## 4 Discussions  
The present paper studied the scalability of configurable electrical interfaces for two cases. In the first case, a 3U-size configurable backplane prototype was designed to support 13 mission payloads. The backplane contained four CPLDs that served as multiplexers, allowing the bus system to control all payloads with a limited number of digital interfaces. Several tests were conducted to verify its functionality and performance. The results showed that the backplane would only consume 28 mWh of energy per cycle in ISS orbit. This is considerably low and does not affect the overall power consumption of a CubeSat. A total transmission delay of up to 20 ns was measured, which is acceptable for serial communications such as UART and SPI. The short transmission delay also ensured data integrity after it was transmitted through the CPLDs. This was validated further in the bit error check, where the transmitted signal (of up to 4 Mbps) was compared to the received signal, and no bit errors were detected throughout the test.  
In the second case, a configurable backplane was used in a W6U satellite that carried out complex missions. The electrical bus system was modified as needed to meet the mission requirements. Additional power lines, integration of commercial ADCS and high-speed transceivers, and an autonomous bus system for the 1U SPATIUM-II section were among the modifications. According to the on-orbit results, no anomalies were detected on any of the 21 digital connections in the main bus or on any of the 17 digital connections in SPATIUM-II, which passed through the CPLDs. Furthermore, the power consumption of the satellite's main bus and SPATIUM-II sections in orbit was comparable to the ground results.  
The advantage of this study is that the scalable standard bus allows more time for integration and system-level verification, which is critical for a reliable and successful mission. The design concept could also benefit satellite developers who provide hosted payload services where bus resources are maximized to accommodate as many payloads as possible. UART and SPI communications were extensively used in the study since the bus system's command and data handling are based on these protocols. However, other protocols have not been supported by the configurable backplane. According to [PERSON] et al. [2], other protocols, such as I2C, CAN, USB, and Ethernet, can be used and areexpected to be used by CubeSat developers and vendors. A programmable backplane that supports these protocols should be developed in future work.  
[PERSON] et al. [14] recommended that the backplane is reconfigurable after the satellite has been fully assembled or even after the satellite has been launched into orbit. The former is easily implemented by inserting the JTAG pins into the umbilical. The 3U backplane prototype partially meets the latter recommendation. The preconfigured multiplexing function of the CPLDs in the backplane allows the selected payload(s) access to the bus system by sending ground commands to the satellite. Full on-orbit reconfiguration of the backplane CPLD as a contingency is still under investigation.  
[PERSON] et al. [24] stated that the flexibility of the BIRDS 1U electrical bus system is one of its key features. The BIRDS bus system was designed to be compatible with up to 3U platforms, with minimum modifications. The BIRDS bus command and data-handling architecture were retained in the 3U backplane prototype and the KITSUNE backplane. UART and SPI are the interfaces between the bus system and the payload. However, other protocols may be required by mission payloads. A bridge circuit capable of translating different protocols between the bus and payload is needed to keep the existing architecture. The bridge circuit can be implemented in the configurable backplane.