1) PROM: - (Programmable Read Only Memory): In this memory it may be programed once and once only, by the user/programmer. The user than programs the chip to his/her requirements. No erasures are possible. To change program in a programmed PROM, you may throw it away and replace it with a new un-programmed PROM. The PROM is rarely used because it requires special programming circuits .It does however the advantages of being an unalterable backup to a ROM
2) EPROM: (Erasable Programmable Read Only Memory): It is a PROM that can be erased. The EPROM is erased by subjecting a window in its top to ultraviolet light for a few minutes. It is also called as Ultra Violet RAM. When exposed to UV light the chips memory bits are reset to 0 the chips window is covered during normal use to prevent unwanted eraser. When the EPROM is exposed to ultraviolet light all of memory locations are erased. This can be reused.
3) EEPROM: (Electrically Erasable Programmable read only memory): It is similar to EPROM instead of UV light exposer for eraser through an electrical signal is applied to the chip. The EEPROM advantages over the EPROM is the easy and speed with which it is reset & erased. The EEPROM is used in place of RAM when they want to fast eraser without using time for individual reprogramming of each part of the chips memory. EEPROM is the memory of choice for storing, backing up and transferring PLC programs.
(ii) List any four advantages of PLC
2) Implementing changes & correcting errors.
3) Speed of operation.
4) Reliability & maintainability.
5) PLCS are smaller in size & can operate Number of devices at a time.
6) Logic change can be very easily done by just adjusting the ladder logic.
7) Operation of PLC can be displayed on CRT or LCD screen.
8) Error occurred in operation can be very easily observed.
9) PLC can keep records of status of Input & Output signals.
10) PLC can provide security for ladder programming.
11) Speed of PLC is in milliseconds.