Controlling a 4 wire servo
Usually RC servos work on a pulse principle. The input idles low. A pulse (TTL-level) of width 1.5ms puts the servo at mid-position (using feedback from an internal pot), 1ms brings it to one extreme and 2ms to the other. 



The pulses need to repeat at some frequency, 50Hz for example, the exact frequency is not too important. The same receiver can supply, in turn, pulses to several servos since the maximum pulse time of 2ms is much less than the 20ms period.