Siemens Simatic S7 Series Manual page 11

BCD.
Hexadecimal.
While.it.is.necessary.for.PLCs.to.use.binary.values,.humans.
often.need.to.see.values.represented.in.decimal..As.a.result,.
some.input.and.output.devices.provide.a.decimal.display.where.
each.decimal.digit.corresponds.to.four.PLC.binary.inputs.or.
outputs..The.most.common.system.used.by.input.and.output.
devices.of.this.type.is.referred.to.as.binary-coded decimal
(BCD).
One.example.of.a.BCD.device.is.a.type.of.four-digit.
thumbwheel.switch..Each.thumbwheel.digit.controls.four.
PLC.inputs..This.means.that.for.a.four-digit.thumbwheel,.6.
inputs.are.required..Because.each.thumbwheel.digit.only.
needs.to.represent.decimal.values.from.0.through.9,.only.ten.
corresponding.binary.values.are.required.for.each.digit..
0 0
2
0000 0010 0000
Hexadecimal.is.another.system.used.in.PLCs...The.ten.digits.
of.the.decimal.system.are.used.for.the.first.ten.characters.of.
the.hexadecimal.system..The.first.six.letters.of.the.alphabet.are.
used.for.the.remaining.six.characters.
The.hexadecimal.system.is.used.in.PLCs.because.it.allows.the.
status.of.a.large.number.of.binary.bits.to.be.represented.in.a.
small.space.such.as.on.a.computer.screen.or.programming.
device.display..Each.hexadecimal.character.represents.the.exact.
status.of.four.binary.bits.
Hexadecimal Number System
16 digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F
Base 16
Weights Powers of base 16 (1, 16, 256, 4096, ...)
Hexadecimal Example
Binary Equivalent 0 0 1 1 1 0 1 0 0 0 1 0 1 1 1 1
0
Decimal
5
0101
3 A 2 F
BCD
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
Binary
Hexadecimal
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
9 1 0 0 1
A 1 0 1 0
B 1 0 1 1
C 1 1 0 0
D 1 1 0 1
E 1 1 1 0
F 1 1 1 1