Rockwell Automation 1771-VHSC , D17716.5.74 Very High S - Manual (Page 71)

Questions and Answers D–5
Q. What happens if I change my BTW length after power up
to save block transfer time?
A. As long as the length is valid, the module will retain the data
previously sent to it as long as backplane power to the module
is present. If you power up with a block transfer length of 64
words to configure the module, and later change to 2 words,
the module will behave in the manner prescribed in the 64
word transfer. It will do this until you power down, and power
back up. This configuration is not affected by switching your
processor from RUN to PROG mode.
Q. How do I know what length to make my BTW file?
A. There are 3 approaches to consider: speed, functionality and
occasional usage.
When considering speed, you would want to configure the
module once, and then have access to certain BTW data only.
You would power up with a configuration word length of 64
words for access to all data, and then change the length to just
2 words allowing you to access specific commands, such as
preset or enable outputs. This would save BTW time by not
sending data which is not changing.
In the second approach, functionality, you might not be using
your outputs, so you would not need to have a 64 word BTW
length. For example, if you just needed to preset counter 3, you
would only need 20 words. You could then perform presets
without sending unnecessary words.
Using the occasional usage approach, you could do BTWs only
on occasion, such as when you needed to change output values.
You could reanable the BTW long enough to send new data,
and disable it thereafter.
Any or all of these approaches can be used to best suit your
individual needs.