Programmable logic controllers, or PLCs, are computers that have no monitor, mouse, or keyboards but do have microprocessors. It is used in controlling things like elevators, traffic lights, washing machines and has many other civil applications. To learn to use a PLC, it is recommended to get training on RSLogix 5000 or similar systems.
The basic architecture of a PLC consists of the processor with its power supply and the input/output modules. The processor has a central processing unit (CPU) and adequate memory. Interfaces are also required to programming devices and the I/O units. This programming allows for the continuous monitoring of inputs from sensors, that then create the necessary outputs that enable the operation of the actuators that are linked to any program. These PLCs can be programmed using any programming language that is standards-based. The most commonly used is Ladder logic also known as LD or Ladder diagram. These programs largely resemble electrical control diagrams.
PLC architecture resembles that in any general-purpose computer that has specialized input and output modules. However, they do have some characteristics that differentiate them form computers, the most important of which is that they have greater reliability than a general computer, having a mean time between failures that runs into years. Another characteristic that keeps it ahead of computers is that it can be placed in industrial environments that have a lot of electrical noise, humidity, extreme temperatures, and vibration. General purpose computers do require climate control and stable environments with no vibration and no interfering electrical circuits.
Depending on the amount of input and output that needs processing, I/O modules can be either on the same chassis as the CPU or housed elsewhere. Some PLC systems may have an architecture that houses a number of processors in the same chassis. Micro PLCs will have all the various units as part of a single package and can also have a panel that allows interface with the operator. Size limitations often limit the amount of input and output and this may not be expandable.
The CPU acts as a control for all operational processes within the PLC. It has a clock that operates at frequencies between 1 and 8 Mhz. The speed of operations within the PLC will depend on the frequency used and it is the frequency that is responsible for synchronization and timing of all the system’s elements. Digital signals carry information within the PLC on buses which are internal paths. A bus is essentially a conductor along which electrical signals are enabled to flow. These can be wires in a ribbon cable or tracks on a printed circuit board. Buses can be used for data (data bus), address buses to access locations for accessing any stored data or system buses that communicate between the I/O ports and I/O units.
CPUs will have arithmetic and logic units that carry out all the addition and subtraction, and the logic operations of AND, OR and NOT which will result in any manipulation of data. They will also have memory or registers that will be used to store any information that is needed for the program execution. They will, in addition, have control units that monitor and execute the timing of any required operations.
Information is transmitted in PLCs in binary form. The storage capacity of memory units depends on the number of binary words that it can store. Programs are stored in the memory. Memory elements can be ROM or read-only, or, RAM or random access that is used for the program and for data storage. Programs and data in the RAM can be changed by the user. PLCs may also have batteries that enable the RAM to prevent loss of programs when power is switched off.
