FX Series Programmable Controllers Devices in Detail 4
4-34
4.12.1 General Use Registers
Data re
isters, as the name su
ests, store data. The stored data can be interpreted as a
numerical value or as a series of bits, bein
either ON or OFF.
A sin
le data re
ister contains 16bits or one word. However, two consecutive data re
isters
can be used to form a 32bit device more commonl
known as a double word.
If the contents of the data re
ister is bein
considered numericall
then the Most Si
nificant Bit
(MSB) is used to indicate if the data has a positive or ne
ative bias. As bit devices can onl
be
ON or OFF, 1 or 0 the MSB convention used is, 0 is equal to a positive number and 1 is equal
to a ne
ative number.
The dia
ram above shows both sin
le and double re
ister confi
urations. In the dia
ram
identified as
¡
, it should be noted that the ‘lower’ re
ister D0 no lon
er has a ‘Most Si
nificant
Bit’. This is because it is now bein
considered as part of a 32bit-double word. The MSB will
alwa
s be found in the hi
her 16 bits, i.e. in this case D1. When specif
in
a 32 bit data
re
ister within a pro
ram instruction, the lower device is alwa
s used e.
. if the above example
was to be written as a 32bit instructional operand it would be identified as D0. The second
re
ister, D1, would automaticall
be associated.
Once the data is written to a
eneral data re
ister, it remains unchan
ed until it is overwritten.
When the PLC is turned from RUN to STOP all of the
eneral data re
isters have their current
contents overwritten with a 0 (zero).
Data retention:
• Data can be retained in the
eneral use re
isters when the PLC is switched from RUN to
STOP if special auxiliar
rela
M8033 is ON.
Data register updates:
• Writin
a new data value to a data re
ister will result in the data re
ister bein
updated
with the new data value at the end of the current pro
ram scan.
D1 D0
D0
0:
1:
0:
1:
2
1
MSB - Most Significant Bit
MSB - Most Significant Bit
