## 4 Evaluation of Serial Bus Network System
### Serial Communication Analysis with Adaptation Function  
For each sensor platform LSI, eight switches were used to set a specific physical address for configuration. Except for ID number, the configurations of all sensor platform LSIs were identical. The communication speed was set to 1 MHz. \"Data Wait Increment\", \"Data Wait Times\", \"Bus Wait Increment\" and \"Bus Wait Times\" were configured to 80, 16, 15 and 8, respectively. Threshold values of the event-driven function were set to zero. After sending configuration commands, 48 sensor platform LSIs operated simultaneously with the adaptation function. This is one of the worst cases in terms of data congestion on the bus because all sensor platform LSIs were triggered and tried to send packets simultaneously. We received data packets by the PC-host for 30 s every 5 min, and the data recording was continued for 110 min in total. CRC-16 was used to check the correctness of a received data packet. To illustrate the number of received correct data packets in 23 times of 110 min measurement sequences, a box whisker chart was drawn as shown in Figure 12. The whisker lower end, box bottom, band inside the box, box top and whisker upper end represent the minimum, first quartile, second quartile (median), third quartile and the maximum values of the numbers of correct data packets received in 23 timing slots, respectively. For all sensor platform LSIs, the number of received data packets per 30 s is between 1890 and 2315,  
Figure 11: Signal waveform sent by the middle sensor platform LSI and recorded near its bus pin: (**a**) rising edge and (**b**) falling edge under 110 \(\Omega\) termination; (**c**) rising edge and (**d**) falling edge under 10 k\(\Omega\) termination. For each sensor platform LSI, difference in the number of received data packets among different experiments is less than 12.76%. These results confirm that Carrier Sense Multiple Access in addition of Collision Avoidance (CSMA/CA) was working as designed. As shown in Figure 13, the error rates of received data packets are less than 0.42% in 23 timing slots, which shows the reliability of our network system. Note that the residual error rate of CRC-16 designed in this study is on the order of \(10^{-9}\), and negligible compared to the error rate of received data packets [24]. Figure 14 shows the number of received data packets in 30 s along with elapsed time for four sensor platform LSIs. There is no obvious changing trend of the number of received data packets along with elapsed time, suggesting that the network system was stable. To evaluate the sampling frequency of the network system, we extracted the time interval between two data packets received by the FPGA-based relay node successively from the same sensor platform LSI within 30 s. The time information was provided by the relay node, since the relay node attached a time stamp (in 0.1 us) to each received data packet. Figure 15a shows the time intervals of 48 sensor platform LSIs in 30 s. Figure 15b illustrates the time intervals of No. 8 sensor platform LSI along with elapsed time in 23 timing slots. Among the time intervals of each sensor platform LSI extracted in each data receiving, the median is around 10 ms, and more than 75% of the time intervals are less than 21 ms.  
Figure 12: Number of correct data packets received in 30 s from each sensor platform LSI. Figure 13: Error rate of the data packets received in 30 s from 48 sensor platform LSIs along with time. Among the time intervals of all sensor platform LSIs extracted in 23 timing slots (2,361,558 values in total), the first quartile, median, third quartile and mean values are 4.85 ms, 9.76 ms, 18.39 ms and 13.58 ms, respectively. The minimum value is 267.2 us, which means that there is a possibility that a sensor platform LSI sends two data packets successively. The maximum value is 189.1 ms which means that the worst sampling frequency is 5.3 Hz.