Relay Circuits: The Roots Of Pc Logic; Pc Terminology - Omron SYSMAC CV Series Operation Manual

PC Terminology

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Relay Circuits: The Roots of PC Logic

Relay vs. PC Terminology
1-3 PC Terminology
PC
Inputs and Outputs
PCs historically originate in relay-based control systems. And although the inte-
grated circuits and internal logic of the PC have taken the place of the discrete
relays, timers, counters, and other such devices, actual PC operation proceeds
as if those discrete devices were still in place. PC control, however, also pro-
vides computer capabilities and accuracy 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 con-
cepts, however, have come in from computer terminology.
The terminology used throughout this manual is somewhat different from relay
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
contact
coil
NO relay
NC relay
Actually there is not a total equivalence between these terms. The term condi-
tion is only used to describe ladder diagram programs in general and is specifi-
cally equivalent to one of a certain set of basic instructions. The terms input and
output are not used in programming per se, except in reference to I/O bits that
are assigned to input and output signals coming into and leaving the PC. Nor-
mally open conditions and normally closed conditions are explained in 4-3 Basic
Ladder Diagrams .
Although also provided in the Glossary at the back of this manual, the following
terms are crucial to understanding PC operation and are thus introduced here.
Because CV-series PCs are Rack PCs, there is no single product that is a CV-
series PC. That is why we talk about the configuration of the PC, because a PC is
a configuration of smaller Units.
To have a functional PC, you would need to have a CPU Rack with at least one
Unit mounted to it that provides I/O points. When we refer to the PC, however, we
are generally talking about the CPU and all of the Units directly controlled by it
through the program. This does not include the I/O devices connected to PC in-
puts and outputs. The term PC is also used to refer to the controlling element of
the PC, i.e., the CPU.
If you are not familiar with the terms used above to describe a PC, refer to Sec-
tion 2 Hardware Considerations for explanations.
A device connected to the PC that sends a signal to the PC is called an input
device; the signal it sends is called an input signal. A signal enters the PC
through terminals or through pins on a connector on a Unit. The place where a
signal enters the PC is called an input point. This input point is allocated a loca-
tion in memory that reflects its status, i.e., either ON or OFF. This memory loca-
tion is called an input bit. The CPU, in its normal processing cycle, monitors the
status of all input points and turns ON or OFF corresponding input bits accord-
ingly.
There are also output bits in memory that are allocated to output points on Units
through which output signals are sent to output devices, i.e., an output bit is
Section 1-3
PC equivalent
input or condition
output or work bit
normally open condition
normally closed condition
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