## 2 BIRDS 1U Standard Bus System
This backplane connects all internal and external boards electrically and mechanically. Figure 2 shows the top side of a backplane board. The internal boards are connected to the backplane via 50-pin female connectors with a 2 mm pitch (C101 to C106). The pins are assigned as power lines, digital lines (for programming, debugging, and data communication), and analog lines. The connector has two pins assigned to each power line and the system ground. The same pins are set to all the connectors. Since the maximum current for each pin is rated as 1 A, the power line current is limited to 2 A. The remaining pins of the connectors are for digital lines and analog lines. The antenna panel and four solar panels are connected to the backplane via 12-pin male connectors with a 2.54 mm pitch (SP1 to SP5). The fifth solar panel is connected directly to the FAB. The connectors in the solar panels route the generated power and temperature readings to the FAB, and the connector on the antenna panel provides an unregulated power line to the antenna deployment system. In addition, the backplane also includes two-pin male connectors (SW1 to SW4) for the deployment switches connected to the satellite structure. The space between the COM (C103) and RAB (C104) connectors is allocated for the mission payload. The maximum board thickness of the payload that can be accommodated is 22.35 mm. Up to two mission boards (C104, C105) can be placed in the given space. It is possible to customize the backplane board to reduce the number of 50-pin female connectors to one and shift its position. A satellite ICD contains the overall information on the mechanical and electrical interfaces between the components. The connector pin assignment is an example of an electrical interface specification found in this document, and the satellite developer can use the pin assignment to determine how components are connected. Table 1 shows the pin assignment of the 50-pin connector on the OBC/EPS board. In addition, it specifies the name of each pin, the connected microcontroller, the pin route, the rated voltage and current, and the protocol and baud rate for digital pins. The pins are named according to  
Figure 2: Backplane board (top side). their function or connection. For example, PROG_GIO_1 denotes the first general I/O pin for programming. Another example is OBC-COM_1, which denotes the first pin connecting the OBC/EPS board and COM board. Pins 9-12, 21, 22, 27, 28, 37, 38, and 42 are linked to the main PIC and routed to the mission board. These 11 pins serve as digital interfaces for the bus and the payload, whereas pins 31-34 are serial peripheral interface (SPI) connections to the flash memory (FM) that the main PIC and mission payload share. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Pin No.** & **Pin Name** & **MCU** & **Destination** & **Voltage** & **Current** & **Baud Rate** & **Protocol** \\ \hline  
PLD\_18 & Main & Mission & 3.3 V & \textless{}30 mA & - & DIO \\
43 & OBC-COM\_3 & COM & No connection & 3.3 V & \textless{}30 mA & - & DIO \\
44 & OBC-COM\_4 & COM & COM board & 3.3 V & \textless{}30 mA & - & DIO \\
45 & OBC-COM\_5 & COM & COM board & 3.3 V & \textless{}30 mA & - & DIO \\
46 & OBC-COM\_6 & COM & COM board & 3.3 V & \textless{}30 mA & - & DIO \\
47 & OBC-COM\_7 & COM & COM board & Analog & \textless{}30 mA & - & Analog \\
48 & OBC-COM\_8 & COM & COM board & Analog & \textless{}30 mA & - & Analog \\
49 & SUP\_3 V3\_1 & - & All boards & 3.3 V & 1 A & - & Power \\
50 & SUP\_3 V3\_1 & - & All boards & 3.3 V & 1 A & - & Power \\ \hline \end{tabular}
\end{table} 
Table 1: OBC/EPS board pin assignment. Figure 3 shows the bottom side of the backplane, where all active devices are placed. This is to avoid interference with the components at the top. One of the active components is a CPLD that can be programmed to perform specific logical functions. The CPLD is a lattice semiconductor ispMACH LC4256 ZE, an ultralow power device that uses 1.8 V LVCMOS (low-voltage CMOS) technology with a standby current of 13 uA.