FX Series Programmable Controllers Devices in Detail 4
4-42
4.14.2 Word Devices
Word devices such as T, C, D, V and Z can store data about a particular event or action within
the PLC. For the most part these devices are 16 bit re
isters. However, certain variations do
have 32 bit capabilities, as can pairs of consecutive data re
isters or combined V and Z
re
isters.
It ma
seem stran
e to quote the size of a word device in bits. This is not so stran
e when it is
considered that the bit is the smallest unit of data within the PLC. So b
identif
in
ever
thin
in bit format a common denomination is bein
used, hence comparison etc is much easier.
Additional consequences of this bit interpretation is that the actual data can be interpreted
differentl
. The ph
sical pattern of the active bits ma
be the important feature or perhaps the
numerical interpretation of the bit pattern ma
be the ke
to the pro
ram. It all comes down to
how the information is read.
4.14.3 Interpreting Word Data
As word data can be read in man
wa
s the si
nificance of certain parts of the word data can
chan
e. PLC’s can read the word data as:
- A pure bit pattern
- A decimal number
- A hexadecimal number
- Or as a BCD (Binar
Coded Decimal) number
The followin
examples will show how the same piece of data can become man
different
thin
s dependin
wholl
on the wa
the information is read or interpreted.
a) Considerin
a bit pattern
The followin
bit pattern means nothin
- it is simpl
16 devices which have two states.
Some of the devices are randoml
set to one of the states. However, if the header
notation (base 2) is added to the 16 bit data the sum, decimal, total of the active bits can
be calculated, e.
.,
Decimal value = (2
0
x 1) + (2
2
x 1) + (2
4
x 1) + (2
5
x 1)
+(2
5
x 1) + (2
9
x 1) + (2
10
x 1) + (2
11
x 1) + (2
12
x 1)
Decimal value = 7797
This is in fact incorrect!
There is one bit device which has been shaded in. If its header notation is studied
carefull
it will be noted that it sa
s MSB. This is the Most Si
nificant Bit. This sin
le bit
device will determine if the data will be interpreted as a positive or ne
ative number. In
this example the MSB is equal to 1. This means the data is ne
ative.
The answer however, is not -7797.
1 0 0 1 1 1 1 0 0 1 1 1 0 1 0 1
1 0 0 1 1 1 1 0 0 1 1 1 0 1 0 1
222222222222222
01234567891011121314
MSB
