear on the communication cables due to nearby lightning strikes, so they should protect easily against 500V. Many of them are designed for high-energy, long-duration transients. \$\endgroup\$
    – DerStrom8
    Commented Dec 14, 2017 at 13:16
  • \$\begingroup\$ Littelfuse has a series of GDTs rated for ramp rates of 1V/microsecond or less, which they call "DC breakdown" devices. I'm sure the same is true for other manufacturers as well. \$\endgroup\$
    – DerStrom8
    Commented Dec 14, 2017 at 13:24
  • \$\begingroup\$ Hi. I've reached the same solution for both power (GDT+L+TVS, except for added ideal diode after TVS for reverse protection) and serial comms protection. Q1: Do you have any experience of accidental wiring of AC to power input in such configuration? As far as I understand, ideal diode should cut off negative half-cycle, while positive would be dealt with by GDT+ TVS combo. Q2: What's with PI filter (CLC combo)? That would mess with higher baudrates. \$\endgroup\$
    – stiebrs
    Commented Apr 8, 2022 at 13:48
  • \$\begingroup\$ @stiebrs I do not have personal experience with applying AC by accident, but provided the components are rated to handle it, I imagine what you describe would prevent damage under such a condition. As for the pi filters, they cut out the high frequency noise which passes through the rest of the protection circuitry which could negatively affect the receivers. They are sized so that they do not impede the transmitted signal. Please note that it is not the baud rate which is the critical factor, but the slew rate. \$\endgroup\$
    – DerStrom8
    Commented Apr 9, 2022 at 1:50
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I would be inclined to use pulse transformers at both ends of the RS-485 pair, which would allow it to "float" and continue to operate even if there is leakage from the 500V.

Transformers won't pass DC levels, of course, but this can be handled by a combination of hysteresis and biasing of the receivers at each end, possibly enhanced by modifying the communication protocol to minimize DC in the first place.

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I suggest using isolated RS485. That generally requires 3 wires to establish a ground reference. Yes, it's bidirectional which can cause issues, but they are usually surmountable depending on your protocol and whether you have control over it.

For example, we use RS-232 to 485 converters with isolation. They require a separate DC power source for the 485 part. If you are designing at the component level, DC-DC converter, optoisolators (or other isolation scheme such as magnetic transformer, capacitive or magnetic GMR) and handling the data direction (sometimes just a one-shot from the input data- even a, gasp, 555).

Beware cheap-a** converters (looking at you middle kingdom) which do not drive the bus actively in both directions. It sort of works, for short distances sans EMI, but it's far from ideal.

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