Hitachi PLC User Manual

2
1. Introduction
What is a PLC?
“PLC” stands for Programmable Logic Controller. PLC is an industrial controller, which enables the
control of many devices and signals easily and flexibly. The PLC is “Programmable” by PC or hand
held programming unit. The PLC’s basically work in 3 steps : reading input data, calculating in the
CPU and writing output data according to user program in the PLC.
How it works?
The PLC has a CPU. The CPU executes (scans) a user program. When the CPU reaches the end of
the program, the CPU will return to the program top and execute again, this process is continuous. In
every cycle, input data is read, and output data is written once each.
Input and output data is effective once in a scan.
This is called “
Refresh Processing
”.
* Scan time (cycle time) depends on your program. (Normally 5 to 30ms.)
Time
1 cycle
Program Scanning
Read Input data Write Output data
System processing
and Communication
1 cycle
1 cycle
Input
- Switches
- Sensors
- Encoders, etc.
PLC
Output
- Valves
- Relays
- Alarms, etc.
CPU
OutputInput
Calculating
Program

Hitachi PLC Specifications

General

Brand	Hitachi
Model	PLC
Category	Controller
Language	English