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142 lines
8.3 KiB
Markdown
142 lines
8.3 KiB
Markdown
# bacBus v4r1
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`bacBus` is a custom M.2-based Key-M interconnect standard used to connect subsystems within the **Build a CubeSat** (BAC) architecture.
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The system is designed to have two largely identical and independent buses: **bacBus A** on the Ym side and **bacBus B** on the Yp side. The buses can be used for redundancy or for independent capabilities, such as dedicating an entire bus to payload operations. This adds flexibility to the architecture.
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The currently available boards support a single bacBus only, with the exception of the EPS, which supports both A and B. Due to the symmetrical nature of the architecture, boards can be connected to either bus. Future iterations will include dual-bacBus capable boards.
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The BAC architecture supports a 5mm PCB pitch, hence the interconnect boards are made lengths of 12.5mm to 52.5mm with increments of 5mm:
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- 12.5mm
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- 17.5mm
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- 22.5mm
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- 27.5mm
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- 32.5mm
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- 37.5mm
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- 42.5mm
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- 47.5mm
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- 52.5mm
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Longer lengths are possible (feel free to open an [issue](https://codeberg.org/buildacubesat-project/bac-hardware/issues)). KiCad projects for the currently available lengths can be found in the [interconnect folder](https://codeberg.org/buildacubesat-project/bac-hardware/src/branch/main/bacbus/interconnect/), Gerber files are available in the [releases tab](https://codeberg.org/buildacubesat-project/bac-hardware/releases).
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To reach from the EPS to the first board on the Zm side, the 32.5mm long interconnect is adequate. Single-bacBus PCBs offer a passthrough on the unconnected side – the same can be implemented on custom boards or models. For PCB-to-PCB distances of less than 5mm, the use of common board-to-board connectors is recommended (pin headers, mezzanine connectors). Search the [structure repo release section](https://codeberg.org/buildacubesat-project/bac-structure/releases) for "PCB", "Mezzanine" and "Module Volume" for starting points.
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---
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## Key Changes in v4r1
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Version **v4r1** introduces a simplified and more robust system architecture with **CAN-based intra-satellite communication**, improved **power distribution**, and additional **system-wide control and safety signals**.
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### Redundant CAN architecture
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* All intra-sat communication now uses **dual redundant CAN buses**
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* Legacy buses (I2C, SPI, etc.) have been removed from the interconnect
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* CAN routing has been optimized for node placement and signal integrity
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### Improved power distribution
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* **VBAT is now exposed for power delivery**, not only voltage sensing
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* Improved routing for **power rails on the EPS**
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* Dedicated auxiliary rail support
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### System-wide control signals
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Several control and status signals were added to simplify system coordination:
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* `EPS_OK`: indicates EPS rails are enabled and nominal
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* `SAFE_MODE`: forces nodes into safe configuration
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* `3V3_AUX_EN`: enables auxiliary 3.3V rail
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* `HDRM_EN`: enables hold-down release mechanism deployment
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* `RF_EN`: enables RF subsystems
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* `PAYLOAD_EN`: enables payload subsystem
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### Timing synchronization
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* `SYNC_PULSE` provides a **system-wide timing reference** for timestamp synchronization.
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### Redundant deployment detection
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* A second deployment switch group `DEPLOY_SW_2` provides redundancy.
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---
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## Pinout
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### Table
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| Pin number | Pin name | Type | Bus domain | Notes |
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| --- | --- | --- | --- | --- |
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| 1 | <span style="background-color:#6f8fbf; padding:4px 6px; border-radius:4px;">CAN_1_H</span> | Differential signal | per bus | Primary CAN bus 1, high |
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| 2 | <span style="background-color:#6f8fbf; padding:4px 6px; border-radius:4px;">CAN_1_L</span> | Differential signal | per bus | Primary CAN bus 1, low |
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| 3, 4 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 5 | <span style="background-color:#6f8fbf; padding:4px 6px; border-radius:4px;">CAN_2_H</span> | Differential signal | per bus | Secondary CAN bus 2, high |
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| 6 | <span style="background-color:#6f8fbf; padding:4px 6px; border-radius:4px;">CAN_2_L</span> | Differential signal | per bus | Secondary CAN bus 2, low |
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| 7, 8 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 9 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">EPS_OK</span> | Flag / status | per bus | Main power rails status flag |
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| 10 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">SYNC_PULSE</span> | Control / timing | shared | System-wide timestamp sync |
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| 11 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">SAFE_MODE</span> | Flag / status | shared | System safe-mode control |
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| 12 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">3V3_AUX_EN</span> | Control / power | per bus | Auxiliary 3.3V enable |
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| 13 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">HDRM_EN</span> | Control / inhibit | shared | Hold-down release enable |
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| 14 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">RF_EN</span> | Control / inhibit | shared | RF subsystem enable |
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| 15 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">PAYLOAD_EN</span> | Control / payload | per bus | Payload power / enable control |
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| 16-23 | <span style="background-color:#59c959; padding:4px 6px; border-radius:4px;">GPIO 1-8</span> | GPIO | per bus | User-defined |
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| 24-29 | <span style="background-color:#c96b6b; padding:4px 6px; border-radius:4px;">3V3_MAIN</span> | Power | per bus | Main 3.3V rail |
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| 30-34 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 35 | <span style="background-color:#c96b6b; padding:4px 6px; border-radius:4px;">3V3_AUX</span> | Power | shared | Auxiliary 3.3V rail |
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| 36-37 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 38-41 | <span style="background-color:#c96b6b; padding:4px 6px; border-radius:4px;">VBAT</span> | Power | shared | Battery rail, exposed for power and sense |
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| 42-43 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 44 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">RBF</span> | Flag / inhibit | shared | High: Remove-before-flight pin removed |
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| 45 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">DEPLOY_SW_1</span> | Flag / inhibit | shared | High: Deployment switch group 1 open |
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| 46 | <span style="background-color:#bfa34a; padding:4px 6px; border-radius:4px;">DEPLOY_SW_2</span> | Flag / inhibit | shared | High: Deployment switch group 2 open |
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| 47-49 | <span style="background-color:#b0a7c9; padding:4px 6px; border-radius:4px;">RESERVED 1-3</span> | Reserved | per bus | Do not use |
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| 50-57 | <span style="background-color:#59c959; padding:4px 6px; border-radius:4px;">GPIO 9-16</span> | GPIO | per bus | User-defined |
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| 58-63 | <span style="background-color:#9e9e9e; padding:4px 6px; border-radius:4px;">GND</span> | Ground | shared | Return path |
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| 64-67 | <span style="background-color:#c96b6b; padding:4px 6px; border-radius:4px;">5V_MAIN</span> | Power | per bus | Main 5V rail |
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### Diagram
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---
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## Reference Files
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The following files are provided to simplify hardware design using bacBus.
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### Pinout definition
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- [bacBus-v4r1-pinout.pdf](./bacBus-v4r1-pinout.pdf)
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- [bacBus-v4r1-pinout.csv](./bacBus-v4r1-pinout.csv)
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Contains the full pin definition including signal descriptions and electrical constraints.
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### Pin-to-position lookup table
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- [bacBus-v4r1-pin-lut.csv](./bacBus-v4r1-pin-lut.csv)
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Provides mappings between:
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- bacBus pin numbers
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- connector pin number
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- schematic net names
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- hierarchical label names
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This simplifies schematic integration and automated symbol generation.
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### KiKit panel configuration
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- [interconnect-panel-4x4-kikit-conf.json](./interconnect-panel-4x4-kikit-conf.json)
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KiKit configuration for panelizing **bacBus interconnect boards**.
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---
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## KiCad Integration
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Updated **KiCad symbol and footprint presets** for bacBus will be provided soon.
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These will likely be released alongside the **KiCad 10** update. |