OMRON C200H-SNT31 C200H Programmable Controllers

C200H Programmable Controllers

A PC (Programmable Controller) is basically a CPU (Central Processing Unit)

containing a program and connected to input and output (I/O) devices. The pro

gram controls the PC so that when an input signal from an input device turns ON,

the appropriate response is made. The response normally involves turning ON

an output signal to some sort of output device. The input devices could be photo

electric sensors, pushbuttons on control panels, limit switches, or any other de

vice that can produce a signal that can be input into the PC. The output devices

could be solenoids, switches activating indicator lamps, relays turning on mo

tors, or any other devices that can be activated by signals output from the PC.

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, posi

tioned 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 example 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.

To achieve proper control, the C200H uses a form of PC logic called ladder-dia

gram programming. This manual is written to explain ladder-diagram program

ming and to prepare the reader to program and operate the C200H.

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.

Relay vs. PC 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

PC equivalent

input or condition

contact

coil output or work bit

normally open condition

NC relay

NO relay

normally closed condition

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 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