I am designing medical data acquisition equipment that requires moderately fast data transfer. As part of the design we are using a custom 100BASE-T PoE architecture (and cabling) to the data acquisition headstage, together with an intermediate "dumb" device to (1) provide medical-level isolation, (2) inject PoE power, (3) connect ethernet to the computer, (4) connect additional non-isolated equipment (via two extra ethernet pairs from the headstage, that's why we are limiting to 100BASE-T).
The only medical-grade isolated magnetics I can find are the HXU6200NL by Pulse, which will serve to connect to the computer and will reside inside the dumb device isolating the headstage itself.
The question is: Would it be ok to use two sets of ethernet magnetics back-to-back inside the dumb device?
That is, to use a second set of PoE magnetics to inject power and connect to the headstage with no intermediate circuitry (besides, perhaps, some common mode terminations for both cables).
There will be a standard set of magnetics in the headstage bringing the total count in the link to 4 (the absolute minimum to provide the needed isolation would be 3)
The total cabling length (headstage-box-computer) would be less than 10m, very likely less than 5m.
One reason to want to use a second set of magnetics is to provide "standard" common-mode termination (i.e., Bob Smith termination) to each cable segment.
Another reason is that ethernet magnetics are not symmetrical, as common-mode chokes and autotransformers are commonly present.