ARCHITECTURE OF PLCS                

Architecture of PLCs

The PLC is essentially a microprocessor-based real-time computing system that often has to handle significant I/O and Communication activities, bit-oriented computing, as well as normal floating-point arithmetic. A typical set of components that make a PLC System

Central controller

The central controller (CC) contains the modules necessary for the main computing operation of the Programmable Controller (PC). The central controller can be equipped with the following:

1. Memory modules with RAM or EPROM (in the memory sub-modules) for the program (main memory);

2. Interface modules for programmers, expansion units, standard peripherals

3. Communications processors for operator communication and visualization, communication with other systems, and configuring of local area networks.

A bus connects the CPUs with the other modules.

Central Processing units

 The CPUs are generally microprogrammed processors sometimes capable of handling multiple data widths of either 8, 16, or 24 bits. In addition, sometimes additional circuitry, such as for bit processing is provided since much of the computing involves logical operations involving digital inputs and auxiliary quantities. Memory with battery backup is also provided for the following:

 1. Flags (internal relays), timers, and counters;

 2. Operating system data

 3. Process image for the signal states of binary inputs and outputs.

 The user program is stored in memory modules. During each program scan, the processor reads the statement in the program memory, executes the corresponding operations. The bit processor, if it exists, executes binary operations. Often multiple central controllers can be configured in hot standby mode, such that if one processor fails the other can immediately pick up the computing tasks without any failure in plant operations.

 Communications processors autonomously handle data communication with the following:

 1. Standard peripherals such as printers, keyboards, and CRTs,

2. Supervisory Computer Systems,

3. Other Programmable controllers

The data required for each communications processor is stored in a RAM or EPROM sub-module so that they do not load the processor memories. A local area network can also be configured using communications processors. This enables the connection of various PLCs over a wide distance in various configurations. The network protocols are often proprietary. However, over the last decade, interoperable network protocol standards are also supported modern PLCs.

Program and Data memory

The program and data needed for execution are stored in RAM or EPROM sub-modules. These sub-modules are plugged into the processors. Additional RAM memory modules can also be connected.

 Expansion units

 Modules for the input and output of signals are plugged into expansion units. The latter is connected to the central controller via interface modules. Expansion units can be connected in two configurations.

A  Centralized configuration

 The expansion units (EU) are located in the same cabinet as the central controllers or in an adjacent cabinet in the centralized configuration, several expansion units can be connected to one central controller. The length of the cable from the central controller to the most distant expansion unit is often limited based on data transfer speeds.

B.     Distributed configuration 

The expansion units can be located at a distance of up to 1000 m from the central controller. In the distributed configuration, up to 16 expansion units can be connected to one central controller. Four additional expansion units can be connected in the centralized configuration to each distributed expansion unit and to the central controller.

Input/Output Units

 A host of input and output modules are connected to the PLC bus to exchange data with the processor unit. These can be broadly categorized into Digital Input Modules, Digital Output Modules, Analog Input Modules, Analog Output Modules, and Special Purpose Modules.

Digital Input Modules

The digital inputs modules convert the external binary signals from the process to the internal digital signal level of programmable controllers.

 Digital Output Modules

The digital output modules convert the internal signal levels of the programmable controllers into the binary signal levels required externally by the process.

 Analog Input Modules The analog input modules convert the analog signals from the process into digital values which are then processed by the programmable controller.

 Analog Output Modules

The analog output modules convert digital values from the programmable controller into the analog signals required by the process.

 Special Purpose Modules

 These may include special units for:

 • High-speed counting

 • High accuracy positioning

 • On-line self-optimizing control

• Multi-axis synchronization, interpolation

 

 Programmers

 External programming units can be used to download programs into the program memory of the CPU. The external field programmers provide several software features that facilitate program entry in graphical form. The programmers also provide comprehensive aids for debugging and execution monitoring support logic and sequence control systems. The printer can be connected to the programmers for the purpose of documenting the program. In some cases, special programming packages that run on Personal Computers can also be used as programming units. There are two ways of entering the program:

 A. Direct program entry to the program memory (RAM) plugged into the central controller. For this purpose, the programmer is connected to the processor or to the programmer interface modules.

 B. Programming the EPROM submodules in the programmer without connecting it to the PC. THE MEMORY SUB-MODULES ARE THEN PLUGGED INTO THE CENTRAL CONTROLLER.