Siemens 020-011 User Manual

Industry Sector, IA&DT

SCE Training Curriculum

Page 18 of 79

To be used only in Educational and R&D Facilities

TIA Portal Module 020-011, Edition 04/2014

SCE_EN _020-011_R1404_Startup Programming with SIMATIC S7-1500

5.3

Where does the PLC obtain the information regarding process states?

The PLC receives information regarding the process from the so-called signal transmitters that are

connected to the

PLC’s inputs. These signal transmitters can be sensors that detect whether a work

piece is located at a certain position, or simple switches and buttons that can be open or closed. Here,

we differentiate between NC contacts (normally closed contacts) that are closed without being

operated, und NO contacts (normally open contacts) that are open without being operated.

5.4

What is the difference between break (NC) and make (NO) contacts?

Regarding signal transmitters, we differentiate between NC contacts and NO contacts.

The switch shown here is a normally open contact (make contact); i.e., it is closed if it was operated.

The switch shown here is a normally closed contact (break contact); i.e., it is closed if it was not

operated.

24V

The PLC inputs record the information

regarding states in the process!

PLC

Inputs

Switch closed

Switch open

24V

NO
contact
not
operated

NO
contact
open

NO
contact
operated

NO contact
closed

NC
contact
not
operated

NC contact
closed

NC
contact
operated

NC
contact
open