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CPU Specifications
and Operation
4–20
CPU Specifications and Operation
PLC Numbering Systems
If you are a new PLC user or are using
PLC
Direct PLCs for the first time, please
take a moment to study how our PLCs use
numbers. You’ll find that each PLC
manufacturer has their own conventions
on the use of numbers in their PLCs. We
want to take just a moment to familiarize
you with how numbers are used in
PLC
Direct PLCs. The information you
learn here applies to all of our PLCs!
1482
0402
1001011011
7
3
3A9
?
?
?
?
BCD
binary
decimal
octal
hexadecimal
ASCII
1011
–961428
177
?
–300124
A
72B
?
As any good computer does, PLCs store and manipulate numbers in binary form:
just ones and zeros. So why do we have to deal with numbers in so many different
forms? Numbers have meaning, and some
representations are more convenient
than others for particular purposes. Sometimes we use numbers to represent a size
or amount of something. Other numbers refer to locations or addresses, or to time. In
science we attach engineering units to numbers to give a particular meaning.
PLCs offer a fixed amount of resources, depending on the model and configuration.
We use the word “resources” to include variable memory (V-memory), I/O points,
timers, counters, etc. Most modular PLCs allow you to add I/O points in groups of
eight. In fact, all the resources of our PLCs are counted in octal. It’s easier for
computers to count in groups of eight than ten, because eight is an even power of 2.
Octal means simply counting in groups of
eight things at a time. In the figure to the
right, there are eight circles. The quantity
in decimal is “8”, but in octal it is “10” (8 and
9 are not valid in octal). In octal, “10”
means 1 group of 8 plus 0 (no individuals).
Decimal 1 2 3 4 5 6 7 8
Octal 1 2 3 4 5 6 7 10
In the figure below, we have two groups of eight circles. Counting in octal we have
“20” items, meaning 2 groups of eight, plus 0 individuals Don’t say “twenty”, say
“two–zero octal”. This makes a clear distinction between number systems.
Decimal 1 2 3 4 5 6 7 8
Octal 1 2 3 4 5 6 7 10
9 10 11 12 13 14 15 16
11 12 13 14 15 16 17 20
After counting PLC resources, it’s time to access PLC resources (there’s a
difference). The CPU instruction set accesses resources of the PLC using octal
addresses. Octal addresses are the same as octal quantities, except they start
counting at zero. The number zero is significant to a computer, so we don’t skip it.
Our circles are in an array of square
containers to the right. To access a
resource, our PLC instruction will address
its location using the octal references
shown. If these were counters, “CT14”
would access the black circle location.
0 1 2 3 4 5 6 7
2 X
1 X
X
X=
PLC Resources