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Fault protected RS485, constant short circuit to power supply
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ajb:

--- Quote from: nctnico on January 27, 2022, 05:55:07 pm ---I'd use the 36V TVS diodes only and call it a day. Putting fuses in series isn't necessary; these might cause impedance mismatch problems in case you are using long wires.
--- End quote ---
Without fuses a constant short circuit, which is one of OP's stated fault conditions, is going to just cook any TVS diode that trips.  A few ohms of series resistance from a polyswitch or these TBU dealies won't be a big deal for most systems.


--- Quote ---The pull-up / pull-down are also not necessary. The receiver has build-in pull-up & pull down resistors.
--- End quote ---
  Those are line biasing resistors, which are most definitely NOT built into RS485 transceivers.  They should only be installed at one point in the line, and typically form part of the termination for one end.  There are plenty of 'fail safe' receivers available now that will treat an undriven line as idle, but with non-failsafe receivers you ideally want to bias the line to a definitively idle voltage state if it's ever going to be undriven.  Having those resistors available via jumper or switch is a nice convenience for the user. 

RS485 receivers don't really have pull-up or down resistors at their inputs.  The inputs do need to be divided down before they make it to the input comparator--how else can an IC running at 3-5V deal with the -7 to +12V common mode range the standard requires?--and those input dividers do pull on the line a little bit.  But because the valid input common mode range extends into the negative, they have to pull to somewhere between the IC's supply, and whether they pull 'up' or 'down' depends on the conditions on the line.  And in any case, they don't pull the line in the direction of any particular logical state, if anything they pull it towards zero differential voltage, which isn't a valid logical state at all, hence the need for a line biasing network. 


--- Quote from: langwadt on January 28, 2022, 12:16:59 am ---
--- Quote from: chris_leyson on January 28, 2022, 12:04:52 am ---Don't think the ADM2587E driver will toletate an A or B short to 48V. Nice part though, has good CMRR

--- End quote ---

if the pair is floating it can't see 48V

--- End quote ---

Sure it could.  You still need to run the bus ground between devices (well, sometimes you can get away without it, but noise immunity suffers), and it's very common for that to be tied to a larger system/building ground somewhere in the system.  It's also very common for DC supplies to be tied to ground, possibly in the same cabinet as the RS485 bus.  Isolating the transceiver only breaks fault current/voltage reference paths through the device, you still have to deal with problematic voltages that appear within the interface. 

In the current market something fancy and specific like the all-in-one isolated transceivers are especially risky, but even in normal times they're awfully expensive compared to a standard transceiver, a digital isolator IC (can't see a good reason to use optocouplers anymore) and even a prepackaged isolated DC-DC converter, all of which can be multiply sourced. 
abomin3v3l:
In my thinking, right at the moment of a short of +23V or more at A or B, there will be a current greater than 400mA, that is the maximum current required to guarantee that the TBU will trigger, and in about 1us it will trigger and limit the amount of current to a safe level, like few miliAmps (the datasheet shows 1mA @ 50V), and after triggered, when the voltage difference between its pins goes below a determinated value of voltage, that can be in the range from 12V to 20V according to the datasheet, then it untriggers. Maybe I'm thinking wrong.

Regards.
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