A PLC consits of a Center Processing Unit (CPU) containing an application program and Input and out put interface modules, with is directly connectedto the field I/O devices. The program controls the PLC so that when an input signal from an input device turns ON, the appropriate response is made. The response normally involves turning ON an out put signal to some sort of output devices.
Center Processing Unit
The central Processing Unit (CPU) is a microprocessor that co-ordinates the activities of the PLC system. It executes the program, processes I/O signal and communicatea with external devices.
Memory
The are various types of mamory unit. It is the area that hold the operating system and user memory. The operating system is actually a system software that co-ofdinates the PLC. Ladder program, timer and counter Values are stored in the user memory, Depending on user’s need various types of memory are available for choice.
- Read-only Memory (ROM)
ROM is non – volatile memory that can be programmed only once. It is there fore unsuitable. It is least popular as compared with others memory type.
- Random Access Memory (RAM)
RAM is commonly used memory type for storing the user program and data. The data in the volatile RAM would normally be lost if the power source is removed. Hoever, this problem is solved by backing up the RAM with a battery.
- Erasable Programmable Read Only Memory (EPROM)
EPROM holds data permanently just like ROM.
- Electrically Erasable Programmable Read Only Memory (EEPROM)
EEPROM combines the access flexibility of RAM and the non-volatility of EPROM in one. Its contents can be eased and reprogrammed electrically, however, to a limit number of times.
Scan time
The process of reading the inputs, executing the program and updating the outputs is known as scan. The scan time is normally a continous and sequential process of reading the status of inputs, evaluating the control logic and updating outputs. Scan time specification indicates how fast the controller can react to the field inputs and correctly solve the control logic.
Factors fluencing Scan Time
The time required to make a single scan ( scan time) varies from 0,1ms to tens of ms depending on its CPU processing speed and the length of the user program. The user of remote I/O subsystems increase the scan time as a result of having to transmit the I/O updates to remote subsystem. Monitoring of the control program also adds overhead time to the san because the controller’s CPU has to send the status of coils and contacts to the CRT or other monitoring device.
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