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Chapter 3: CPU Specifications and Operation
PLC Numbering Systems
1
1
If you are a new PLC user or are using AutomationDirect
PLCs for the first time, please take a moment to study how our
PLCs use numbers. You'll find that each PLC manufacturer
2
2
has their own conventions on the use of numbers in their
PLCs. We want to take just a moment to familiarize you
3
3
with how numbers are used in AutomationDirect PLCs. The
information you learn here applies to all of our PLCs.
4
4
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.
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5
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
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6
a particular meaning (see Appendix I for numbering system details).
PLC Resources
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7
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.
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8
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,
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9
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
10
10
circles. The quantity in decimal is "8", but in octal it
is "10" (8 and 9 are not valid in octal). In octal, "10"
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11
means 1 group of 8 plus 0 (no individuals).
In the figure below, we have two groups of eight circles. Counting in octal we have "20" items,
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12
meaning 2 groups of eight, plus 0 individuals Don't say "twenty", say "two–zero octal". This
makes a clear distinction between number systems.
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13
Decimal
14
14
Octal
a
A
After counting PLC resources, it is time to access PLC resources (there is a difference). The
CPU instruction set accesses resources of the PLC using octal addresses. Octal addresses are the
b
B
same as octal quantities, except they start counting at zero. The number zero is significant to a
computer, so we don't skip it.
c
C
Our circles are in an array of square containers to
the right. To access a resource, our PLC instruction
D
D
will address its location using the octal references
shown. If these were counters, "CT14" would access
the black circle location.
3-20
DL05 Micro PLC User Manual, 6th Edition, Rev. D
1
2 3 4
5
6
1
2 3 4
5
6
octal
?
1001011011
–300124
Decimal 1 2 3 4 5 6 7 8
Octal
7 8
9 10 11 12 13 14 15 16
7 10
11 12 13 14 15 16 17 20
X=
0
X
1 X
2 X
binary
?
BCD
1482
3
?
0402
3A9
ASCII
–961428
7
hexadecimal
177
1011
?
decimal
A
72B
1 2 3 4 5 6 7 10
1
2 3
4
5
6
?
?
7
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