Rockwell Automation 57C570 AutoMax PC3000 User Manual - Manual (Page 73)

4Ć15
4.4.3 Calculating the WorstĆCase Remote I/O Scan Time
Since it is impossible to predict within which remote I/O scan a
blockĆtransfer will occur, you only can calculate the worstĆcase
remote I/O scan time. To calculate the worst case time:
D Determine the normal I/O time (without blockĆtransfers)
D Add the time of the longest blockĆtransfer to each entry in the
scan list. (The PC3000 can only perform one blockĆtransfer per
entry in the scan list per I/O scan.)
For example:
BT
10
words
115.2 kbps
Rack 1
Rack 2
Rack 3
No BTs
BT
30
words
BT
20
words
WorstĆcase I/O scan:
(3 x 7)
+ (20 x .14) + 2.5
+ 0
+ (30 x .14) + 2.5
3 racks at 115.2 kbpsĊ
normal I/O scan
longest BT in rack 1
no BTs in rack 2
longest BT in rack 3
21 + 5.3 + 0 + 6.7 = 30 ms
AĆB RIO
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4.5 Optimizing System Performance
You can also optimize blockĆtransfer performance. You can
blockĆtransfer to only one blockĆtransfer module per entry in the
scan list per I/O scan. If you have three blockĆtransfer modules in
one I/O rack, it takes a minimum of three I/O scans to complete the
blockĆtransfers to all of the modules:
Adapter
BT BT
Adapter
Adapter
With this arrangement, only one blockĆtransfer can
occur to each BT module for every 3 discrete I/O scans.
Maximum scan time
Minimum time to complete
a blockĆtransfer to all modules
= 3 discrete scans + 1 blockĆtransfer
= 3D + 1BT
= 3 (3D + 1BT)
= 9D + 3BT
AĆB RIO
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BT
Figure 4.8 Ć System Optimized for DiscreteĆData Transfer