3
For example, a sensor detecting a passing product 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. Another
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 ex-
ample is much more complex than it may at first appear because of the tim-
ing that would be required, i.e., “How does the PC know when to activate
each pusher?” Much more complicated operations, however, are also possi-
ble. The problem is how to get the desired control signals from available in-
puts at appropriate times.
To achieve proper control, the SP-series PCs use a form of PC logic called
ladder-diagram programming. The next few sections will explain ladder-dia-
gram programming and to prepare you to program and operate the SP-series
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 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
concepts, 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 terms used for OMRON PCs.
Relay term
PC equivalent
contact
input or condition
coil
output or work bit
NO relay
normally open condition
NC relay
normally closed condition
The terms used for PC will be described in detail later.
1-2-1
PC Terminology
Although also provided in the Glossary at the back of this manual, the follow-
ing terms are crucial to understanding PC operation and are thus explained
here.
A device connected to the PC that sends a signal to the PC is called an in-
put 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 allo-
cated a location in memory that reflects its status, i.e., either ON or OFF. This
memory location is called an input bit. The CPU, in its normal processing
cycle, monitors the status of all input points and turns ON or OFF corre-
sponding input bits accordingly.
Relay Circuits: The Roots
of PC Logic
Relay vs. PC Terminology
Inputs and Outputs
PC Basics
Section 1-2