## 4 The SensoTube Architecture's Implementation Reference Model
### Expandability and Inter-Layer Services Mechanisms
#### 4.2.1 Inter-Layer Signals Management Service Mechanism  
According to the SensoTube architecture, signals management includes not just the physical connection among the various shields of the system, but also a mechanism for signal isolation and  
Figure 10: Top (**a**), bottom (**b**), and three-dimensional views (**c**), of the SensoTube PCB model with its physical expansion and stacking BEC-based mechanisms. other auxiliary signals connections alternatives. In the proposed PCB reference model two 1 \(\times\) 20-pins signals BECs, namely S-BEC 1 and S-BEC 2, have been used, as illustrated in Figure 11. The same figure also depicts the proposed types of signals that have been decided to be included in these BECs. As the colors denote, the signals have been conceptually grouped into four functional categories, namely the communication signals (green color), the digital and analog input and output signals (orange color), the signals for programming and debugging through the JTAG standard interfaces (blue color) [106], and the power supply signals (red color). The predefined positioning of the signals on the BECs ensures the standardization for the design of various new shields from conglomerate developers. Forty pins of different types can completely cover the requirements of any WSAN functional shield. Furthermore, the introduction of four exclusive pins to serve interrupt signals can significantly support the design of multi-processor applications, e.g., an MCU-enabled shield can wake up other shields from deep sleep mode, which is a technique, in order to reduce energy consumption. On the other hand, this provision can also support the adoption of challenging embedded systems design techniques, such as event-driven programming, and Synchronous Finite State Machines (SFSM) [142], which can contribute to energy optimization at the system level. Another novelty is the introduction of ten pins devoted to the power management of the expansion functional shields. In particular, there are five different voltage signals alternatives, two with predefined values, i.e., +3.3 Vdc and +5 Vdc, and three that can be defined by the developer, i.e., the V_IN, the V_BAT, and the V_AUX, respectively. Each one of these voltage input signals has its own ground pin, which is isolated from the rest of the ground pins. This is very useful for mixed-signal circuits design, because it allows the reduction of electric noise interference. Any connections between different ground signals can be implemented at the PCB of any functional shield. The voltage signals should derive from one, or multiple, shields of PML type. The two S-BEC signals pins are passing through all the connected functional shields. To overcome the aforementioned signals management constraints that exist in the expandable architectures (see Section 2), SensoTube S-BECs provide polymorphism in terms of the signals connections and routing. In particular, as Figure 12 depicts, two rows of through-hole pads have been added in parallel with the pads of S-BEC 1 and S-BEC 2. The pads of the internal rows are directly connected to the pads of the S-BECs, while the pads of the external rows can be connected to the signals of the shield's circuits. In this way, the signals coming from the S-BECs are mechanically disconnected from the signals of the shield. Figure 11: The SensoTube signals BECs (S-BECs) grouped into four functional categories. These pins are physically common among all of the functional expansion shields’ boards. As illustrated in Figure 13, by placing of dual male pin-headers at the available two rows of pads, and through the use of shorting jumpers, the signals of the shield can be selectively connected to the signals of the S-BECs. This option is very critical for the system's reconfigurability. Furthermore, in cases where the signals of the S-BECs must be remapped, with regard to the signals of the shield, then instead of the male pin-headers, the developers can make their own wiring at the two rows of pads.