Cross-Board Rules

The container holds ERC checks and DRC rule types that apply to cross-board nets. ERC checks run on the MBS document; DRC rules run on each sub-board's DRC pass.

ERC checks

Cross-board ERC runs in two places:

MBSCH ERC (most checks): run Inspect → Electrical Rules Checker (or Ctrl/Cmd+Shift+I) with the MBS open.
Sub-project ERC (one check): a single cross-board check fires during a normal sub-project schematic ERC pass.

Severity for each check is configurable in Schematic Setup → Violation Severity on a sub-project (the MBSCH's setup dialog is titled "Multi-Board Schematic Setup" and hides the Violation Severity panel). The severities apply project-wide.

Check	Runs in	Default	Triggers
Module pin's target pad is missing	MBSCH	Warning	A module pin points at a connector pad (component ref + pin number) that no longer exists on the sub-project PCB. Run Refresh from sub-projects to rescan.
Module pin not wired to a cross-board net	MBSCH	Error	A module pin has no wire attached or sits on no named net. Wire it to a pin on a different block.
Net touches only one module block	MBSCH	Warning	All pins on the net belong to the same block, so it's not actually cross-board. Wire to another block or remove the wiring.
Power pin not driven	MBSCH	Error	A power-input pin on a cross-board net has no power-output source. Catches `GND ↔ 5V` shorts and missing supply connections.
Pin-type conflict	MBSCH	Error or Warning	Two pins of incompatible electrical types are connected on a cross-board net (e.g. output ↔ output). Uses the standard ERC pin-type matrix.
Label shadows a cross-board net	MBSCH	Warning	A sub-project has a local PCB net with the same name as a cross-board net it doesn't participate in. The next sync would silently merge the two; rename one side first.
Connector pin unconnected	Sub-project SCH	Warning	A connector pin participates in a cross-board net per the container's topology, but is dangling on this board's local schematic. Wire it up locally.

DRC rules

The container holds five DRC rule types beyond standard sub-board DRC. Each rule fires during sub-project DRC, not container DRC. All five run together in the same DRC pass.

Set them from the container project's setup dialog. The Cross-Board Rules panel is a five-tab notebook with one grid per rule type. To validate cross-board rules from the agent without opening every sub-project, use pcb_cross_board action="validate".

Rule	What it checks	Violation code
Min Power Pins	Connector pin count for a named net is below the minimum on a sub-board	`DRCE_CROSS_BOARD_POWER_PINS`
Max Length	Total trace length of a cross-board net across all sub-boards exceeds the limit	`DRCE_CROSS_BOARD_LENGTH`
Differential Pairs	Pair members are out of sync (one isn't cross-board, or they touch different sub-projects)	`DRCE_CROSS_BOARD_DIFF_PAIR`
Current	Expected current exceeds pin count × per-pin rating	`DRCE_CROSS_BOARD_CURRENT`
Voltage Drop	Estimated DC drop across the cross-board net exceeds the maximum	`DRCE_CROSS_BOARD_VOLTAGE_DROP`

Configuring DRC rules

Each rule type has its own configurable fields.

Cross Board Rules

Min Power Pins

Counts unique connector pins on a sub-board that carry the named cross-board net. Fires when the count is below the minimum. Use this to enforce minimum pin redundancy on power and ground nets.

Field	Type	Notes
`net_name`	string	Cross-board net (e.g. `+5V`)
`min_pins`	int ≥ 1	Minimum number of pins

Max Length

Sums the total trace length of a cross-board net across every sub-board. The test lazy-loads each sibling sub-board's PCB to read its traces, so the rule counts contributions from every sub-project even if only one editor is open.

Field	Type	Notes
`net_name`	string	Cross-board net
`max_length`	nm in proto, mm in dialog	1 mm = 1e6 nm

Differential Pairs

Checks two invariants per declared pair:

Both pair members are cross-board nets. If the positive is cross-board but the negative is local-only on a sub-board, the pair is broken.
Both members touch the same set of sub-projects. If one ends at board A while the other continues to board B, the differential signal converts to single-ended at a connector.

Field	Type	Notes
`p`	string	Positive net name
`n`	string	Negative net name

Current

Multiplies the per-pin current rating by the number of connector pins on a sub-board to get safe carry capacity. Fires when expected_amps exceeds capacity.

Field	Type	Notes
`net_name`	string	Cross-board net
`expected_amps`	double (A)	DC current draw on the net
`pin_rating_amps`	double (A)	Safe carry per connector pin

Zero values skip the check silently.

Voltage Drop

Estimates DC voltage drop across a cross-board net with a lumped-resistance model:

R_trace   = (length_mm / width_mm) × sheet_R_mΩ_per_sq      (per sub-board, summed)
R_contact = pin_R_mΩ / number_of_parallel_pins              (per connector crossing)
drop_mV   = (R_trace + R_contact) × expected_amps

Fires when drop_mV exceeds max_drop_mv.

Field	Type	Default if 0
`net_name`	string	required
`expected_amps`	double (A)	required
`max_drop_mv`	double (mV)	required
`trace_width_um`	double (µm)	250
`trace_sheet_r_milliohm_per_sq`	double (mΩ/sq)	0.5
`contact_r_per_pin_milliohm`	double (mΩ)	20

The defaults model 1 oz copper with a standard pin header. Override for non-standard stackups.

From the agent and Python

Agent: mbs_setup with rules payload reads or mutates the container's rules.
Python: get_rules, set_rules.