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2.13 Memory Types
The FX3U comes with enough internal RAM to hold a program of 64K steps.
The 64K memory in the FX3U is NOT expandable.
The FX2N comes with 8K steps of program memory. By using an 8K RAM
memory module, the PLC’s memory can be expanded to 16K steps.
The FX2NC comes with 8K steps of program memory. By using a 16K memory
module, the PLC’s memory can be expanded to 16K steps.
The FX1N comes with 8K of program memory, which is not expandable.
The FX1S comes with 2K of program memory, which is not expandable.
A cassette that contains non-volatile memory can add portability to a program.
The program is saved in the cassette, and when the cassette is connected to
another PLC, the program in the cassette overrides the program in the PLC
RAM. The program in the RAM is retained; the PLC simply uses the program on
the cassette instead of the memory. A special memory card with a program
loader option allows the memory card to backup or overwrite the program in the
PLC.
Note that since the PLC uses the cassette instead of the PLC RAM, the cassette
memory is not cumulative with the memory of the PLC.
Depending on the PLC type, there are 4 types of program memory. RAM,
EPROM, EEPROM and FLROM modules are available. Each type has its
advantages and disadvantages. Your PLC type, the application, and the level of
security you require will dictate which type of memory you use.
RAM (Random Access Memory)
RAM memory is volatile, which means it relies on a battery to keep the program
in memory. It is easy to make changes to a program that is kept in RAM memory.
On-line program changes are allowed with RAM.
EPROM (Erasable Programmable Read Only Memory)
EPROM memory is permanent; it retains the program memory without a battery.
It is difficult to make changes to programs stored in EPROM, because an
ultraviolet light is required to erase it. An EPROM burner is required. Online
changes are NOT allowed
EEPROM (Electrically Erasable Programmable ROM)
EEPROM is permanent memory also; the program is retained with no battery
connected. It is easy to make changes to EEPROM, because it is electrically
erasable. On-line changes are allowed with FX1N and newer.