Page Appendix G Appxg
User Manual: Appendix G D3-350 CPU User Manual
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Appendix G
PLC Memeory
G--2 PLC Memory
DL350 User Manual, 2nd Edition
DL350 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 DL350
CPU: RAM and EEPROM. RAM is Random Access Memory and EEPROM is
Electrically erasable Programmable Read Only Memory. The PLC programis stored
in EEPROM, and the PLC V--memory data is stored in RAM. There is also a small
range of V--memory that can be copied to EEPROM which will be explained later.
The V--memory in RAM can be configured as either retentive or non--retentive
memory.
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 are maintained by a “Super--Capacitor”. If the
Super--Capacitor ever discharges, the contents of RAM will be lost. The data
retention time of the super--Capacitor backed RAM is 3 weeks maximum, and 4 1/2
days minimum (at 60°F). An optional batery, D2--BAT--1, can be added to maintain RAM
retentive memory if the DL350 is ever without external power (see page 3--6 for a detailed
explanation).
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 indefinately.
PLC user V--memory is stored in both volatile RAM and non--volatile EEPROM
memory. Data being stored in RAM uses V1400 -- V7377 and V10000 -- V17777.
Data stored in EEPROM uses V7400 -- V7777
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 tothe
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.
Appendix G
PLC Memory
G--3
PLC Memory
DL350 User Manual, 2nd Edition
There are two types of memory assigned for the non--volatile V--memory area. They
are RAM and flash ROM (EEPROM). They are sharing the same V--memory
addresses; however, you can only use the MOV instruction, D2--HPP and
DirectSOFT to write data to the flash ROM. When you write data to the flash ROM,
the same data is also written to RAM. If you use other instructions, you can only write
data to RAM. When you read data from the nonvolatile V--memory area, the data is
always read from RAM.
Writing Data
RAM Flash RAM
V4000--V4377 V4000--V4377
Reading Data
RAM Flash RAM
V4000--V4377 V4000--V4377
Other instructions
(OUT, OUTD...)
MOV
D2--HPP
DirectSOFT
MOV
D2--HPP
DirectSOFT
There is no way to read data
from the Flash ROM directly.
After a power cycle, the PLC always copies the data in the flash ROM to the RAM.
If you use the instructions except for the MOV instruction to write data into the
non--volatile V--memory area, you only update the data in RAM. After a power cycle,
the PLC copies the previous data from the flash memory to the RAM, so you may
think the data you changed has disappeared. To avoid trouble such as this, we
recommend that you use the MOV instruction.
Copy
Cycle power
LD K2222
OUT V4000
Not changed
RAM Flash RAM
V4000 = 1111 V4000 = 1111
V4000 = 2222 V4000 = 1111
V4000 = 1111 V4000 = 1111
This appears to be previous data returning.
Non--volatile
V--memory in the
DL350
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