Chapter 164: Shielding Lines and Heartbeats

At 5:40 a.m., the alarm never went off. Lin Chen woke on his own. His left foot hung over the edge of the bed; the numbness had faded into a dull, heavy swelling, as if half a sack of wet sand had been poured into it. He slowly sat up and lowered it to the floor, testing his weight on it. Pain crawled up from the arch of his foot, but it was still within a controllable threshold. He twisted on the desk lamp and dug last night's shopping list out of the drawer: two meters of shielded twisted-pair cable, one pack each of Φ3 and Φ5 heat-shrink tubing, ten female-to-female Dupont wires, one backup USB-to-TTL module. In red pen at the bottom, it said: total budget not to exceed 45. He opened his expense notebook. Balance: 35.3 yuan. The number looked light, but it carried weight on the page.

At 6:30, the dormitory building had only just opened. He slung his canvas bag over his shoulder and headed downstairs, walking with a slight limp, his weight almost entirely on his right leg. The early autumn morning wind carried a chill that cut straight through his thin, washed-out shirt. The bus to the electronics market required one transfer. He stood at the stop waiting, clutching loose change in his hand. The bus was packed with vegetable vendors heading to the morning market and salesmen rushing off to work. Sweat, scallions, garlic, and cheap perfume blended into one dense smell. Lin Chen leaned against the handrail by the door, closed his eyes, and ran through the soldering sequence in his head: strip the wire, tin it, slide on the heat-shrink tubing, shape it with the hot air gun, and finally use the multimeter to test continuity. The order couldn't be mixed up. If it was, he'd have to redo it. Redoing it would waste time, and wasting time meant his foot wouldn't hold out. He evened out his breathing and treated the pain like a background parameter—neither resisting it nor amplifying it.

At 7:40, he arrived at South City Electronics Center. The rolling shutters had only been pulled halfway up, and the shop owners were carrying cardboard boxes inside. The air smelled of rosin, dust, and damp old cartons. Lin Chen turned down the second-floor accessories section with the ease of habit. At the first stall, he crouched and pointed at the shelf. “Boss, how much for RVVP 2×0.5? Two meters.”

“Eight yuan a meter retail. It's cheaper if you buy the whole roll.” The owner didn't even look up; he was wrapping a pile of resistors in newspaper.

“Just cut me two meters,” Lin Chen said quietly. “One pack each of Φ3 and Φ5 heat-shrink tubing, ten female-to-female Dupont wires, and a TTL module with a CH340 chip. Don't give me a PL2303—its driver isn't compatible.”

The owner finally looked up and sized him up. Seeing he was a student, his tone softened. “Student project? Fine, I'll give you a real price. Cable, fifteen. Tubing, six. Wires, eight. Module, twelve. Forty-one total.”

“Thirty-eight. Cash.” Lin Chen pulled a few crumpled bills from his bag and counted them out one by one before handing them over.

The owner clicked his tongue, took the money, and briskly cut the cable and bagged everything. Lin Chen tucked the items into the inner compartment of his canvas bag and zipped it shut. Bargaining forty-one down to thirty-eight saved him three yuan—enough for two meat buns. Every account had to be calculated down to the last cent if he wanted the road to stretch to the end. He turned and headed downstairs. The voice-activated light in the stairwell was broken, so he felt his way down along the handrail, his steps steady.

At 9:15, he was back in the dormitory. His roommates had already gone to class, leaving him alone in the room. He cleared the desk, laid out an anti-static mat, and plugged in the soldering iron. The smell of rosin and solder spread through the air—familiar, steadying. He started with the sensor interface. The pins on the salvaged secondhand component were badly oxidized, so he gently scraped off the surface layer with a small knife, dabbed on flux, and set the soldering iron tip to 350 degrees. The instant the solder wire touched, white smoke rose, and the joint came out full and smooth. He slid on the heat-shrink tubing and swept a lighter's outer flame across it; the tubing contracted tightly around the connection. He wasn't fast, but every step landed on rhythm. His left foot trembled slightly under the desk, but he ignored it and kept all his attention on the millimeter-scale movement at the tip of the tweezers. Sweat slid down from his temples. He wiped it away with the back of his hand and stripped the next wire.

By 11:00, the hardware assembly was complete. The Raspberry Pi 4B was fixed to an acrylic board, the ADC expansion board connected through pin headers, and three shielded cables led to the temperature-and-humidity sensor, vibration sensor, and current sensor respectively. He connected the power. The screen lit up. System startup logs popped into the terminal window. He typed sudo python3 v3_main.py and hit Enter.

The data stream began to scroll. The heartbeat packets held steady at 2.1-second intervals. The log window displayed: [INFO] Sensor init OK. [INFO] Data pipeline active.

But in the third minute, the vibration sensor readings began to drift. The standard deviation jumped from 0.02 to 0.15. Lin Chen frowned. He unplugged the USB cable and rerouted it with the shielded wire, bundling the signal line separately from the power line. He ran it again. The drift was still there, but smaller. He picked up the multimeter and tested the ground terminal. Impedance was too high. The electromagnetic interference in the workshop environment was strong; software filtering alone wouldn't be enough. He had to do hardware isolation. He dug out a spare roll of copper foil tape, applied it between the Raspberry Pi baseboard and the expansion board, and grounded it again. On the third run, the readings stabilized within 0.03. In his mistake notebook, he wrote:

10:47 Vibration signal drift. Cause: common-mode interference + poor grounding. Countermeasure: copper foil shielding + independent ground wire. Verification passed.

His handwriting was neat, without emotion—only facts. He knew that no matter how beautifully code was written, once it landed in the mud, it first had to survive electromagnetic interference.

At 1:00 p.m., he soaked a bowl of instant noodles and added a braised egg. He ate slowly, chewing thoroughly. Only once there was food in his stomach did his body's feedback become clear again. The heavy swelling in his left foot began spreading up into his calf, and the muscles showed faint signs of cramping. He stood up, braced himself against the wall, and slowly walked two laps, stretching his gastrocnemius. The pain sharpened for an instant, then eased. He returned to the desk and opened his PPT. On the appendix page, the hardware modification diagram had already been replaced with actual photographs, annotated with the routing of the shielding layer and the grounding points. He changed “theoretical anti-interference capability” to “measured signal-to-noise ratio improved by 18 dB,” and changed “software filtering” to “hardware isolation + moving average algorithm.” The language continued to be stripped of decoration until only verifiable parameters remained. He checked the timing of the animations and reduced them all to the bare minimum. No showing off. No empty promises.

At 3:20, his phone vibrated. Old Zhao had sent a text:

Data received. 30,000 rows, settled at 0.8 yuan each, 24,000 total. I'll transfer it to your card tomorrow. Defense is Friday—don't drop the ball.

Lin Chen stared at the screen. His fingers hovered above the keyboard for two seconds before he replied:

Received. Script has been packaged, deployment document attached.

Twenty-four thousand. He opened his banking app. The balance showed 35.30. The number was about to jump, but for now it was only pixels on a screen. He closed the app and pulled his attention back to the code. The V3.0 log module still needed one more exception handler: if the network was interrupted for more than 30 seconds, it had to automatically cache locally and resume from the breakpoint after recovery. He typed out the try...except block and filled in the retry logic. Compile, run, simulate network loss. The log showed: [WARN] Network timeout. Caching locally... [INFO] Connection restored. Resuming.

Passed. He shut the laptop and rubbed his stiff neck. Outside the window, a few more leaves had fallen from the plane tree. The wind slipped through the gap in the window frame, carrying the dry chill of late autumn.

At 6:00 in the evening, darkness came down. The dormitory's main light was off; only the pool of light from the desk lamp covered the desktop. Lin Chen packed the USB drive, backup USB drive, printed application documents, and defense script into a document pouch. He checked his shirt collar. The frayed edge had already been taped down on the inside with transparent tape and couldn't be seen. He tidied his hair in front of the mirror. The bluish shadows beneath his eyes couldn't be hidden, but his gaze was calm. He knew that tomorrow there would be no code in the conference room—only people. What people cared about was not how perfect the technology was, but whether the thing could stand steady in the mud, whether it could make an account come out right, whether it could keep an old farmer's son from having to grind away in a workshop until dawn.

At 8:00 p.m., he ran the final stress test. The script continued running, simulating data throughput in a 42-degree high-temperature workshop environment. The fan was at full speed, and the Raspberry Pi casing was hot to the touch. The log window scrolled smoothly. But at the 47th minute, one heartbeat packet showed a 0.8-second delay. Lin Chen immediately pulled up system monitoring. CPU usage had spiked to 89%, and signs of a memory leak were beginning to emerge. He traced it to an old-version compatibility issue in the xlrd library, replaced it with openpyxl, and rewrote the data-reading logic. Recompile, run. The delay disappeared, and the CPU stabilized at 65%.

He added another line to his mistake notebook:

20:14 Memory leak. Old xlrd version did not release handles. Replaced with openpyxl. Stress test passed.

He closed the notebook and turned off the desk lamp. His left foot had gone completely numb, like a block of wood that no longer belonged to his body. He lay down, his breathing gradually smoothing out. From outside the window came the distant whistle of a train, long and piercing through the autumn night.

The phone screen lit up once in the darkness. Professor Zhou had sent a WeChat message:

A temporary follow-up question has been added to the Science and Technology Department's enterprise review: if the on-site condition includes both network and power outage for more than two hours, how will your system guarantee that data is not lost, duplicated, or contaminated? 8:00 tomorrow morning. Conference room.

Lin Chen stared at the line of text without replying at once. He sat up and opened the computer again. The screen's cold white light reflected on his face. He created a new document and typed the title:

Local Caching and Consistency Verification Plan Under Network and Power Outage Conditions

The night was still long. But the road had already been laid beneath his feet.