#8 Interface Control and Integration Planning

As systems engineers, we know that systems aren’t just built they’re assembled. And the seams where components or subsystems meet are the most common points of failure.

This step in the lifecycle focuses on making those seams explicit. It transforms high-level architecture and requirement allocation (see [Dispatch #7]) into concrete, testable, and owned interface definitions, backed by a strategy to integrate those elements predictably and safely.

Lifecycle Placement: This activity begins once architectural decomposition is complete and requirements are allocated. It feeds into Verification Planning and Integration Readiness activities.

1. What is an Interface?

An interface is a point of interaction between two separate system elements which could be hardware, software, humans, external systems, or the environment.

There are four classic types of interfaces in system engineering:

2. What Is Interface Control?

Interface Control is the discipline of defining, documenting, and governing the characteristics of an interface.

It ensures that:

• All parties involved in development understand what needs to connect, when, and how

• No assumptions are made about "how it should work"

• Interfaces can be tested independently

• Changes are traceable, reviewed, and managed

Without interface control, integration becomes an uncontrolled experiment.

3. Interface Control Document (ICD)

An ICD is the primary deliverable of this phase. It is the agreed specification for an interface between two entities.

Key Fields in a Professional ICD:

Minimum Maturity for Engineering: All critical ICDs should be defined at L2 maturity  protocol plus behavior before integration begins.

4. Integration Planning

With interfaces defined, we prepare for planned, staged integration not big-bang assembly.

Key Concepts:

• Integration Sequence Plan:

  Defines which subsystems get brought together, in what order, and what each milestone should verify.

• Interface Readiness Mapping:

  Confirms that both sides of each interface are ready, including hardware, firmware, and test tools.

• Integration Test Environment:

  Lab rigs, emulators, or virtual environments used to validate interfaces before full deployment.

5. Dependency and Ownership Management

Interface success depends on ownership clarity and inter-team coordination.

Use a RACI Matrix:

Example: The Mechanical Team is Responsible for lock housing dimensions. Software Team is Responsible for interpreting limit switch signals.

6. Interface Risk Identification

Interfaces carry failure modes mismatched expectations, bad timing, overloads, or unclear error recovery. These must be assessed early.

Use FMEA (Failure Mode and Effects Analysis) to:

• Identify likely failure points

• Assess severity and detectability

• Plan mitigations or redesigns

Risk Examples:

7. Deliverables from This Step

These deliverables become part of the Integration Readiness Review (IRR) pack.

8. Exit Criteria for Interface Control Phase

You’re ready to proceed to physical integration when:

• All ICDs are at L2 maturity (protocol + behavior)

• Each ICD has assigned ownership and test responsibility

• Integration sequencing is defined and agreed

• All interfaces have test access plans

• Interface risks are mitigated or under control

Locker System Example

• LTE Module ICD defines HTTPS endpoint, response codes, timeout policy

• Lock Solenoid ICD defines 12V ±5% power spec, actuation current, return spring feedback

• Integration plan sequences LTE→Controller→Solenoid in test rig before site deployment

Closing Reflection

Interface control isn't just documentation it's engineering discipline that turns architectural structure into testable, integrable, traceable subsystems.

Handled well, integration becomes an engineering checkpoint.Handled poorly, it becomes a minefield of rework.

Next : Verification and Validation Planning

We’ll shift focus from how things connect to how we prove they work:

• Writing verification methods

• Building the V&V matrix

• Mapping test plans to contractual needs

• Handling safety-critical test scenarios