Siemens Simatic S7 Series Manual page 10

Bits, Bytes, and Words.
Logic 0, Logic 1.
The.process.of.converting.a.binary.number.to.an.equal.decimal.
value.is.as.simple.as.adding.the.equivalent.decimal.value.
for.each.position.in.the.binary.number.where.a..is.shown..
Positions.with.a.0.do.not.add.to.the.number.value..
2 7
2 6 2 5
128 64 32
0 0 1
Decimal Value = 32 + 8 + 1 = 41
Each.position.in.a.binary.number.is.called.a bit..The.number.
of.bits.used.to.represent.numbers.varies.with.the.device..
However,.instructions.and.data.are.usually.grouped.in.bytes.and.
eight.bits.make.up.one.byte..Two.bytes,.or.6.bits,.make.up.one.
word.
Bit
While.PLCs.are.capable.of.sensing.and.generating.analog.
values,.programmable.controllers.internally.use.signals.that.are.
on.or.off..These.on.and.off.conditions.correspond.to.the.binary.
values..and.0..For.example,.a.binary.0,.also.called.logic.0,.can.
be.used.to.indicate.that.a.switch.is.off,.and.a.binary..(logic.).
can.be.used.to.indicate.that.a.switch.is.on.
Off
Logic 0
On
Logic 1
9
2 4 2 3 2 2 2 1
16 8 4 2
0 1 0 0
Word
Input 1
PLC
24 VDC
Input 1
PLC
24 VDC
2 0
1
1
Byte