Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 205)

ASCII Module
PLC-2 Family Processors
Appendix A
AĆ37
4. Compute the approximate read or write block transfer time for
channel 1 and channel 3 (Figure A.15).
Figure A.15
Computing Block Transfer for Each Channel
Step 4
Program Scan
Scanner Scan
Block Transfer Timer per Channel
Compute the read or write block transfer time. Example values have been added.
Time (program) = 2.5ms/K words x 20K words
= 2.5ms x 20
= 50ms
Time (read or write) = 231ms for channel 1 and
77ms for channel 3 (from step 3)
Channel 1 = Program Scan + 2[Scanner Scan]
50ms + 2[231ms]
50ms + 462ms
512ms
Channel 3 = Program Scan + 2[Scanner Scan]
50ms + 2[77ms]
50ms + 154ms
204ms
Reducing Scan Time
Due to the asychronous scan relationship between program and scanner,
and the serial operation of each channel in the scanner, we suggest that
you optimize the overall scan time. Although recommendations are
application dependent, we make the following recommendations as
general guidelines:
Whenever possible, control the manner in which block transfer
instructions are enabled. For example, if only a few block transfer
modules require frequent transfer of data, program them to run
continually. Inhibit block transfer instructions of those modules that
require less frequent transfer until enabled by a timer and/or some
application dependent condition.