Latching
2.7 Safety Tip: How to control a machine shutdown when a wire breaks
Getting Started - Advanced
Training Documents, 09/2007, A5E01469795B
21
2.7 Safety Tip: How to control a machine shutdown when a wire breaks
Because the circuit uses a NC switch as the input to the Reset logic, a broken wire on that
input forces a reset.
,
,
,
,
6
6
9
6
4
4
6
4
5
1HWZRUN
Switch with NC contact that
supplies the signal "0" when
operated.
In LAD, this signal is
reversed by the NC contact
I0.1. This means that if you
operate the switch S3, Q0.0
is reset.
Note
Safety notes
• In the above example, an NC switch S3 was used for resetting.
When I0.0 is operated, output Q0.0 is set with a steady state. If there is +24 V at I0.1, the
"NC contact" supplies the state "0" in LAD. Output Q0.0 is not reset. The LAD "power
flow" is interrupted, and the coil for resetting is deactivated. If there is no signal (0V) at
I0.1 (S3 is open), the NC contact of I0.1 in LAD = "1" and the output is reset.
When an NC switch is used at I0.1, the latching output Q0.0 is reset (switched off again)
on either of the following conditions:
– If switch S3 is operated (I0.1 = "0")
– If there is a break in the connecting cable between I0.1 and the NC switch. Even in the
event of wire break, it is guaranteed that a plant component will operate in a safe
state, for example, a motor is switched off.
• The operation "Reset Q0.0" network is executed after the operation 'Set Q0.0' This
means that in the event of both switches being operated simultaneously, clearing the
latch takes priority.
In STEP 7-Micro/WIN, open the exercise example 2hr_pr_02.mwp and test the functions!
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