PLC Components
Chapter-1
Most PLC'S have a modular structure. Some PLC'S have a block-type modular structure, but the majority have a traditional modular structure.
Despite their variation in appearance, modular PLC'S have a similar organization which includes the following components:
The Central Processing Unit (CPU) is the brains of the PLC.
The CPU stores and runs the user program.
The input/output system (often referred to as the I/O system) includes modules with I/O channels that connect to input devices such as switches and sensors and output devices that are controlled by the PLC.
A programming device is used to load the program into the CPU memory. The programming device is usually an industrial personal computer with software that allows you to create and modify PLC programs.
The programming device is only connected to the PLC when it is needed. Although not part of a PLC, one or more human machine interfaces (HMI'S) are often connected to a PLC to allow factory personnel to monitor or alter a machine or process.
How a PLC Works
PLC /O devices are sometimes the same as devices used in hard-wired control circuits, but the relationship of these devices is determined by software referred to as the user program which is stored in the CPU.
This program is executed repetitively as part of a process referred to as a scan.
After startup, a PLC scan includes the following sequence of operations. The CPU reads the statuses of input devices and records this information in its memory.
The CPU executes the user program and changes internal memory values as needed. The CPU performs internal diagnostic and communication tasks.
The CPU sends updated status information to output devices through the 1/O system.
This process is repeated continuously as long as the PLC is in the run mode.
The time required for a scan depends on the capabilities of the CPU, the size of the user program, the number of I/Os, and the amount of communication required; however, because PLC'S are very fast, a typical PLC scan is measured in milliseconds.
This means that the PLC response time from reading an input to reacting by changing an output is also very fast.
PLC Inputs and Outputs
Input devices are usually switches or sensors that send electrical signals to the CPU through input channels on input signal modules.
Similarly, output devices receive electrical signals from the CPU through output channels on output signal modules.
These output devices are sometimes called actuators and are used to control a machine or process.
PLC'S have two broad categories of I/0, digital I/O and analog I/0. Digital I/O devices, also called discrete I/O devices, are either on or off and are connected to digital I/ O channels.
- Analog I/O devices use variable voltage or current signals and are connected to analog I/O channels.
Analog I/O is used in applications where variable quantities, such as temperature, pressure, rate of flow, etc., are being controlled or monitored.
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