* with "slot" you mean "connector" ... and not a "time slot" in the bus protocol?
* I don't understand why you consider to use optocouplers. Please explain.
* with "distance longer", "very slow" and "few bytes" only you have a clue ... but we need values with units to find a suitable solution.
* hardware based handshake: how many signal lines to each client?
* if microcontroller (=ARM?) is not involved in the UART problem, please put it aside for now. Let's focus on the UART.
* why UART at all? Why not SPI, since you seem to have experience with it.
* I don't understand what "position" means.
* You talk about "position" and "protocol". Why do you think both don't fit? For me this are two independent things.
* "immedate" again is a vague or even impossible description. It leaves no (zero) room for a delay. Not even a picosecond.
* I don't understand what "hot plug" has to do with "optocouplers". Please explain.
* still missing information: do you expect the FPGA to handle 90 independent UART (HW) channels?
> Do these 90 UARTs have to work simultaneously or can they be multiplexed so that only one or small group from the whole operate at the same time?
> Squeezing 90 physical connection in 50cm of wiring is a challenge in itself!
> Optocoupler is an "isolating" part, not a "protection" part.
Every module (which themselves are equipped with microcontrollers) will have an identifier, they will start to talk with the FPGA during startup.More and more confusing: “during the registration / startup phase they will talk simultaneously, afterwards it can be serialized.” How do you propose to have 90 channels in parallel and then miraculously put them all in serial?
Once you have mentioned the hotpluggable opperation and electrical level being in the LVTTL range, you mingh be aware that in general RS-485 connectors are made in such a way that the body is first connected yielding signal shielding in some extent. I would bet due to space restrictions you'll not use e.g 90x standard DB-9 connectors, so this should be considered on selecting the connector itself.In case something's going wrong on a slave module it won't damage the FPGA on the other side.
which datasheet exactly did you read to say so?in case of the RS485 transceivers they wouldn't have any protection either,
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