DL06 Micro PLC User Manual, 3rd Edition, Rev. E
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Chapter 3: CPU Specifications and Operation
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PLC Numbering Systems
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 (see Appendix I for numbering system details).
PLC Resources
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 (see Appendix I for more details).
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.
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.
1482
0402
1001011011
7
3
3A9
?
?
?
?
BCD
binary
decimal
octal
hexadecimal
ASCII
1011
–961428
177
?
–300124
A
72B
?
0 1 2 3 4 5 6 7
2 X
1 X
X
X=
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
Decimal 1 2 3 4 5 6 7 8
Octal
1 2 3 4 5 6 7 10
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 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
AutomationDirect
PLCs.
The information you learn here applies to all of our
PLCs.
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)
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.
