1. I want to power up the RB750UP from -48V supplied from a telecom BTS. Does the Mikrotik converter 48V to 24V sort this. I mean inserting the Telecom ethernet drop into the Mikrotik 48V POE converter and the output from converter into the blue port of the RB750UP
= Right or Wrong

2. I have some devices which power up with 24V and some like the TPLINK accesspoint to be powered up with 12 V so :-
= can the RB750UP judge the voltage required by itself.
= do i need to manually set the voltage output for 24V or 12V (is it possible)
= if so can i check which port is drawing 24V and which one is drawing 12V

As I understand it the 750UP provides the same voltage out as it receives in. You would therefore be unable to power 12V PoE devices if you provide the UP with 24V

http://routerboard.com/RBPOE-CON-HP

Seems to indicate that you are right with your plan to power the UP from a telecom PoE supply at -48V. However please note that that page also mentions it can provide 24W of power.

The 750UP comes with a 2.5A power supply @ 24V for a total of 60W of power. Initially this was 0.5A for each port with PoE out and 0.5A for powering the UP itself.

The 750UP draws a maximum of 3W which is nice and low leaving you 21W of power left from your 48V PoE converter.

So be careful with your power budget if you plan to make use of this system

Regards
Alexander

it means 24Watt from a Mikrotik POE can just power up say one 433ah/411ah with a mini PCI.

OK how about powering the 750UP from 12V x 2 = 24V & 7AH batteries. Has neone tried this.

Or why doesnot Mikrotik come up with a Routerboard which directly powers up through POE on 48V from a telecom BTS