THERNET MDI 1000BASE-T pairs length on pcb vs long cable

[violat]

Hi ,

I see sometimes MDI pairs length limitations in Datasheets of components of a few inches (regarding the PCB routing length).
Why is there such a strict limitation if after that the ETHERNET cable can be 100 meters long? Will it not work correctly even if the MDI pairs length on the PCB will be 20 inch or even more ?

Thanks.

 

[FvM]

Which datasheets do specifically refer to? I think there are several reason why one would avoid long PCB traces between PHY and ethernet transformer and also between transformer and connector (if they are separate components), e.g. reducing EMC considerations, difficulties of impedance controlled routing. But none is absolute. If you see a necessity to implement long ethernet connections on PCB, it should be possible.

 

[violat]

Which datasheets do specifically refer to? I think there are several reason why one would avoid long PCB traces between PHY and ethernet transformer and also between transformer and connector (if they are separate components), e.g. reducing EMC considerations, difficulties of impedance controlled routing. But none is absolute. If you see a necessity to implement long ethernet connections on PCB, it should be possible.

Datasheets of PHY'S or ETHERNET SWITCHES with integrated PHY'S sometimes specify max length of several inches for the MDI pairs.
but , it seems to me (like you said) that longer routing on the pcb is possible as long as you route it with controled impedance, clearance from other signals , minimum via's and so on...

is it possible to simulate it ? using which tool ?