Programming instructions

51
D – Data Registers
D devices are data registers. Data registers can be used for any purpose.
All data registers are 16 bit, meaning the limits of numeric data is from -
32768 to 32767. In ladder you can perform 32 bit operations, in that case
2 consecutive D registers are used together and the maximum numeric
value can be 2,147,483,647 to -2,147,483,648.
In GX-Developer, it is possible to configure D devices to be battery-
backed. This means the bits will maintain their current value in the event
of power loss. It is possible to retain data in all registers by turning on
special relay M8033.
File registers are data registers that are stored with the program, rather
than with the PLC. They are declared in the Parameters section in groups
of 500. Each group of 500 declared registers reduces the amount of
program steps by 500.
Data registers D8000~D8511 are dedicated for PLC diagnosis and special
functions. Their meanings can be seen in the FX3U Programming Manual
chapter 36.
In FX3U, the PLC has the ability to directly access bits within a word. By
placing a decimal point between the word address and bit address, you
can reference the status of a single bit within a word. An example would
be D100.0 which references the least significant bit in word D100.
R and ER – File Registers and Extension File Registers
R registers are file registers. File registers are an extended set of data
registers which are battery backed. They are stored in the PLC’s internal
memory, and can be accessed from within the ladder logic program.
ER registers are extension file registers. Extension file registers are only
available if a memory cassette is installed in the PLC, as the ER data is
stored in the memory cassette. There are dedicated commands in the
PLC for access to data in the extension file registers. ER registers are
accessed by copying their contents into the R file registers.