Rockwell Automation Allen-Bradley 1715-AENTR - Page 229

Rockwell Automation Publication 1715-UM001J-EN-P - December 2020 229
Chapter 8 Using SIL 2 Add-On Instructions with 1715 Redundant I/O Modules
2. Set the priority to 1.
3. Set the Task Watchdog based on the following.
a. How long it takes to run the SIL 2 code (see AOI Scan Times on page
235
).
b. How small the task watchdog must be to help verify that safety
reaction times are met (see Safety Reaction Time Calculations on page
236
).
If you do not have the information that is required to complete the watchdog
time, leave the default of 500 ms and adjust it later when you have more
information.
4. Click OK.
IMPORTANT
• All 1715 SIL 2 Add-On Instructions have unique timer presets
set to the module RPI / 2, so each Add-On Instruction
processes packets at a rate that is based on its module RPI.
In Table 45
, the AENTR only processes a packet at 180/2=90
ms; every third task period. The analog modules 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 periodic task rate affects the actual rate
the Add-On Instructions process packets 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 can be needed if many 1715 modules are used, as this
adds more Add-On Instructions. Any increase to the safety
task period impacts the safety reaction time. See Worst
Case Reaction Time Calculations on page 237.