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 is shown in Fig.
Programmable Logic Controllers Architecture
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:
Memory modules with RAM or EPROM (in the memory sub modules) for the program
Interface modules for programmers, expansion units, standard peripherals etc;
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 width of either 8, 16 or 24 bits.
In addition some times 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:
Flags ( internal relays), timers and counters;
Operating system data
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:
Standard peripherals such as printers, keyboards and CRTs,
Supervisory Computer Systems,
Other Programmable controllers,
The data required for each communications processors 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 in 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.
Modules for the input and output of signals are plugged into expansion units. The latter are 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.