Features:
1) Normally controls change of direction by external mechanical force
2) When the external mechanical force disappears, valve will automatically 
    reset and change direction
3) Its knob type and push-block type structure has memory function
4) 2 kinds available: 2-position, 3-port; 2-position, 5-port in function
5) 2-position and 3-port valve is used for the control of signal output in 
    pneumatic system, while 2-position and 5-port valve can directly drive 
    air cylinder