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Memory Types and Size Kilo, abbreviated k, normally refers to 1000 units. When talking
about computer or PLC memory, however, 1k means 1024. This
is because of the binary number system (2
10
=1024). 1k can refer
to 1024 bits, bytes, or words, depending the context.
Random Access Memory (RAM) is memory that allows data
to written to and read from any address (location). RAM is used
as a temporary storage area. RAM is volatile, meaning that
the data stored in RAM will be lost if power is lost. A battery
backup is required to avoid losing data in the event of a power
loss.
Read Only Memory (ROM) is a type of memory used were
it is necessary to protect data or programs from accidental
erasure. The original data stored in ROM can be read, but
not changed. In addition, ROM memory is
nonvolatile. This
means that information will not be lost as the result of a loss of
electrical power. ROM is normally used to store the programs
that define the capabilities of the PLC.
Erasable Programmable Read Only Memory (EPROM)
provides a level of security against unauthorized or unwanted
changes in a program. EPROMs are designed so that data
stored in them can be read, but not easily altered. Changing
EPROM data requires a special effort. UVEPROMs (ultraviolet
erasable programmable read only memory) can only be erased
with an ultraviolet light. EEPROM (electronically erasable
programmable read only memory), can only be erased
electronically.
Software, Hardware, and Software is the name given to computer instructions,
Firmware regardless of the programming language. Essentially, software
includes the instructions or programs that direct hardware.
Hardware is the name given to all the physical components
of a system. The PLC, the programming device, and the
connecting cable are examples of hardware.
Firmware is user or application specific software burned into
EPROM and delivered as part of the hardware. Firmware gives
the PLC its basic functionality.
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