Appendix
G
Numbering
S
ystems
G--5
Numbering Systems
DL105 PLC User Manual, 3rd Edition
Using the powers of expansion, the example below shows octal 4730 converted to
decimal.
Binary Coded Decimal (BCD) Numbering System
BCD is a numbering system where four bits are used to represent each decimal digit.
The binary codes corresponding to the hexadecimal digits A--F are not used in the
BCD system. For this reason numbers cannot be coded as efficiently using the BCD
system. For example, a byte can represent a maximum of 256 different numbers (i.e.
0--255) using normal binary, whereas only 100 distinct numbers (i.e. 0--99) could be
coded using BCD. Also, note that BCD is a subset of hexadecimal and neither one
does negative numbers.
BCD Bit Pattern
Bit #
15 14 13 12 11 10 9
8
7
6
5
4
3
2
1
0
Power
10
3
10
2
10
1
10
0
Bit Value
8
4
2
1
8
4
2
1
8
4
2
1
8
4
2
1
Max Value
9
9
9
9
Table 5
One plus for BCD is that it reads like a decimal number, whereas 867 in BCD would
mean 867 decimal. No conversion is needed.
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