Bradley Smoker PLC-5 user manual 10Maximizing System Performance, Maximum scan time

C-10Maximizing 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:

System Optimized for Discrete-Data Transfer

With this arrangement, only one block-transfer can occur to each BT module for every 3 discrete I/O scans.

PLC

Adapter	BT	BT	BT
Adapter
Adapter

Maximum scan time	= 3 discrete scans + 1 block-transfer
	= 3D + 1BT
Minimum time to complete
a block-transfer to all modules		= 3 (3D + 1BT)

= 9D + 3BT

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.

Publication 1785-UM012D-EN-P - July 2005