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PLC Hardware

Chapter-4

• A PLC is made up of both hardware and software. Hardware is the part of a system that you can see, and software is the collection of computer programs that control what the hardware does.

• PLC hardware also connects to other hardware, such as input and output devices and human machine interfaces (HMl'S).

•  Although not shown in the graphic, a programming device can be considered as PLC hardware, but it is only connected to the PLC when needed.

• Because PLC applications vary in complexity, there is considerable variation in the appearance of PLC hardware.
  

• In a typical application, small- and medium-sized PLC'S are often used to control individual machines or manufacturing cells.

• These PLC'S typically communicate via a network to one or more larger PLC'S that collect data and coordinate the operation of some or all of the system.
  

• Additionally, because the needs of customers vary widely.

Siemens SIMATIC PLC'S are available as

1. Modular controllers,

2. Embedded controllers, or as

3. PC-based controllers. Of these three categories, modular controllers are the most common.

Modular PLC'S

• SIMATIC S7 Modular PLC'S include a variety of models.

• For example, S7-300 and S7-400 PLC'S have been in service for many years.

S7-300 and S7-400 PLC'S

• More recently, S7-1200 and S71500 have been introduced.

• Regardless of the model, SIMATIC S7 modular PLC'S are optimized for control tasks and can be adapted to meet application requirements by adding modules. Typical PLC Module types are:

1. CPU modules,

2. power supply modules,

3. signal modules,

4. communication modules, and

5. special purpose modules.

Central Processing Unit (CPU)

• PLC CPU modules all contain: A microprocessor, which is an integrated circuit that runs the computer software and memory circuits that store the user program and related of data and may include a removable memory card.

• A CPU may also have an internal power supply that converts the supplied power to the tittered, low- voltage DC needed by internal circuits.
  

• If the CPU does not have an internal power supply, a system power supply module is needed to provide power CPU'S typically have the following three operating modes:

• In the STOP mode, the CPU is not executing the user program, and a new program can be downloaded to the CPU.

• In the STARTUP mode, the CPU executes any startup logic, if present.

• In the RUN mode, the CPU executes the PLC scan repeatedly.

• The specific mode the PLC is in at any given time depends on a software setting and whether or not a critical CPU error has occurred In the event of a critical error, the CPU is designed to go into the STOP mode.

• The current mode for S7- 1200 and S7-1500 PLC'S is shown by the CPU RUN/STOP LED.
  

• The current mode for S7-1500 PLC'S is also shown on the CPU's integrated display.

Chapter 5 we will discuss about Memory Types,CPU memory Organization ,Power Supply Module...

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