Allen-Bradley Powermonitor 3000 User Manual page 91

Allen-Bradley HMIs
Total Scan Time
Where:
R = Foreground to Background Poll Ratio
D = Interscan Delay
Change of State I/O messaging (COS) reports data only when the
content of the I/O table changes. COS messaging can be more
efficient for discrete applications because it tends to reduce the
network traffic. If you have configured the input message table to
include metering data, however, COS may reduce the network
efficiency because the data constantly changes.
Cyclic I/O messaging reports data periodically according to a time
increment you configure.
COS and Cyclic messaging typically reduce the network bandwidth
loading compared with Polled messaging. To optimize explicit
messaging performance, use a Background Polled I/O connection
with a high foreground to background poll ratio.
To help obtain optimal network operation, verify the following
settings using RSNetworx for DeviceNet, looking at the scanner
Properties dialog:
• For Polled I/O messaging, verify that the effective polling rate
(or scan time) is less than the expected packet rate (EPR) to
prevent time-out errors. You may find the EPR on the Module
tab, Advanced button.
• For COS or Cyclic I/O messaging, verify that the COS/Cyclic
Inhibit Time is less than the EPR and that the ACK time out is set
appropriately. You may find these parameters on the Scanlist
tab, Edit I/O Parameters... button.
Please contact Rockwell Automation technical support if you find that
the default settings do not result in adequate network performance.
Explicit Messaging
Use explicit messaging to read and write all data tables other than the
I/O messaging table. The specific details of explicit messaging depend
upon the master device that initiates the message. The example in this
section uses an Allen-Bradley SLC 500 controller and DeviceNet
Scanner (1747-SDN) as the master. Refer to the DeviceNet Scanner
Module Installation Instructions, publication 1747-5.8, for a detailed
description of explicit message programming in the SLC 500. Please
refer to the Rockwell Automation Knowledge Base for other examples
of explicit messaging to a Powermonitor 3000.
Communications
( ) D •
= 1 + R
Publication 1404-UM001D-EN-E - October 2004
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