Bradley Smoker PLC-5 Calculating Worst-Case Remote I/O Scan Time, Optimizing Remote I/O Scan Time

Maximizing System Performance C-9

Calculating Worst-Case Remote I/O Scan Time

Since it is impossible to predict within which remote I/O scan a block-transfer will occur, you only can calculate the worst-case remote I/O scan time. To calculate the worst case time:

1.Determine the normal I/O time (without block-transfers)

2.Add the time of the longest block-transfer to each entry in the scan list. (The controller can only perform one block-transfer per entry in the scan list per I/O scan.)

For example, if your system is:

PLC

115.2 kbps

Rack 1

BT BT

1020

words words

Rack 2

No BTs

Rack 3

BT 30 words

Worst-case I/O scan:
	(3 x 6)	3 racks at 115.2 kbps*normal I/O scan
	+ (20 x .14) + 2.5	longest BT in rack 1
+ 0		no BTs in rack 2
	+ (30 x .14) + 2.5	longest BT in rack 3

18 + 5.3 + 0 + 6.7 = 30 ms

Optimizing Remote I/O Scan Time

The best way to optimize your scan time is to place your most time-critical I/O on a separate channel from non-critical I/O. If you have only one channel available for I/O, however, you can still optimize the scanning by using the controller’s configurable scan list.

In a normal 4-rack system, the scan list would be:

rack 1 rack 2 rack 3 rack 4

If you are using 57.6 kbps, the normal I/O scan is 4 racks x 10 ms = 40 ms. Each entry is of equal priority, so each rack is scanned every 40 ms.

Publication 1785-UM012D-EN-P - July 2005