did you read the RB800 user manual?
http://www.routerboard.com/pricelist/do ... ile_id=133
Fan Connectors
You can connect up to four fans to the RouterBOARD, but only one pair of them will work at a time. They will
receive either 3.3V or 5.5V DC power. The board supports fan speed feedback signaling. RouterOS can be
configured to change the active fan, if the current active one is not rotating (note that if a fan does not have
rotation sensor, it will be considered failed).
JP1 and JP3 are 3.3V connectors, only one of them will power a cooling fan at the same time, other one will
be started when the first stops.
JP2 and JP4 are 5.5V connectors, only one of them will power a cooling fan at the same time, other one will
be started when the first stops.
To enable rotation of two fans simultaneously, simply connect one 3.3V fan to JP1 and a 5.5V fan to JP4
(outside of the pair).