Central Furnace blower alwas on, even with thermostat disconected
The limit switch isn't on the control board; it is an external part that is mounted in the plenum above the burners.  Wires run from the control board to the limit switch.

If you also have no heat and no AC, then check that there's 24 volts present on the control board.  Some control boards have a normally closed relay that turns off the fan by pulling their switch open after power is applied.