Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 70)

Chapter 4
Data File Organization and Addressing
4–14
Creating Data for Indexed Addresses
Data tables are not expanded automatically to accommodate indexed
addresses. You must create this data with the memory map function as
described in chapter 6. In the example on the previous page, data words
N7:3 through N7:12 and N11:6 through N11:15 must be allocated.
Important: Failure to allocate these data file elements will result in an
unintended overwrite condition or a major fault.
Crossing File Boundaries
An offset value may extend operation to an address outside the data file
boundary. You can either allow or disallow crossing file boundaries. If you
choose to disallow crossing file boundaries, a runtime error occurs if you use
an offset value which would result in crossing a file boundary.
You are allowed to select crossing file boundaries only if no indexed
addresses exist in the O: (output), I: (input), or S: (status) files. This
selection is made at the time you save your program. The file order from
start to finish is:
•
B3:, T4:, C5:, R6:, N7:, x9:, x10: . . .
•
x9: and x10: . . . are application-specific files where x can be of types B,
T, C, R, N.
Example
The figure below indicates the maximum offset for word address #T4:3.ACC
when allowing and disallowing crossing file boundaries.
#T4:3.ACC
T4:9.ACC
T4:0.ACC
Crossing file boundaries is disallowed.
Crossing file boundaries is allowed.
#T4:3.ACC
End of Highest File Created
B3:0
Maximum.
positive of 6
Maximum negative
of –3
Crossing file boundaries disallowed: In the example above, the highest
numbered element in the timer data file is T4:9. This means that #T4:3.ACC
can have a maximum negative offset of –3 and a maximum positive offset of
6.
Crossing file boundaries allowed: The maximum negative offset extends
to the beginning of data file 3. The maximum positive offset extends to the
end of the highest numbered file created.