Latching; Introduction - Siemens SIMATIC S7-200 Getting Started

Latching

2.1

Introduction

You should now be familiar with the standard latching function and here you will learn how to
program it.
In STEP 7-Micro/WIN, open the first practice project "2h_pr_01.mwp" from the Programming
Exercises CD. There are still a few elements missing in the program. Add the missing LAD
elements now as a short exercise.
First add a contact Q0.0 at the point indicated as a parallel circuit to I0.0. To enter the
contact:
1. Click on the ladder diagram, (in the blank space just below the I0.0 contact and just
above "Network 2"). Then, click on the toolbar button for a contact (F4). Now click on the
normally open contact in the drop list. (As indicated on the tooltip, you can also use
function key F4 instead of the mouse). Finally, click on the address field, replace ??? with
Q0.0, then press enter.
2. To enter the vertical line, place the box cursor on the I0.0 contact, and then click on the
line down toolbar button.
Getting Started - Advanced
Training Documents, 09/2007, A5E01469795B
For example:
Output Q0.0 is to be activated
as soon as S1 at input I0.0 is
operated. With latching, Q0.0
is to remain active until S2 at
input I0.1 is operated and thus
interrupts the latch.
To allow the latching function to work,
the output (Q0.0 in this case), must itself
ensure, as soon as it is activated, that it
retains its "true" state and therefore
remains active.
This is achieved by switching the output
(Q0.0 in this case) as a contact in
parallel to the tripping input in the same
way as a conventional contactor circuit
(Q0.0 can be compared to our contactor
K1).
Output Q0.0 as an input ensures
latching.
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