Page 57 - Programmable Logic Controllers, Fifth Edition

to develop and modify control programs. This memory is
made nonvolatile so that if power is lost, the PLC holds UV EPROMs (ultraviolet erasable programmable read-
only memory) can only be erased with an ultraviolet light.
its programming. EPROM memory is used to back up, store, or transfer
Read Only Memory (ROM) stores programs, and PLC programs.
data cannot be changed after the memory chip has been Electrically erasable programmable read-only
manufactured. ROM is normally used to store the pro- memory (EEPROM) is a nonvolatile memory that offers
grams and data that define the capabilities of the PLC. the same programming flexibility as does RAM. The
ROM memory is nonvolatile, meaning that its contents EEPROM can be electrically overwritten with new data
will not be lost if power is lost. ROM is used by the PLC instead of being erased with ultraviolet light. Because the
for the operating system. The operating system is burned EEPROM is nonvolatile memory, it does not require bat-
into ROM by the PLC manufacturer and controls the sys- tery backup. It provides permanent storage of the program
tem software that the user uses to program the PLC. When and can be changed easily using standard programming
Allen Bradley burns the operating system into memory it devices. Typically, an EEPROM memory module is used
is called PROM (programmable read-only memory). to store, back up, or transfer PLC programs (Figure 2-45).
Random Access Memory (RAM), sometimes re- Flash EEPROMs are similar to EEPROMs in that they
ferred to as read-write (R/W) memory, is designed so that can only be used for backup storage. The main difference
information can be written into or read from the memory. comes in the flash memory: they are extremely fast at sav-
RAM is used as a temporary storage area of data that may ing and retrieving files. In addition, they do not need to
need to be quickly changed. RAM is volatile, meaning be physically removed from the processor for reprogram-
that the data stored in RAM will be lost if power is lost. A ming; this can be done using the circuitry within the pro-
battery backup is required to avoid losing data in the event cessor module in which they reside. Flash memory is also
of a power loss (Figure 2-44). Most PLCs use CMOS- sometimes built into the processor module (Figure 2-46),
RAM technology for user memory. CMOS-RAM chips where it automatically backs up parts of RAM. If power
have very low current draw and can maintain memory fails while a PLC with flash memory is running, the PLC
with a lithium battery for an extended time, two to five will resume running without having lost any working data
years in many cases. Some processors have a capacitor after power is restored.
that provides at least 30 minutes of battery backup when
the battery is disconnected and power is OFF.
Erasable Programmable Read-Only Memory RAM EEPROM
(EPROM) provides some level of security against unau- (volatile) (nonvolatile)
thorized or unwanted changes in a program. EPROMs are Executed Program
designed so that data stored in them can be read, but not program backup
easily altered without special equipment. For example, Current
data Parameters
Memory
bits,
timers,
counters

Figure 2-45 EEPROM memory module is used to store,
+ back up, or transfer PLC programs.

–

Processor Module

Flash
Card

Figure 2-46 Flash memory card installed in a socket on
Figure 2-44 Battery used to back up processor RAM. the processor.

38 Chapter 2 PLC Hardware Components

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