N2 Diagram

N2 Interface Diagram (systems-engineering-safety/mbse/n2-diagram)

Use when the task is interface analysis for a model-based systems engineering program: building the N2 interface matrix from the element list and interface pairs, counting interfaces per element, and reviewing the model for missing data links and isolated elements.

Domain quick reference

• An N2 diagram arranges the system functions or components along the matrix diagonal; every off-diagonal cell (row i, column j) records the data interfaces from element i to element j.
• Cell value: the number of interface pairs from the row element to the column element (1 for a single link, 2 for a duplicated pair, 0 for none).
• Interface count per element: row sum (outgoing) plus column sum (incoming); each interface is counted once per endpoint.
• Total interface entries: sum of every off-diagonal cell; equals the number of interface pairs when the pairs are unique.
• Missing data link: a required interface pair whose cell value is zero; the review lists each absent pair in the required order.
• Isolated element: an element with zero outgoing and zero incoming interfaces; the review flags it as unconnected.
• N2 interface analysis supports the interface requirements capture in the ARP4754A system development process: functions and their data interfaces are defined and reviewed before integration.

Workflow

1. Collect the ordered element list (functions or components) and the interface pair list (source, target).
2. Build the matrix with build_matrix; validate that every endpoint exists and no self interface is present.
3. Compute the per-element counts with interface_counts and the total with total_interfaces.
4. Compare the matrix against the required interface list with missing_links; list every absent pair.
5. Identify elements with no connections with isolated_elements.
6. Render the matrix with render_matrix and gate the interface requirements review on the missing link and isolation report.

Pitfalls

• Reversing the pair order: an interface from A to B sits in row A column B, not row B column A; the direction matters for the review.
• Double counting by design: the per-element count is the row plus column sum, so an element that sends k and receives m links has count k + m.
• Treating the diagonal as an interface cell: the diagonal holds the element itself, and a self interface pair is an input error, not a link.
• Forgetting the required list: missing_links only compares against the required pairs you provide; the function cannot invent requirements.
• Mixing decomposition levels in one matrix: the N2 diagram is a single-level view, so keep every element at the same level.
• Ignoring isolated elements: a zero-count element is a modeling gap, not a harmless empty row.

Behavior contract (gate 3)

The matrix build, count, and review logic is exercised by the gate 3 contract test: scripts/test_n2_diagram.py against scripts/n2_diagram_logic.py (stdlib unittest, offline). Run: python3 scripts/test_n2_diagram.py

Compliance

• Standards referenced, not reproduced: ARP4754A text is proprietary (SAE); summary-only per standards-map.yaml. The N2 chart method is common systems engineering methodology.
• compliance: STANDARDS-REF, gated: false.