LibreCube Board Specification
This page defines the normative specification of LibreCube boards. These boards are used in stacked configuration and typically embed system components or entire subsytems (for example, a power system) or larger sensors and actuators. All LibreCube board projects must follow this specification.
The reason for defining such a board specification is to enforce a consistent and modular design of LibreCube elements. Much of this specification is inspired by the typical design of CubeSat boards, which however were never formally defined. This is quite a paradox because the success of the CubeSat program is clearly due to its standardized form factor. The internal electronic boards were never specified by the CubeSat program, which has led to many incompatible CubeSat boards, even among vendors. This shows that it is extremely important to define a formal specification of such boards.
- The board shall have a rectangular shape with dimension of 95.89 mm x 90.17 mm and thickness of 1.57 mm (standard PCB FR4 thickness).
- The origin of the coordinate system shall be placed on the bottom left corner with X and Y axis in the directions as shown in Illustration 1, and Z axis in the direction to complete a right-handed coordinate system.
- The board shall have cutouts on its four edges as shown in Illustration 1. Specifically, the cutouts shall span the following rectangles:
- bottom: (36.07, 0) to (56.39, 2.92)
- top: (36.07, 92.97) to (56.39, 95.89)
- left: (0, 37.81) to (2.92, 58.08)
- right: (87.25, 37.81) to (90.17, 58.08)
- All corners of the board shall be rounded with a radius of 1.2 mm.
- Four mounting holes with hole diameter of 3.18 mm and a pad diameter of 6.35 mm shall be located at position (5.08, 5.08), (85.09, 5.08), (8.89, 90.81), and (82.55, 90.81).
- A total of 104 pin holes shall be provided, in a grid of 4×26 as shown in Illustration 1. Each hole shall be of diameter 40 mil (1.016 mm) and with a pad of 68 mil (1.7272 mm). The pitch (distance) between pin hole centers shall be 100 mil (2.54 mm). The most bottom left pin center shall be located at (13.97, 83.19).
- Spacing between boards is 15 mm.
- The preferred side to mount components is the top side (A).
- Components shall not exceed a height of 8.76 mm above the top side (A).
- Components shall not exceed a height of 4.83 mm below the bottom side (B).
- Connector: SAMTEC ESQ-126-39-[P]-D and ESQ-126-39-[P]-S or compatibles. The plating option [P] may be either gold or tin.
- Standoff: 15mm hex standoff aluminum, for example from here.
The naming of the headers (H1 and H2) and the pin numbering is shown in the Figure.
The pin definitions is shown the Table.
|1||CAN_A_L||Nominal CAN Bus (Low Signal)|
|3||CAN_A_H||Nominal CAN Bus (High Signal)|
|31-32||CHARGE||Battery charge input (5 Volt)|
|2||CAN_B_L||Redundant CAN Bus (Low Signal)|
|4||CAN_B_H||Redundant CAN Bus (High Signal)|
|25-26||5V||Main Bus Power|
|45-46||VBAT||Direct Battery Output|
CAN System Bus
CAN was chosen as the control and monitoring bus (also refered to as system bus). CAN is a reliable and robust bus that has many years of heritage in automotive and industrial applications and is also qualified for space use. The CAN bus is redundant and has a nominal and redundant bus line.
The system bus is composed of a single master (with node ID 0) and up to 127 slaves (with node ID 1 to 127). The node IDs are typically hard-coded in software and do not change during operation. Node IDs with lower value have higher priority in communication. That means, critical systems must be given lower IDs.
The master node can talk to slaves and the slaves can talk to the master. The slave DO NOT talk with each other.
The master node defines the bus to be considered active by periodic transmission of heartbeat messages on the active bus. The slave nodes monitor the presence of the heartbeat message from the master to determine the active bus.
serial or parallel
The master nodes can send a SYNC frame (CAN id = 0x080, no data) periodically (for example every 5 seconds). This can be used as the system reference pulse, upon which slave nodes synchronize their activities (for example, the collection of measurement data, sending of housekeeping data etc.).
Nodes use messages for exchange of commands (master to slave) and monitoring data (slave to master). A message can have a size of up to 4095 bytes. Messages of 7 bytes or less are sent in a single CAN frame. Larger messages are segmented into smaller chunks by the sending side and reassembled at the receiving side. For this, the ISO-TP protocol is used.