Communication Chasis and Backplane

[EDALearner]

It seems that Communication chasis and and the physical size of card and connrctors to be standard! Some thing like ATX for PC. It is not clear why the -48v became a standard voltage on communication systems?!

 

[Vicent Yang]

You have two reasones for using -48V :
1- The long distance between the exchange and the user which may reach 3Km. So the DC voltage for the bias of the telephone must be high to avoid cable DC resistance voltage DROP. Also its a requirement that the telephone set to work from the telephone line voltage.

2- The negative voltage is used to avoid crossion of the telephone cables