depression of a selected pushbutton indicates to computer 126 the selected operation to be performed and subsequent depression of trigger switch 136 sends to computer 126 a trigger signal which is processed in computer 126 to cause the selected operation to be performed after a pushbutton switch has been pressed it can remain depressed until another pushbutton switch of the same group has been pressed for this purpose the pushbutton switches of each group can be mechanically or electromechanically interconnected each pushbutton switch 132 contains a lamp which is connected via cable 128 to computer 126 to receive illumination signals therefrom when a switch 132 is pressed computer 126 sends to the associated lamp a train of illuminating pulses which cause the lamp to emit a blinking light signal when the selected operation has been completed i e the carriage hoist or gripper is in the desired position computer 126 sends to the associated lamp an illuminating signal ac or dc which causes the lamp to emit a steady or continuous light signal the lamp continues to emit a steady light signal until the condition which it represents ceases to exist which will occur only when another pushbutton of the same group has been pressed followed by depression of trigger switch 136 thus when the system is stationary the lamp of the 34 position 34 group representing the present position of carriage 6 the lamp of the 34 hoist 34 group representing the present vertical position of gripper 20 and the lamp of the 34 gripper 34 group representing the present gripper.