One of our community members recently asked what would cause his 5380 analog inputs updating so slow?

His application required that each 5380 analog inputs updated every 10ms, but what he was seeing in the field was a much slower.

When working with analog inputs and Logix PACs, two settings will effect the update rate:

1) RPI (Requested Packet Interval)

RPI is how often you'd like the controller to read the values from the Input modules.

It is named “Requested” because it's a target the controller uses, but guaranteed update rate.

Because of this, Rockwell suggestions setting this rate at twice as fast as you actually need the data.

So in our Member's case, to insure his Analog Inputs update no slower than 10ms, he should set it to a 5ms RPI.

2) Filters

Many PLC Input Modules have a built-in feature which allows the user to “Filter” the input signals.

A side effect of these filters is that the input which is being filtered updates at a slower rate.

In the case of the 5069-IF, the manual has a table which helps users understand how the RPI and Notch filter interact:

Table Eleven in the above manual shows the Notch Rate filter on the left, and then the suggested RPI on the right.

In this member's case, I pointed out that with the 5000 Hz Notch Filter for all channels, the table recommends a 4ms RPI for faster updates.

In the field he actually used slightly faster settings, and was very happy with updates coming in reliably at under 4ms.

If you have a related question or comment please post it as a reply to this post, or if you have a different question for our community, please post it as a new topic.

Sincerely,

Shawn Tierney
Automation Instructor and Blogger

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