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3
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 4-4 Basic
Ladder Diagrams.
1-3 PC Terminology
Although also provided in the Glossary at the back of this manual, the following
terms are crucial to understanding PC operation and are thus explained here.
PC Because the C200HX/HG/HE PCs are Rack PCs, there is no one product that is
a C200HX/HG/HE 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 Unit and all of the Units directly controlled by
it through the program. This does not include the I/O devices connected to PC
inputs and outputs.
If you are not familiar with the terms used above to describe a PC, refer to Sec-
tion 2 Hardware Considerations for explanations.
Inputs and Outputs 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 Unit, in its normal processing cycle, monitors
the status of all input points and turns ON or OFF corresponding input bits ac-
cordingly.
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 turned ON to send a signal to an output device through an output point. The
CPU Unit periodically turns output points ON or OFF according to the status of
the output bits.
These terms are used when describing different aspects of PC operation. When
programming, one is concerned with what information is held in memory, and so
I/O bits are referred to. When talking about the Units that connect the PC to the
controlled system and the places on these Units where signals enter and leave
the PC, I/O points are referred to. When wiring these I/O points, the physical
counterparts of the I/O points, either terminals or connector pins, are referred to.
When talking about the signals that enter or leave the PC, one refers to input
signals and output signals, or sometimes just inputs and outputs. It all depends
on what aspect of PC operation is being talked about.
The Control System includes the PC and all I/O devices it uses to control an ex-
ternal system. A sensor that provides information to achieve control is an input
device that is clearly part of the Control System. The controlled system is the
external system that is being controlled by the PC program through these I/O
devices. I/O devices can sometimes be considered part of the controlled sys-
tem, e.g., a motor used to drive a conveyor belt.
Controlled System and
Control System
PC Terminology Section 1-3