Introduction; Relay Circuits: The Roots Of Pc Logic - Omron SYSMAC C20P Operation Manual

Relay Circuits: The Roots of PC Logic

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Introduction

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Relay Circuits: The Roots of PC Logic

Relay vs. PC Terminology
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A Programmable Controller (PC) is basically a central processing unit (CPU)
containing a program and connected to input and output (I/O) devices (I/O
Devices). The program controls the PC so that when an input signal from an
input device turns ON, the appropriate response is made. The response nor-
mally involves turning ON an output signal to some sort of output device. The
input devices could be photoelectric sensors, pushbuttons on control panels,
limit switches, or any other device that can produce a signal that can be input
into the PC. The output devices could be solenoids, switches activating indi-
cator lamps, relays turning on motors, or any other devices that can be acti-
vated by signals output from the PC.
For example, a sensor detecting a product passing by turns ON an input to
the PC. The PC responds by turning ON an output that activates a pusher
that pushes the product onto another conveyor for further processing. An-
other sensor, positioned higher than the first, turns ON a different input to
indicate that the product is too tall. The PC responds by turning on another
pusher positioned before the pusher mentioned above to push the too-tall
product into a rejection box.
Although this example involves only two inputs and two outputs, it is typical of
the type of control operation that PCs can achieve. Actually even this exam-
ple is much more complex than it may at first appear because of the timing
that would be required, i.e., "How does the PC know when to activate each
pusher?" Much more complicated operations, however, are also possible.
The problem is how to get the desired control signals from available inputs at
appropriate times.
Desired control sequences are input to the P-type PCs using a form of PC
logic called ladder-diagram programming. This manual is written to explain
ladder-diagram programming and to prepare the reader to program and oper-
ate the P-type PCs.
PCs historically originate in relay-based control systems. And although the
integrated circuits and internal logic of the PC have taken the place of the
discrete relays, timers, counters, and other such devices, actual PC opera-
tion proceeds as if those discrete devices were still in place. PC control, how-
ever, also provides computer capabilities and consistency to achieve a great
deal more flexibility and reliability than is possible with relays.
The symbols and other control concepts used to describe PC operation also
come from relay-based control and form the basis of the ladder-diagram pro-
gramming method. Most of the terms used to describe these symbols and
concepts, however, originated as computer terminology.
The terminology used throughout this manual is somewhat different from re-
lay terminology, but the concepts are the same. The following table shows
the relationship between relay terms and the PC terms used for OMRON
PCs.
Relay term PC equivalent
contact input or condition
coil output or work bit
NO relay condition
NC relay inverse condition
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