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January 17, 2014

Rockwell Point I/O Safety Inputs Issue

Recently we came across a problem involving Point I/O safety inputs that I would like to share. 

The Issue:

We had 4 inputs wired into a Point I/O safety input card. 

The inputs are 2 dual channel EPS signals from an ABB robot, the signals are not using the cards test pulse they are considered a solid state output from the ABB (OSSD) similar to most light curtains.   
The card would read the signals correctly but we were having intermittent faults where one of the channels would drop out causing the software block (DCS) to fault.  You could reset the fault immediately and the cell would continue running normally.

The Solution:

We knew that the EPS signals were not a clean 24V signal but rather a square wave with a duty cycle of 2ms.

Our thought was to use the EPS signal to drive a relay and then use the input card’s test pulse to drive the inputs through the contacts of the relay thus eliminating the pulsed EPS signals from the inputs but retaining the same functionality.
After spending significant time on the phone with Rockwell Tech Support (thanks Tim) we came up with another solution that doesn’t require additional hardware of programming, filtering the inputs.

Essentially by setting the “Input Delay Time” – On to Off to 12ms the card will not consider the signal to be “off” unless the signal has been off for longer than 12ms.  This allowed us to ignore the 2ms duty cycle square wave and treat the signal like a clean 24V input.  As soon as this was implemented the intermittent faults went away.

If you run into this issue and need some help to implement or you would like further explanation please let me know, I would be happy to help!

 


Posted by:
Elvis Engineer at 2:02 PM
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