## 1 Introduction
UWE-3, a 1U CubeSat from the University of Wurzburg in Germany, first carried out the use of a backplane and became the reference for UNISEC Europe's CubeSat specification interface document (CSID) interface [9; 10]. The BIRDS satellite program at the Kyushu Institute of Technology (Kyutech) in Japan also adopted the backplane board approach to its 1U CubeSats as an interface to the bus and the payload [11]. The bus and payload boards have 50-pin male connectors connected to the 50-pin female connectors of the backplane board. These connectors have a smaller form factor, which provides more space for electronic components on the bus and payload boards. The spacing between boards can also be adjusted by moving the female connectors on the backplane board. This provides efficient utilization of the limited volume of a CubeSat. In addition, power lines, as well as analog and digital signals, are routed through the PCB. This greatly reduces the use of the wiring harness, which is considered one of the fundamental reasons for satellite failure. Lastly, the backplane approach makes satellite assembly and disassembly significantly more straightforward. Therefore, it allows integration and troubleshooting to be performed in a shorter time. A survey was conducted to determine the interface method used in CubeSats launched from 2003 to 2019 [12]. Of the 397 CubeSats surveyed, only 170 CubeSats had an identified interface since the sources of information (e.g., web pages, papers, conference papers, etc.) did not provide the details. A total of 137 CubeSats used the PC/104 interface, which is about 80% of the total identified CubeSats. However, the use of backplanes on CubeSats started gaining favor in 2013, with 24 satellites launched from then until 2019. For example,CalPoly's Aerospace Engineering Department developed its own kit as an educational platform for satellite development known as CalPoly CubeSat Kit MK1. Its internal configuration uses a backplane that can connect five boards through its 48-pin female connectors [13]. However, this backplane's design lacks flexibility because of changes in interface definition from one CubeSat project to the other. These changes lead to the complete reproduction of the backplane, adding cost and development time [14]. The third generation of the BIRDS project introduced a standard software-configurable backplane board as one of its technology demonstrations [14]. A complex programmable logic device (CPLD) was placed on the backplane, and digital signals between the bus and the payload were routed through the CPLD. By reprogramming the chip, rerouting can be performed without changes in the hardware design. This makes the backplane flexible and reusable by future satellites with minimal modification while saving time and cost. The configurable backplane was proven to work in space during satellite deployment at the International Space Station (ISS) in June 2019 until it was deorbited in October 2021. The software-configurable backplane has become an integral part of the BIRDS 1U bus architecture. The standard bus, however, has limited digital interfaces that constrain the number of payloads a CubeSat can carry out. The demand for launching CubeSats is on a continuous uptrend, and most CubeSats being launched are 3U and 6U platforms mainly because they can accommodate multiple and complex payloads. Implementing a configurable interface board on larger CubeSat platforms could provide benefits similar to those in the 1U platform, such as a reduction in development throughput and cost. This paper aims to demonstrate how the configurable interface board can be scaled up and adapted to different CubeSat sizes using the BIRDS 1U standard bus system. The novelty of the present work is described as a confirmation of the configurable interface board's flexibility in absorbing challenges encountered when scaling up to larger CubeSat platforms.