Provide 3 possible
implementations for Path Separation (Section 3.1.2.1.5)
Requirement
Path Separation (Section
3.1.2.1.5): Alternate or redundant functional paths shall
be separate or protected such that any single credible event which causes the loss
of one functional path will not result in the loss of the redundant functional
path.
Implementation Strategy
1. Physical
Path Separation:
·
Design Approach: Ensure
redundant functional paths are physically separated. For instance, cables,
buses, and hardware components must be routed along different physical pathways
to mitigate single-point failures from physical damage (e.g., fire, impact, or
radiation exposure).
·
Verification:
§ Conduct
engineering drawing inspections to confirm physical separation.
§ Perform
Failure Modes and Effects Analysis (FMEA) to validate that no single physical
event impacts multiple paths.
2. Logical
Independence:
·
Design Approach: Implement
software and logical diversity between redundant paths. Use independent
software modules or distinct logic sequences to reduce the likelihood of
common-mode failures caused by software bugs.
·
Verification:
§ Perform
Software Hazard Analysis (SHA) to confirm logic and functional independence.
§ Conduct
controlled fault injection tests to validate system responses.
3. Electromagnetic
Isolation:
·
Design Approach: Protect
redundant paths from electromagnetic interference (EMI) through shielding,
filtering, and ensuring adequate spacing between parallel signal lines.
·
Verification:
§ Use
EMI/EMC testing (Electromagnetic Compatibility) to certify immunity to
interference.
§ Inspect
engineering schematics for compliance with spacing and shielding requirements.
Tools
and Technologies
·
Design Tools: Computer-Aided
Design (CAD) software for mapping physical pathways and ensuring separation.
·
Analysis Tools: FMEA
and SHA tools such as ReliaSoft XFMEA or similar.
·
Testing Equipment: EMI/EMC
compliance testing setups, e.g., anechoic chambers and spectrum analyzers.
Challenges
1. Common
Mode Failures:
·
Mitigation: Employ
diversity in hardware (e.g., different manufacturers for redundant components)
and software (distinct programming languages or paradigms for logic paths).
2. Cost
and Complexity:
·
Mitigation: Prioritize
separation for critical and catastrophic hazards, focusing on components
directly tied to safety-critical functions.
References
·
SSP 50038 Rev B Section 3.1.2.1.5: Alternate
or redundant functional paths.
·
Verification Guidance: Engineering
drawings and test results outlined in Section 4.3.1.2.1.5.
Provide example Python code to
implement Logical Independence for this requirement
Below is an example of Python
code implementing logical independence for redundant control
paths in a computer-based control system (CBCS). The design uses two separate
logic modules for redundant paths, ensuring they process commands
independently. Each path uses different logic to avoid common-mode software
failures.
# Define redundant logic paths
def control_path_A(input_signal):
"""
Control Path A: Uses a simple state machine
approach.
"""
if input_signal
== "START":
return "Path A: Starting
process"
elif input_signal == "STOP":
return "Path A: Stopping
process"
else:
return "Path A: Invalid
command"
def control_path_B(input_signal):
"""
Control Path B: Uses a conditional
branching logic.
"""
response_map = {
"START": "Path B:
Starting process",
"STOP": "Path B:
Stopping process"
}
return response_map.get(input_signal, "Path B: Invalid command")
# Define an independent
decision arbiter
def decision_arbiter(input_signal):
"""
Combines and evaluates the outputs of both
paths.
Ensures logical independence by processing
each path's result separately.
"""
result_A = control_path_A(input_signal)
result_B = control_path_B(input_signal)
# Verify consistency between paths before
proceeding
if "Invalid" in result_A or "Invalid" in result_B:
return "Arbiter: Command rejected
due to invalid input."
if result_A == result_B:
return f"Arbiter:
Command executed successfully. [{result_A}]"
else:
return "Arbiter: Command conflict
detected between paths."
# Main function to demonstrate
logical independence
if __name__ ==
"__main__":
# Simulate commands
test_commands =
["START", "STOP", "INVALID"]
for cmd in test_commands:
print(f"\nProcessing command: {cmd}")
print(decision_arbiter(cmd))
Explanation
1.
Logical Diversity:
·
control_path_A:
Implements logic using a simple state machine.
·
control_path_B:
Implements logic using a dictionary-based approach.
2.
Decision Arbiter:
·
Independently evaluates the results from both
paths.
·
Validates the consistency of the outputs to
ensure fault tolerance.
·
Rejects commands if there's a mismatch or
invalid input.
3.
Testing:
·
The script demonstrates
handling START, STOP, and invalid commands.
·
Ensures redundancy and highlights the
importance of independent paths.
Use Case
This approach is applicable in
safety-critical CBCS systems where redundancy is required to ensure system
reliability. If one path fails or produces inconsistent results, the arbiter
can flag the discrepancy, preventing hazardous conditions.