Session III Architecture of PLC By K.SAMBA SIVA RAO

PLC Architecture

PLC Architecture contd.. The main parts of a PLC are divided into: Input/ Output (I/O) section Power supply Central Processing Unit (CPU) Programming device The PLC follows closed architecture (Proprietary), so one must be sure that any generic hardware or software we may use is compatible with our particular PLC. Although the principle concepts are the same in all methods of programming, there might be slight differences in addressing, memory allocation, retrieval, and data handling for different models. Consequently, PLC programs cannot be interchanged among different PLC manufacturers.

I/O Module I/O module is responsible for accepting the inputs from input field devices (sensors, switches etc.) and conveying the output to the output field devices (solenoids, valves, indicators etc.). Inputs from the field devices are connected to the input module and from the output module output is conveyed to output field devices. I/O modules can be of two types: Fixed and Modular

Fixed I/O Vs Modular I/O

Fixed I/O Module Fixed I/O is typical of small PLCs that come in one package with no separate, removable units. The processor and I/O are packaged together, and the I/O terminals will have a fixed number of connections built in for inputs and outputs. The main advantage of this type of packaging is lower cost. The number of available I/O points varies and usually can be expanded by buying additional units of fixed I/O. Disadvantages: Lack of flexibility; you are limited in what you can get in the quantities and types dictated by the packaging. If any part in the unit fails, the whole unit has to be replaced

Modular I/O Module Here the PLC is divided by compartments into which separate modules can be plugged, thereby increasing the flexibility. One can choose from the modules available from the manufacturer and mix them any way they desire. The basic modular controller consists of a rack, power supply, processor module (CPU), input/output (I/O modules), and an operator interface for programming and monitoring. The modules plug into a rack. When a module is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack. (Modular I/O)

Time for some activity? (Funny Riddles)

Power Supply The power supply supplies DC power to other modules that plug into the rack

Power Supply contd.. For small PLC systems, the power supply may be used to power the field devices as well. For large PLC systems, the power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies.

Central Processing Unit (CPU) The processor (CPU) is the “brain” of the PLC. A typical processor usually consists of a microprocessor for implementing the logic and controlling the communications among the modules. The processor requires memory for storing user program instructions, numerical values, and I/O devices status.

Programming Device A programming device is used to enter the desired program into the memory of the processor. Programming device can be: Computer or Hand held device A programming device need not be connected to the PLC always. Only when entering or editing the program is required, the programming device has to be connected.

Hand held device Hand-held programming devices can be used to program small PLCs because they are inexpensive and easy to use. They can be used to enter and monitor programs.

PC A personal computer (PC) is the most commonly used programming device. Using the available PLC software, a PC can be used as the programming device. This software allows users to create, edit, document, store, and troubleshoot ladder logic programs. As the computer monitor is able to display more logic on the screen than a hand-held device, interpretation of the program is easy.

Time to Recollect

Quiz 1. 2. 3. 4. The input interface module a) conditions the signal received from field device b) allows the programmer to input the program c) allows CPU to input messages to CRT screen d) provides inputs to motor controllers and similar field devices 1. 2. 3. 4. The output interface module connects to a) sensing devices such as switches or push buttons b) the programming device that connects to the machine process c) load devices such as lamps or solenoids d) the tape or disc drive circuits Field devices refer to a) input devices only b) output devices only c) load devices only d) all devices physically wired to the PLC The programming device for a PLC a) allows the programmer to enter program b) is always a CRT screen device c) is used by the programmer to bring the process signals into output interface  d) is all the above

Quiz The programming device must be connected to the controller a) always b) while monitoring a program c) when entering a program d) both b) and c) 5. 6. 7. 8. __________ i/os are typical of small PLCs that come in one package with no separate removable i/o units a) analog b) digital c) fixed d) modular PLC software that runs on personal computer can be used to a) write a PLC program b) document a PLC program c) monitor and control the process d) all of these The function of input interface module is to a) accept inputs from field devices and convert them into signals that can be used by the processor b) convert signals from processing unit into values that can be used to control the process c) input signals from programming device and convert them into signals that can be used by the CPU d) interpret and execute the user program that controls the process

Time to Conclude Can someone volunteer?