Central Processing Unit (CPU)

A central processing unit (CPU), is the brains of a PLC. The CPU stores the user program, I/O status bits, and other data.
It also repetitively executes the PLC scan as previously described.

As part of this scanning process, the CPU receives input information from input channels and sends output information to output channels.
SIMATIC S7-1200 CPU'S have a small number of I/O channels in the same housing.
Most of these CPU'S also have the capability to connect to I/O signal modules for additional I/0 channels.
Most other SIMATIC PLC'S have al| I/O channels on I/O signal modules.
Both S7-1200 and S7-1500 CPU'S have one or more communication connections, referred to as ports.
A communication port is designed for a specific type of communications.

When a different type of communications is required or when additional communication ports are required, one or more communication modules are used.

Input Devices

Digital input devices provide an on or off signal to a digital input signal module in response to an operator input or a change in condition in a machine or process.
The input device may operate on direct current (DC) or alternating current (AC), and the voltage and current level is dependent on the type of device.

A digital input signal module converts this value to a low-level DC signal that represents the on or off condition of the input device.
For each digital input device, the latest on or off condition is stored in the CPU.
Analog input devices provide a variable voltage or current to an analog input signal module.
This variable voltage or current represents a condition in a machine or process such as temperature, pressure, vibration, rate of flow, weight, fluid level, etc.

An analog input signal module converts each current or voltage value to a low-level DC signal and sends it to the CPU.
For each analog input device, the latest value received from the device is stored as a number in the CPU.

Digital output devices are turned on or off by signals from digital output signal modules.
For example, an indicator light on an operator panel may be turned on to indicate that a machine is running.
Often, however, the digital output device is used to cause a change in a machine or process, such as starting or stopping a motor or pump.
Analog output devices receive a variable voltage or current signal from an analog output signal module.
For example, an analog meter on an operator panel could be used as an analog output device to show a condition in a machine or process.
Often, however, an analog output device is used to control a machine or process.
For example, analog outputs from a PLC could be used to control AC drives which, in turn, control pumps that fill and drain a storage tank.

Input and output devices are often used to interconnect panels with lights and switches to one or more PLC'S so that operation and maintenance personnel can monitor and control a machine or process.
Increasingly, human machine interfaces (HMI'S) are also used for similar purposes.
HMI panels range in capability from simple keypad devices to panels with the ability to graphically display an entire process.
An HMI can also be a mobile device that allows freedom of movement for operation or maintenance personnel.