Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 13)

An Introduction to Programmable
Controllers
Chapter 2
2Ć6
Data Table
The area of memory, where data is controlled and utilized, 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:
Input image table
Output image table
Processor work areas (2)
Timer/Counter accumulated values and internal storage
Timer/Counter preset values and internal storage
Data Table
Input Image Table
Output Image Table
Program Storage
At this time, we will only discuss the input and output image tables. Chapter 4
discusses the remaining areas.
Image Tables
The input image table reflects the status of the input terminals. The output
image table reflects the status of the output terminals.
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 CPU 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 CPU detects a voltage at an input terminal, it records that information
by turning the corresponding bit on. Likewise, when the CPU 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 corresponding to the on or off status of an I/O terminal.
When people who work with PCs talk about turning a bit on, they use the term
“set.” For example, “The CPU sets the bit.” means “turns it on”. On the other
hand, they use the term “reset” when they talk about turning the bit off. For
example, “The CPU reset the bit.”