Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual User Manual

Page 137

Chapter 10

Maximizing System Performance

10-5

However, if rack 2 has the most time-critical I/O, use the configurable scan
list to specify:

Using this scan list, rack 2 is scanned every other rack. There are 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 60ms. 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:

PLC

Adapter

BT BT BT

Adapter

With this arrangement, there is only one block transfer

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 DiscreteĆData Transfer

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. To optimize
your system layout for block-data transfers, use an arrangement similar
to the following:

PLC

Adapter

With this arrangement, there is a block transfer

to each BT module every discrete I/O scan.

Maximum scan time

Minimum time to complete

a block transfer to all modules

= 3 discrete scans + 3 block transfers
= 3D + 3BT

= 1 * (3D + 3BT)

= 3D + 3BT

System Optimized for BlockĆData Transfer

rack 1

rack 2
rack 3

rack 4

rack 2