Page 314 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Counter parameters
Accumulative value(ACC)-number of false to true transitions that have Occurred since the counter was last reset

Preset value(PRE)- Specifies the value that the counter must reach, before The controller sets the done bit. When
the accumulator value becomes equal To or greater than the preset value, the done status bit is set. This can be
Used to control an output device
PLC memory elements
Memory elements are used to store intermediate values. Memory function are achieved using flags ( bit memory
locations) and system memory. Specified bit memory can be set or reset using a set coil. A latch in ladder logic
uses one instruction to latch and a second instruction to unlatch, as shown in Figure 1.9. The output with an S
inside will turn the output D on when the input A becomes true. D will stay on even if A turns off. Output D will turn
off if input B becomes true and output with a R inside becomes True.

Both set coil and reset coil can be combined in one box as shown in Figure 1.10. Following instructions are to be
followed while writing a program for memory function
Setting up a memory location
Resetting up a memory location
The memory address locations vary from manufacturer to manufacturer. For sake of simplicity

We shall use M1, M2 …To represent memory. Number of bit memories available in PLC for memory are 1024,
2048 8192 and more. If the power flows either momentarily or continuously to the set coil, the specified memory
address is set to signal state 1. If power flows momentarily or continuously to the reset coil , the corresponding
memory address is reset to signal state 0. If there is no power in the set input or reset input, the memory address
remains unaffected. The output of the memory function can be accessed through either NO or NC program element.
Set and reset functions are combined in one memory box as shown in Figure 1.10. They can be further classified
into two categories
a Memory box with set priority

b Memory box with reset priority
The functions of a memory box are similar to the memory coils. In the memory box with set priority, the associated
memory address is set when signal state 1 appears simultaneously at both the set and reset inputs. In the memory
box with reset priority, the associated memory address is reset when signal state 1 appears simultaneously at
both reset and set inputs.

Wiring of field devices to various module in PLC:-
A programmable logic controller is an industrial computer control system that monitors the input device continuously
and it takes a decision based upon the custom program to control the state of output devices.

During the wiring of PLC, the wire size is an important factor. One or more conductors can be accepted by the
input-output terminal of a particular wire size we need to check that if the wire is the proper size and is correct
gauge so that it can handle the maximum possible current.

Wiring interlocks need a good understanding of the logic to be implemented as well as the electrical standards
that need to be implemented to fulfill it. So, it is said the one who is a master in designing and understanding
electrical wiring logic and interlock can easily.

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