ARCHITECTURE OF PLCS
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.
No comments:
If you have any doubts, please let me know