Rockwell Automation 1771-QA Stepper Positioning Assembly User Manual - Manual (Page 53)

3–31 Programming and Operation
Publication 1771-UM002A–EN–P – May 2000
When the block length of the read block transfer instruction is set to
the default value, 00, the stepper controller module will
automatically establish the default number of status words to be
transferred; either four, seven or ten words. This number is
determined by the highest address of a pulse output expander module
contained in the chassis. For example, if the highest expander
address were 1, 2, or 3, then the read block transfer would be four
words, seven words or ten words, respectively. The remaining words
of the 64 word default read block not used by the module can be
used for timer/counter accumulated values and/or bit/word storage.
Refer to Figure 3.17.
When the block length of the write block transfer instruction is set to
the default value, 00, the stepper controller module will expect to
receive 64 words. The write block transfer file should also be 64
words long. During a write block transfer operation, 64 words will
be transferred. However, all 64 words need not contain data.
When transferring a moveset of less than 64 words, the unused
balance of the moveset or write block transfer file must be cleared or
voided. This can be done by loading zeros into all unused words or
by programming the skip bit in the single move control word of each
move to be skipped. The write block transfer file will contain data
from the previous transfer. If the next moveset to be transferred
contains fewer words of data than the previous transfer, the balance
of the file will not be over-written but will accompany the new data.
The stepper controller module, unable to distinguish between new
and previous data, will operate on the entire transfer.
!
WARNING: To avoid unpredictable machine
operation with possible damage to equipment and/or
injury to personnel, be sure that the unwanted balance
of transferred data is properly voided.
This can be done using only one of the following methods:
A. Loading zeros into all words remaining unused in the 64
word moveset file. In this case, zeros can be loaded into the
moveset files initially when move data is programmed.
For example, if only the first seven moves of a moveset are
to be used, the last three moves (16 words) can be loaded
with zeros. The zeros will over-write any data that could
remain in the write block transfer file from the transfer of a
previous moveset.