Optimizing Application Response Time

Overview

You can reduce the maximum application response time (ART) for your system, by employing these network design tips:

use only the minimally required number of RIO drops (BM•CRA312•0 X80 EIO adapter modules)
use only the minimally required number of RIO input and output modules
place the RIO drops with the fastest communications capacity nearest to the local rack containing the CPU with Ethernet I/O scanner service

In addition, you can further reduce ART by using the FAST task in your Control Expert logic.

NOTE: In an M580 Hot Standbysystem, plan your topology accordingly so the amount of data exchanged is reduced.

Reducing the Number of RIO Drops

When you reduce the number of RIO drops in your system, you also reduce:

the number of hops that a packet passes through from an RIO drop to the CPU with Ethernet I/O scanner service in the local rack
the number of packets received by the CPU with Ethernet I/O scanner service

By reducing these values, you also reduce the following elements of ART:

network input/output times
network input/output jitter
CPU with Ethernet I/O scanner service
CPU scan time (the greatest savings)

Reducing the Number of Remote Input and Output Modules

When you reduce the number of RIO input and output modules, you also reduce the size of the packet, which in turn reduces the following elements of ART:

network input/output time
network input/output jitter
BM•CRA312•0 module drop process time

Placing the Fastest RIO Drops Nearest to the Local Rack

When you place the fastest RIO drops nearest to the local rack, you reduce the number of hops that a packet passes through from the RIO drop to the local rack. You also reduce the following elements of ART:

network input/output time
network input/output jitter

Using the FAST Task to Optimize ART

Using the FAST task can result in smaller ART because the I/O data associated with the FAST task can be executed at a higher priority. ART when using FAST task is not degraded because of the task’s priority.

NOTE: These efficiencies of the FAST task are not realized during end-of-scan delays.

	Scan Type	Period (ms) / Default Value	Watchdog (ms) / Default Value	Usage (I/O)
MAST1	cyclic2 or periodic	1...255 / 20	10...1500 by 10 / 250	local and remote racks
FAST	periodic	1...255 / 5	10...500 by 10 / 100	local and remote racks3
AUX05	periodic	10...2550 by 10 / 100	100...5000 by 100 / 2000	local and remote racks3
AUX15	periodic	10...2550 by 10 / 200	100...5000 by 100 / 2000	local and remote racks3
I/O Event5	event (128 maximum devices from 0 to 127)			local rack4
1 The MAST task is mandatory. 2 When set to cyclic mode, the minimum cycle time is 4 ms if there is an RIO network and 1 ms if there is no RIO network in the system. 3 FAST and AUX tasks are supported for the BM•CRA31210 X80 EIO adapter modules only. 4 DDDT syntax is not supported in the I/O event task. 5 Not supported by Hot Standby systems.

The Control Expert help pages further describe the multiple tasks.