Rockwell Automation 1771-DCM USER MANUAL 1771-DCM - Manual (Page 45)

Calculating Transfer Time
Chapter 7
7Ć11
4. Compute the approximate read or write block transfer time for
Channel 1 and Channel 3 (Figure 7.4).
Figure 7.4
Computing Block Transfer for Each Channel
STEP 4: Compute the read or write block transfer time.
Example values have been added.
Program Scan Time (Program)
= 2.5 ms/K Words x 20K Words
Scanner Scan Time (Read or Write) =
2.5 ms x 20 =
= 50 ms
237 ms for Channel 1 and 79 ms
for Channel 3 (from Step 3)
Block Transfer
Time Per Channel
Channel 1 =
= Channel 3
Program Scan + 2 [Scanner Scan]
50 ms + 2 [ 237 ms ]
50 ms + 474 ms
524 ms
Program Scan + 2 [Scanner Scan]
50 ms + 2 [ 79 ms ]
50 ms + 158 ms
208 ms
Reducing PLCĆ3 Scan Time
Due to the asynchronous 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.
Program the read and write block transfer instructions of your
1771-DCM in the same rung (Figure 6.5).
Distribute your block transfer modules equally between all four scanner
channels.