Automating Manufacturing Systems - Process Control and ...

plc timers - 9.5
I/0
O/0
I/0
O/1
L
I/1
O/1
U
I/0
O/2
I/1
O/2
I/0
I/1
O/0
O/1
O/2
Figure 9.5
A Latch Example
A normal output should only appear once in ladder logic, but latch and unlatch
instructions may appear multiple times. In Figure 9.5 a normal output O/2 is repeated
twice. When the program runs it will examine the fourth line and change the value of O/2
in memory (remember the output scan does not occur until the ladder scan is done.) The
last line is then interpreted and it overwrites the value of O/2. Basically, only the last line
will change O/2.
Latches are not used universally by all PLC vendors, others such as Siemens use
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