Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 18)

An Introduction to
Programmable Controllers
Chapter 2
2Ć4
Memory
Memory serves three functions:
stores information in the data table that the CPU may need
stores sets of instructions called a program
stores
messages
Data Table
The area of memory where data is controlled and used, is called the data table.
The data table is divided into several smaller sections according to the type of
information to be remembered. These smaller sections are called:
output image table
input image table
timer/counter
storage
Output Image Table
Input Image Table
Timer/Counter
Storage
Data Table
At this time, we will only discuss the input and output image tables and
program storage.
I/O Image Tables
The input image table reflects the status of the input terminals. The output
image table reflects the status of bits controlled by the program.
Each image table is divided into a number of smaller units called bits. A bit is
the smallest unit of memory. A bit is a tiny electronic circuit that the processor
can turn on or off. Bits in the image table are associated with a particular
I/O terminal in the input or output section.
When the processor detects a voltage at an input terminal, it records that
information by turning the corresponding bit on. Likewise, when the processor
detects no voltage at an input terminal, it records that information by turning the
corresponding bit off. If, while executing your program, the CPU decides that a
particular output terminal should be turned on or off, it records that decision by
turning the corresponding bit on or off. In other words, each bit in the
I/O image tables corresponds to the on or off status of an I/O terminal.
When people who work with personal computers talk about turning a bit on,
they use the term “set.” For example - “The processor sets the bit” means
“turns it on.” On the other hand, we use the term “reset” when we talk about
turning the bit off - for example, “The processor reset the bit.”