Introduction - Hitachi PLC Getting Started

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.
Input
- Switches
- Sensors
- Encoders, etc.
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.
1 cycle
Input and output data is effective once in a scan.
Refresh Processing
This is called "
* Scan time (cycle time) depends on your program. (Normally 5 to 30ms.)
Input
Program Scanning
Read Input data
".
PLC
CPU
Program
Calculating
1 cycle
1 cycle
Write Output data
2
Output
Output
- Valves
- Relays
- Alarms, etc.
Time
System processing
and Communication