Page 312 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

NC contact of PLC
The PLC representation of NC contact is given in Figure 1.3. This contact scans for the signal state OFF (0) at
the specified bit address. Power flows through NC contact if the scanned address has a signal state OFF (0). This
contact is used for scanning the signal state of input devices or output devices or other internal program elements.
Figure 1.4 Shows PLC circuit with NC contact position using NO push button. Figure 1.5 Shows PLC circuit with
NC contact position using NC push button

PLC TIMERS
Many control tasks require the programming of time. For example, cylinder 2 is to extend, if the cylinder 1 is re-
tracted- but only after a delay of few seconds. The timers of a PLC are realised in the form of software modules
and are based on the generation of digital timing. Memory space is allocated in system memory to store the values
of the delay time. The representation of the timer address varies from manufacturer to manufacturer. For sake of
understanding we shall as T1, T2for timer addresses. The typical number of timers available in commercial PLC
are 64, 128, 256 , 512 or even more. To explicitly reset timer, an RLO of 1 has to be applied at the reset port.

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CITS : E & H - Electronics Mechanic - Lesson 180 - 187