User manual

42 Publication 1756-RM093F-EN-P - January 2010
Chapter 5 Characteristics of Safety Tags, the Safety Task, and Safety Programs
SIL 2 Safety Applications
You can perform SIL 2 safety control by using the 1756 or 1768
GuardLogix controller’s safety task.
Because 1756 GuardLogix controllers are part of the ControlLogix
series of processors, you can perform SIL 2 safety control with a 1756
GuardLogix controller by using standard tasks or the safety task. This
capability provides unique and versatile safety control options, since
most applications have a higher percentage of SIL 2 safety functions
than SIL 3 safety functions.
SIL 2 Safety Control in the Safety Task
The 1756 and 1768 GuardLogix safety task can be used to provide SIL
2 as well as SIL 3 safety functions. If SIL 3 safety functions need to be
performed at the same time as SIL 2 safety functions, you must fulfill
the requirements defined in the SIL3 Safety – the Safety Task
, Safety
Programs, and Safety Routines sections of this chapter, as well as the
SIL 2 requirements listed in this section.
SIL 2 Safety Logic
From a GuardLogix safety control perspective, the biggest difference
between SIL 2 and SIL 3 safety-rated devices is that SIL 2 is generally
single-channel, while SIL 3 is typically dual-channel. When using
safety-rated I/O, which is required by the safety task, SIL 2 safety can
be single-channel, reducing system complexity.
Within the safety task, RSLogix 5000 software includes a set of
safety-related ladder-logic instructions. In addition to these
safety-rated ladder logic instructions, GuardLogix controllers feature
application-specific SIL 3-rated safety instructions. All of these logic
instructions may be used in Cat 1…4 and SIL 1…3 safety functions.
For SIL 2-only safety, a safety task signature is not required. However,
if any SIL 3 safety functions are used within the safety task, a safety
task signature is required.
IMPORTANT
If a combination of SIL 2 and SIL 3 safety functions are used at
the same time within the safety task, you must prevent SIL 2
input signals from directly controlling SIL 3 safety functions.
This can be done by using specific safety task programs or
routines to separate SIL 2 and SIL 3 safety functions.