Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 19)

An Introduction to
Programmable Controllers
Chapter 2
2Ć5
Picture memory as a page that has been divided into many blocks. Each block
represents one bit. Since each bit is either on or off, we could show the state of
each bit by writing “on” or “off” in each block. However, there is an easier
way. We can agree that the numeral one (1) means on and that the numeral zero
(0) means off. We can show the status of each bit by writing 1 or 0 into the
appropriate block. For example, you might hear expressions like, “The CPU
responded by writing a one into the bit when the limit switch closed.” Of
course, the processor didn’t really write a one into memory: it simply set the bit
by turning it on.
When the I/O device is: The bit status is said to be:
on
on
1
set
off
off
0
reset
If you heard the expression, “The processor wrote a zero into that bit location.”
What actually happened? If you said the processor merely reset the bit by
turning it off, you’re right.
Program Storage
The other major area of memory, program storage, takes up the largest portion
of memory. You’ll recall that this is where your instructions to the
programmable controller are stored. You’ll also recall that this set of
instructions is called a program.
Program Language
A program is made up of set of statements. Each statement does two things:
It describes an action to be taken. For instance, it might say, “Energize motor
starter number one.”
It describes the conditions that must exist in order for the action to take place.
Statement
Statement
Statement
Statement
Statement
Statement
Program Storage Area
of Memory
Action Conditions
Program
Statement
Program