Rockwell Automation 1715-OF8I Redundant I/O System User Manual - Manual (Page 231)

Rockwell Automation Publication 1715-UM001C-EN-P - March 2014 231
Using SIL 2 Add-On Instructions with 1715 Redundant I/O Modules Chapter 7
Follow these steps to create the SIL 2 periodic task.
1. In the Controller Organizer, right-click the Tasks folder and choose New
Task.
2. Name the Task (SIL 2, for example).
3. Select the Type ‘Periodic’.
1715 SIL 2 Periodic Task ‘Period’ Configuration
The following are recommendations to configure the periodic task ‘period’ for
SIL 2 applications. Set the period to the minimum 1715 SIL 2 module RPI
divided by 2. For example, if the default RPIs are used.
1. Make the period 60/2 = 30 ms.
Table 46 - Module Default RPIs
1715 Module RPI
Adapter modules 180 ms
Digital modules 60 ms (lowest)
Analog modules 120 ms
IMPORTANT • Note that all 1715 SIL 2 Add-On Instructions have unique timer
presets set to the module RPI / 2, so each Add-On Instruction will
process packets at a rate based on its module RPI. In Table 46 , the
AENTR will only process a packet at 180/2=90 ms; every third task
period. The analog modules will process a packet at 120/2 = 60
ms; every other task period. The Add-On Instruction obtains the RPI
from the RPI input parameter on the Add-On Instruction, which is
why it is important to enter this value to match the actual module
RPI.
• Although the timer preset within the Add-On Instructions equals
RPI/2, the actual rate the Add-On Instructions processes packets is
affected by the periodic task rate and the periodic task scan time.
As these values get lower, the timer resolution improves and
packets are processed closer to the RPI/2 timer preset.
IMPORTANT Make sure that the SIL 2 task period allows enough time for the SIL 2
task, including the Add-On Instructions, to complete. Adjusting the
period time above the suggested value may be needed if a large
number of 1715 modules is used, as this adds additional Add-On
Instructions. Any increase to the safety task period impacts the safety
reaction time. See
Worst Case Reaction Time Calculations on page 241 .