Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 180)

Chapter 8
8Ć19
Figure 8.9
BlockĆtransfer Time Computation
Step 4 -
Program Scan:
Time
Scanner Time:
Time (read or write) = 512ms for channel 1 and 77ms for channel 3 (from results of sep 3)
Read/Write
Compute the 1771-M3 controller read/write block transfer time. Example values have been added.
Time (program)
= Program scan + 2[Scanner Timer]
(1771-M3 Controller
in Channel 1
= 50 + 2(512)
= 50 + 1024
= 1074ms
= 1.1 seconds
Time
(1771-M3 Controller
in Channel 3)
= Program scan + 2[Scanner Time]
= 204ms
= 50 + 2[77]
= 2.5ms per 1K words x 20K words
= 2.5 x 20
= 50ms
Reducing Scan Time
Due to the asychronous scan relationship between program scan and I/O
scan, 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 (as the 1771-M3 controller),
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.
Distribute your block transfer modules equally between all four scanner
channels.
Distribute block transfer instructions equally throughout your program.
Place an equal number of non-block-transfer rungs between
block-transfer rungs. Consider the last rung adjacent to the first.
For large numbers of block transfer instructions, distribute groups of
block transfer rungs equally throughout your program. Place no more
than four block transfer rungs consecutively in one group (one block
transfer instruction per rung). Within each group, condition the next
rung using the done bit of the previous block transfer instruction.