Publication 1766-RM001A-EN-P - October 2008
42 Controller Memory and File Types
User Memory
User memory is the amount of controller storage available to store data
such as ladder logic, data table files, and I/O configuration.
User data files consist of the system status file, I/O image files, and all
other user-creatable data files (bit, timer, counter, control, integer, string,
long word, MSG, and PID).
A word is defined as a unit of memory in the controller. The amount of
memory available to the user for data files and program files is measured
in user words. Memory consumption is allocated as follows:
• For data files, a word is the equivalent of 16 bits of memory. For
example,
– 1 integer data file element = 1 user word
– 1 long word file element = 2 user words
– 1 timer data file element = 3 user words
• For program files, a word is the equivalent of a ladder instruction
with one operand. For example
(1)
,
– 1 XIC instruction, which has 1 operand, consumes 1 user word
– 1 EQU instruction, which has 2 operands, consumes 2 user words
– 1 ADD instruction, which has 3 operands, consumes 3 user words
• Function files do not consume user memory.
TIP
Each input and output data element consumes 3 user words due to
the overhead associated with I/O forcing.
(1) These are approximate values. For actual memory usage, see the tables in Appendix A of this manual.
TIP
Although the controller allows up to 256 elements in a file, it may not
actually be possible to create a file with that many elements due to the
user memory size in the controller.
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