Appendix
G
Numbering
S
ystems
G--7
Numbering Systems
DL105 PLC User Manual, 3rd Edition
BCD/Binary/Decimal/Hex/Octal -- What is the Difference?
Sometimes there is confusion about the differences between the data types used in
Direct
LOGIC PLCs. The PLC’s native data format is BCD, while the I/O numbering
system is octal. Other numbering formats used are binary, decimal and Real.
Although data is stored in the same manner (0’s and 1’s), there are differences in the
way that the PLC interprets it.
While all of the formats rely in the base 2 numbering system and bit--coded data, the
format of the data is dissimilar. The formats discussed are shown in Table 7 below.
Binary/Decimal Bit Pattern
Bit #
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Decimal
Bit Value
3
2
6
7
8
1
6
3
8
4
8
1
9
2
4
0
9
6
2
0
4
8
1
0
2
4
5
1
2
2
5
6
1
2
8
6
4
3
2
1
6
8
4
2
1
Max Value
65535
Hexidecimal Bit Pattern
Bit #
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Decimal
Bit Value
8
4
2
1
8
4
2
1
8
4
2
1
8
4
2
1
Max Value
F
F
F
F
BCD Bit Pattern
Bit #
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Decimal
Bit Value
8
4
2
1
8
4
2
1
8
4
2
1
8
4
2
1
Max Value
9
9
9
9
Octal Bit Pattern
Bit #
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Bit Value
1
4
2
1
4
2
1
4
2
1
4
2
1
4
2
1
Max Value
1
7
7
7
7
7
Octal Bit Pattern
Bit #
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
Sign
Exponent
Mantissa
Bit #
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Mantissa (continued from above)
Table 7
As seen in Table 7, the BCD and hexadecimal formats are similar, although the
maximum number for each grouping is different (9 for BCD and F for hexadecimal).
This allows both formats to use the same display method. The unfortunate side
effect is that unless the data type is documented, it’s difficult to know what the data
type is unless it contains the letters A--F.
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