Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 101)

Rockwell Automation Publication 1756-UM058G-EN-P - November 2012 101
Fast Module Features Chapter 5
In this type of application, the Minimum On Time, Extend Cycle, and Stagger
Output configuration options provide these benefits:
• Minimum On Time and Extend Cycle —Ensures that output devices that
require a minimum time to turn On or that cannot react to a short pulse
cycle can react with any given PWM On time calculation rather than not
turning On.
To ensure the output device turns On when the calculated On time is less
than the minimum On time, you must enable the Extend Cycle option.
When Extend Cycle is enabled, the cycle time is extended proportionately
up to 10 times the calculated On time while taking into account the
minimum On time.
If Extend Cycle is not enabled and the calculated On time is less than the
minimum On time, the output of the module will not energize.
• Stagger Output —Mitigates the power surge from outputs that drive high
power loads by preventing the outputs from turning On simultaneously.
Enabling the Stagger Output option for multiple output points addresses
surges by staggering the leading edge of those outputs (
Figure 18 ). When
the Stagger Output feature is not enabled, output points turn On
immediately at the start of the cycle (
Figure 19 ).
The stagger time for an output is calculated when the output turns On. If
the On time and cycle times are changed by large amounts while the
output is On, the stagger times may begin to overlap.
If the cumulative On time of staggered outputs is less than the cycle, each
new On transition is staggered to begin 50 μs after the prior staggered
output turns Off.
EXAMPLE A solenoid requires at least 40 ms to turn On. During configuration, you
enable the output for PWM, specify a minimum On time of 40 ms, and
enable the Extend Cycle option.
If the calculated On time in the PWMOnTime output tag drops below
the 40 ms minimum On time, the module automatically extends the
On time to 40 ms and proportionally extends the cycle time in the
PWMCycleTime output tag.
If the On time drops below 4 ms, the output will turn Off because the
cycle cannot extend beyond 10 times the 40 ms On time.