Initial CAN Physical Layer spec

Architecture/README.md

@@ -125,7 +125,7 @@ Initial intended I/O categories are:
 | Analogue inputs | Air temperature, coolant/water temperature, pressure sensors, and expansion inputs |
 | Engine outputs | One injector and one ignition-coil primary channel |
 | Auxiliary outputs | Starter-enable logic output to an external high-current MOSFET or IGBT switch |
-| Communications | CAN bus with selectable termination |
+| Communications | One CAN FD vehicle bus with selectable ECU termination; see [CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md) |
 
 The starter-enable output only commands an external high-current switch. The
 ECU does not carry the starter's approximately 40 A current.
@@ -141,7 +141,9 @@ selection are finalised:
 3. Required quantity and exact types of spare analogue and digital I/O.
 4. ECU enclosure, board-size, mounting, environmental, and connector
    requirements.
-5. CAN connector and bus topology.
+5. CAN connector family/pin assignment, final cable construction, and
+   endpoint locations; the V1 CAN FD physical-layer basis is defined in
+   [CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md).
 6. Detailed trigger sensor wiring, connector, and cable-length information.
 
 ## Diagram sources