Automating Manufacturing Systems - Process Control and ...

plc design - 10.5
step 3
step 3
top limit switch
L
up
motor
step 4
The ladder logic for step 3 turns on the motor to
raise the flag and when the top limit switch is
hit it goes to step 4.
U
step 3
step 4
flag down button
L
step 1
The ladder logic for step 4 only waits for the
push button to lower the flag.
U
step 4
Figure 10.3
A Process Sequence Bit Design Example (continued)
The previous method uses latched bits, but the use of latches is sometimes discouraged.
A more common method of implementation, without latches, is shown in Figure
10.4.