PLC Memory - Automation Direct
PLC Memory
In This Appendix. . . .
— DL405 PLC Memory
1F
F--2
PLC Memory
Appendix F
PLC Memeory
DL405 PLC Memory
When designing a PLC application, it is important for the PLC user to understand the
different types of memory in the PLC. Two types of memory are used by the DL205
CPU, RAM and EEPROM. This memory can be configured by the PLC user as either
retentive or non--retentive.
Retentive memory is memory that is configured by the user to maintain values
through a power cycle or a PROGRAM to RUN transition. Non--retentive memory is
memory that is configured by the PLC user to clear data after a power cycle or a
PROGRAM to RUN transition. The retentive ranges can be configured with the
handheld programmer using AUX 57 or DirectSOFT (PLC Setup).
The contents of RAM memory can be written to and read from for an infinite number
of times, but RAM requires a power source to maintain the contents of memory. The
contents of RAM are maintained by the internal power supply (5VDC) only while the
PLC is powered by an external source, normally 120VAC. When power to the PLC is
turned off, the contents of RAM can be maintained by an optional battery (See page
3--12). The contents of RAM will be lost when external power is lost without battery
backup.
The contents of EEPROM memory can be read from for an infinite number of times
but there is a limit to the number of times it can be written to (typical specification is
100,000 writes). EEPROM does not require a power source to maintain the memory
contents. It will retain the contents of memory indefinitely.
PLC user V--memory is stored in both volatile RAM and non--volatile EEPROM
memory. See the memory map pertaining to the Data Word range for your particular
CPU (page 3--40 to 3--42).
Data values that must be retained for long periods of time, when the PLC is powered
off, should be stored in EEPROM based V--memory. Since EEPROM is limited to the
number of times it can be written to, it is suggested that transitional logic, such as a
one--shot, be used to write the data one time instead of on each CPU scan.
Data values that are continually changing or which can be initialized with program
logic should be stored in RAM based V--memory.
DL405 User Manual, 4th Edition, Rev. A
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