#AN OUTPUT IN A LADDER LOGIC PROGRAM CAN BE CONTROLLED BY SERIAL#

This unit consists of microprocessor, system memory, serial communication ports and LAN link. Modular PLCs can be extendable to large number of I/O modules because mounting racks are cascadable. This rack also holds the power supply module if processor does not consist a power supply unit (inbuilt) as stated above. The power and data connections are provided by these racks to the processor via the backplane. These racks specify the number of modules that can be required to implement the system. It is a metal framework with PCB backplane consisting of several racks for all the modules. It provides the means for mounting processor, I/O modules, communication modules and power supply module.

In some PLCs, power supply is the separate module that can deliver a current greater than the sum of all currents needed by other modules. In such cases, additional power source is needed if excessive current or voltage is required by some modules (analog and external communication modules). In some PLCs, power supply is the part of processor module that can deliver all required power to the system. The specification of the power depends on the type of PLC being utilized in the application. It converts AC mains voltage to PLC operating low DC voltage and supplies the power to the modules that are plugged into the rack. The following are the main components of a PLC. The components or parts of PLC are housed in a suitable cabinet designed especially for the industrial environment. It has an extendable I/O capacity, larger memory size and communication with remote unit capability. However, modular PLCs offers flexibility to the user in order to match the controller components to the specific application needs. It consists of predetermined digital and analog modules (of a fixed number of channels) along with processor and power supply units.ДЇor an expanding number of I/O channels, fixed PLC requires separate interconnectable components. Fixed PLCs are cheaper, smaller and easier to install. PLCs can be placed into two categories, namely fixed and modular. Since the PLC has some memory, once the program is written, tested and downloaded to the PLC, it takes charge control of a process, thus increases the reliability.ДЄ PLC can communicate with other PLCs or computers or any other smart instruments via a communication network to perform the functions like downloading of programs, remote monitoring the devices, facilitating supervisory control, etc. PLCs continuously monitors various sensor outputs connected to its input modules and produces output decisions to the actuators connected to output modules according to the control function implemented in its program. The principle operation programmable logic controller is illustrated in figure below. PLCs overcomes such hardwiring associated with relay control circuits not only by performing such switching tasks, but also performing the operations like processing analog signals, counting, timing, sequencing, comparing, etc. PLCs replaces the conventional relay logic control where a set of relays is hardwired to perform a specific task and also if the system needs any changes then relay wiring has to be modified accordingly. It can be easily integrated into the industrial control systems to expand and enhance the performance of industrial processes. Programmable Logic Controller (PLC) is a digitally operating electronic system or it can be termed as industrial computer which is designed especially for use in industrial environment. What is PLC (Programmable Logic Controller)? Applications of the PLCs vary from simple stand-alone installations to complex machine control. PLC (Programmable Logic Controllers) are widely used in industrial and manufacturing automation systems for improved operational availability and extendable production rates.ДЄs industry increasing its production rates without compromising product quality, PLCs are widely accepted by the industry so as to minimize the time and cost of developing a large quantity of products and also for developing new products.

Examples of Ladder Logic Diagrams for PLC.

What is PLC (Programmable Logic Controller)?.

An output in a ladder logic program can be controlled by