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CENTRAL PROCESSING
UNlT-CPU |
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CENTRAL PROCESSING UNlT-CPU
Central processing unit (CPU) is the brain of a PLC controller. CPU itself is usually one of the microcontrollers. A foretime these were 8-bit microcontrollers such as 8051, and now these are 16- and 32-bit microcontrollers. CPU also takes care of communication, interconnection among other parts of PLC controller, program execution, memory operation, overseeing input and setting up of an output. PLC controllers have complex routines for memory checkup in order to ensure that PLC memory was not corrupted. Generally speaking, CPU unit makes a great number of check-ups of the PLC controller itself so eventually errors would be discovered early. You can simply look at any PLC controller and see that there are several indicators in the form of light diodes for error signalization.
MEMORY
System memory (mostly implemented in FLASH technology) is used by a PLC for a process control system. Besides this PLC systems also contains a user program translated from a ladder diagram to a binary from. FLASH memory contents can be changed only in case where user program is being changed. PLC controllers used earlier have EPROM memory instead o!' FLASH memory which had to be erased with UV lamp and to be reprogrammed. With the use of FLASH technology this process was greatly shortened. Reprogramming of program memory is done through a serial cable for application development.
User memory is divided into blocks having special functions. Some parts of a memory are used for storing input and output status. The real status of input is stored either as "1" or as "0" in a specific memory bit. Each input or output has one corresponding bit in memory. Other parts of memory are used to store variable contents for variables used in user program.
POWER SUPPLY
Electrical supply is used in bringing electrical energy to central processing unit. Most PLC controllers work either at 24V-dc or 220V ac. Electrical supply can be present as a separate module (bigger PLC controllers). while small and medium series already contain the supply module. This electrical supply is usually not used to start external inputs or outputs. User has to provide separate supplies in starting PLC controller inputs or outputs because then only so called "pure" supply for the PLC controller can be ensured. With pure supply we mean supply where industrial environment cannot affect it damagingly. Some of the smaller PLC controllers supply their inputs with voltage from a small supply source already incorporated into a PLC.
PLC CONTROLLER INPUTS
Intelligence of an automated system depends largely on the ability of a PLC controller to read signals from different types of sensors and inputs devices. In order to detect a working piece, view a mechanism in motion, check pressure or fluid level specific automatic devices such as proximity sensor, marginal switches, photoelectric sensors, level sensors, etc. are required. The input signals can be logical (on /off) or analog. Smaller PLC Controllers usually have only digital inputs lines while larger also accept analogue inputs through special units attach to PLC controllers. One of the most frequent analogue signals are current signals of 4 to 20 ma and millivolt voltage signal generated by various sensors.
Sensors are usually used as inputs for PLCs. They can sense presence of some parts, measures temperature, pressure, or some other physical dimension, etc. Other devices also can serve as inputs to PLC controller. Intelligent devices such as robots, video system, etc. often are capable of sending signals to PLC controller input modules.
INPUT ADJUSTMENT INTERFACE
Adjust interface also called an interface is placed between input lines and a CPU unit. The purpose of adjustment interface is to protect a CPU from disproportionate signals from an outsides world. Input adjustment module turns a level of real logic to a level that suits CPU unit (for example- Input from a sensor which works on 24V DC must be converted to signal of 5V DC in order for a CPU to be able to process it). This is typically done through opto-isolation. Opto-isolation means that there is no electrical connection between external world and CPU unit. They are "optically" separated, or in other words, signal is transmitted through light.
The working of opto-isolation is simple. External device brings a signal which turns LED on, whose light in turn incites photo transistor which in turn starts conducting, and a CPU sees this as logic zero. When input signal stops LED diode turns off, transistor stops conducting, collector voltage increases, and CPU receives logic 1 as information.
PLC CONTROLLER OUTPUT
Automated system is incomplete if it is not connected with some output devices. Some of the most frequently used devices are motors, solenoids, relays, indicators, sound signalization etc. By starting a motor, or a relay, PLC can manage or control a simple system such as system for positioning head of CNC machine. Output can be of analog or digital type.
OUTPUT ADJUSTMENT INTERFACE
Output interface is similar to input interface. CPU brings a signal to LED diode and turns it on. Light incites a phototransistor that begins to conduct electricity, and thus the voltage between collector and emitter falls to 0.7V, and a device attached to this output sees this as a logic zero. Inversely it means that a signal at the output exits and is interpreted as logic one. Phototransistor is not directly connected to a PLC controller output. Between. Phototransistor and an output usually there is a relay.
EXTENSION LINES
Every PLC controller has a limited number of Input/Output lines. If needed, this number can be increased through certain additional modules by system extension through extension lines. Each module can contain extension of both input and output lines. Also extension modules can have inputs and outputs of a different nature from those on the PLC controller.
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