KB 0033
Network is working properly but I am seeing delays in on-demand reads and writes

CATEGORY

ASComm.NET

 

SYMPTOMS

On a properly functioning network, you are seeing significant delays in your on-demand reads and writes.

 

CAUSE

Scheduled reads (Group reads via Group threads or Device reads using your own threads) are overwhelming the controller by reading to agressively.

Rockwell Automation processors use overhead time, or background task time, to perform overhead tasks such as responding to a polled communications message request.

The overhead part of the scan is executed after inputs are read, ladder code is executed, and outputs are written.

 

SOLUTION

First, ensure that you are requesting scheduled reads at the appropriate rate:

• If you are using Group reads, ensure that Group.UpdateRate property is set to the rate that you need the data updated.
• If you are using Device reads within your own thread, ensure that your thread is requesting data at the rate that you need the data updated.
• Setting either of these too agressively can cause transactions to queue up and delay processing of on-demand reads and writes

Next, it may be necessary to adjust the controller's System Overhead Timeslice

The A-B tech note G54059311 outlines how to increase the time or bandwidth in the processor available for communications.

1. Visit the Rockwell Automation Support Center website
2. Select 'Get Support Now'
3. Click the 'Knowledgebase' link
4. Login to the Knowledgebase
5. Query for Knowledgebase article ID 25915 (article titled 'Setting the Overhead Time Slice or Increasing Processor Bandwidth for Communications')
6. Follow the instructions for your processor type.