Rockwell Automation 1785 PLC-5 Manuel d'utilisateur Page 296

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Publication 1785-UM012D-EN-P - July 2005
C-10 Maximizing System Performance
However, if rack 2 has the most time-critical I/O, use the configurable scan list
to specify:
rack 1
rack 2
rack 3
rack 2
rack 4
rack 2
Using this scan list, rack 2 is scanned every other rack. The list has 6 entries,
so the normal I/O scan time is 6 x 10 ms = 60 ms. Since rack 2 is scanned
every other rack, however, the rack 2 effective scan time is 2 x 10 ms = 20 ms.
The remaining racks are scanned every 60 ms. Thus, the tradeoff for the more
frequent scanning of rack 2 (every 20 ms) means that the other racks are
scanned only every 60 ms.
You can also optimize block-transfers within the channel. You block-transfer
to only one block-transfer module per entry in the scan list per I/O scan. If
you have three block-transfer modules in one I/O rack, it takes a minimum of
three I/O scans to complete the block-transfers to all of the modules:
If you place the three block-transfer modules in different racks, however, you
can block-transfer to all three modules in one I/O scan.
PLC
Adapter
BT BT BT
Adapter
Adapter
With this arrangement, only one block-transfer can
occur to each BT module for every 3 discrete I/O scans.
Maximum scan time
Minimum time to complete
a block-transfer to all modules
= 3 discrete scans + 1 block-transfer
= 3D + 1BT
= 3 (3D + 1BT)
= 9D + 3BT
System Optimized for Discrete-Data Transfer
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