PC Operation
Section 1-3
1-3 PC Operation
PCs operate by monitoring input signals and providing output signals. When
changes are detected in input signals, the PC reacts through the user-pro-
grammed internal logic to produce output signals. The PC continually executes
the program in its memory to achieve this control.
Block Diagram of PC
Power Supply
Memory
Signals to
Solenoids,
motors,
etc.
Signals
from
switches,
sensors,
etc.
Input
Output
CPU
Programming
Device
A program for your applications must be designed and stored in the PC. This
program is then executed as part of the cycle of internal operations of the PC.
Execution Cycle
When a PC operates, i.e., when it executes its program to control an external
system,
a series of operations are performed inside the PC. These internal oper-
ations can be broadly classified into the following four categories. Refer to the
CV-series PCs Operation Manual: Ladder Diagrams for details.
1, 2, 3...
1. Common (or overseeing) processes, such as watchdog timer operation and
testing the program memory.
2. Data input and output.
3. Program execution.
4. Peripheral device servicing.
Cycle Time
The total time required for a PC to perform all these internal operations is called
the cycle time.
Timing is one of the most important factors in designing a Control System. For
accurate operations, it is necessary to have answers to such questions as these:
• How long does it take for the PC to execute all the instructions in its memory?
• How long does it take for the PC to produce a control output in response to a
given input signal?
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