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Unidirectional data flow

For a certain project I need a product which guarantees, that any current can flow in one direction only. In other words: a unidirectional flow shall be enforced on the physical layer and it shall not be possible to circumvent or cheat on this (e.g. by increased voltages) unless the hardware itself is altered.

Let's say I have a custom board that receives data via a GbE (RJ-45) connection and after some data conversions outputs a raw payload to a high speed interface which uses 32 GPIO pins (standard 0.1 inch connectors) to transfer raw data to/from that board. The 32 bits can be configured to be inputs or outputs in groups of 8 allowing bidirectional transfers from/to a 'dumb' device. For the purpose of a unidirectional data flow I could configure them accordingly to output to 8, 16, 24 or 32 pins.

These 8, 16, 24 or 32 pins shall be connected to another board which enforces the unidirectional flow physically.
So I guess I need something like a digital isolator here - since optocouplers have some drawbacks and simply using Schottky diodes will not work proper (forward voltage drop, reverse leakage current)?

That isolator shall then pass the data to a second instance of the custom board (mentioned above) to its 8, 16, 24 or 32 pins (here configured as input pins). That second custom board will then do some data conversions again and output the data to another GbE (RJ-45) connection.

So the data flow shall look like this:
[GbE -> custom board logic -> 8-32 GPIO pins] -> [UNIDIRECTIONAL ISOLATOR BOARD] -> [8-32 GPIO pins -> custom board logic -> GbE]

The performance of the isolator board shall not restrict the GbE bit rate.
Since the custom board works with 3.3V, I guess it would come in handy if the isolator board could work with 3.3V as well.

Will you please give me an advice which of your products suits best for this purpose?

Please note: I'm a professional software developer / architect and not a hardware designer (but need this piece of specialized hardware for my project), so any further technical explanations / advices would be appreciated.

Thanks a lot in advance!