Omron SYSMAC C500, Installation Manual

Page: 14 / 112

Section 1
Introduction
5
1-3 How Does a PC Work?
PCs operate by monitoring input signals and providing output signals. When
changes are detected in the signals, the PC reacts, through the user-pro-
grammed internal logic, to produce output signals. The PC continually cycles
the program in its memory to achieve this control.
Block Diagram of PC
Power Supply
Input Output
CPU
Memory
Programming
Device
Signals
from
switches,
sensors,
etc.
Signals
to Sole-
noids,
motors,
etc.
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.
Scanning Cycle
When a PC operates, that is, when it executes its program to control an ex-
ternal system, a series of operations are performed inside the PC. These in-
ternal operations can be broadly classified into the following four categories:
1. Common (or overseeing) processes, such as watchdog timer operation
and testing the program memory.
2. Data input and output.
3. Instruction 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. The flowchart and diagram on page 7 illustrate these
internal operations for a typical PC.
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 mem-
ory?