define keying std and add conn part numbers

backplane/documentation/bom/bom.md

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+# Backplane BOM
+
+## Document Control
+
+- Version: 1
+- Author: Ethan Eggert
+- Approver:
+- Last Revised Date: 07/11/2026
+- Approval Status: Unapproved
+
+#  BOM
+
+| Part Number | Description | Distributor URL | Notes |
+| --------------- | --------------- | --------------- | --------------- |
+| SFM-120-01-L-D-A-TR | Main board connector, backplane side | https://www.samtec.com/products/sfm | 40 position, 2 row, THT, Alignment Pin, Polarization Notch |
+| TFM-120-01-L-D-RA | Main board connector, board side | https://www.digikey.com/en/products/detail/samtec-inc/TFM-120-01-L-D-RA/6678882 | 40 position, 2 row, THT, Right Angle, Polarization Notch |
+

backplane/documentation/spec/keying_standard.md

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+# Backplane Keying Scheme
+
+## Document Control
+
+- Version: 1
+- Author: Ethan Eggert
+- Approver:
+- Last Revised Date: 07/11/2026
+- Approval Status: Unapproved
+
+## Purpose
+
+This document defines how the backplane prevents a board from being inserted incorrectly (reversed, rotated, or into the wrong slot) and is a supporting design doc for the keying/orientation deliverable already required in the Backplane Subsystem Info doc and the ICD template.
+
+## Problem Statement
+
+Because every subsystem board uses the same connector standard (per the connector trade study), a board that shares a connector footprint with another slot's board is not prevented from being physically mated into the wrong place. There are two distinct failure modes to prevent:
+
+1. **Reversed/rotated insertion:** a board flipped 180°, or otherwise mated backwards relative to its own correct slot. Given the shared power lines running through each connector, this is the higher-severity case: a reversed board can put power directly onto a ground or signal pin, risking damage to that board and potentially others sharing the same backplane rail.
+
+2. **Wrong-slot insertion:** a board that mates fine in its own orientation, but into a slot intended for a different subsystem. Lower risk of immediate electrical damage if all slots share an identical pin mapping, but still a risk if any slot carries a subsystem-specific signal, and a risk to mission success even if nothing shorts (e.g., the OBC not finding the board it expects at boot).
+
+## Layered Approach
+
+No single mechanism should be relied on alone. Mechanical prevention first with visual and electronic checks as backups.
+
+### Layer 1: Asymmetric connector placement (prevents reversed/rotated insertion)
+
+Place each board's backplane-facing connector(s) at a position that is not symmetric about the board's centerline or rotation axis. A board that's flipped or rotated then physically cannot align its connector(s) with the backplane's receptacles.
+
+### Layer 2: Slot-specific guide pins (prevents wrong-slot insertion)
+
+Add a mechanical guide pin and matching hole pattern that is unique per board type/slot. For example, a guide pin at a specific position only matches the hole pattern on the board intended for that slot. A board can partially align its connector without the guide pin engaging, but cannot fully seat and reach proper connector mating depth without it.
+
+- **Action item:** define the guide pin position/diameter table per slot as part of the pinout ICD.
+
+### Layer 3: Visual labeling (aid for integration)
+
+Silkscreen each board with its intended slot name/number, and label the backplane itself at each slot to match. This exists purely to help someone doing integration catch a mistake before attempting to seat a mis-mated board.
+
+### Layer 4: Electronic slot-ID check
+
+Reserve a small number of spare pins (already budgeted in the pin-budget methodology) as hard-wired slot-ID lines. For example, a unique ground/no-connect pattern per slot that a board can read on power-up. On boot, each board confirms it's reading the slot ID it expects before proceeding past a minimal safe power-up state.
+
+## Open Items
+
+- Define the specific guide pin position/diameter table per slot, and add it to the pinout ICD
+- Confirm the number of spare pins available for the electronic slot-ID scheme against the existing pin-budget worksheet
+- Decide the exact slot-ID encoding (e.g., grounded pin pattern vs. a small resistor-divider ID vs. a single-wire ID read over an ADC pin)
+
+## References
+
+- Backplane Subsystem Info doc: keying/orientation deliverable, pin-budget methodology
+- Backplane Connector Trade Study: connector standard selection (Samtec SFM/SFH)