After a hot runner communication terminal resistor burns out, replacement must be completed in four steps: "Diagnosis Confirmation → Safe Power Off → Precise Replacement → Verification and Recovery". Incorrect operation may cause secondary faults. The following is the complete handling procedure.
1. Before Replacement: Three-Step Diagnosis Confirms Burnout
① Measure Resistance: Power Off and Check Physical Status
Operation: Stop the machine and disconnect the main power supply to ensure the communication line is de-energized. Use a multimeter in resistance mode to measure the equivalent resistance between bus lines A and B.
Judgment:
Normal: ≈60Ω (120Ω in parallel at both ends)
Abnormal: ∞ (open circuit) or ≈120Ω → Indicates at least one end is burnt out.
Checkpoints: Remove the suspected burnt-out resistor and measure its resistance individually to see if it is 120Ω. Observe the resistor body for signs of overheating such as carbonization, cracking, and discoloration.
Tip: Prioritize checking the terminal resistor of the furthest device, as this location is most prone to overload.
② Observe the waveform: Oscilloscope captures signal anomalies
Operation: Restart the system and connect the oscilloscope online to the A/B differential signal line.
Observe the communication waveform for ringing, overshoot, or blurred edges.
Judgment: If the waveform deteriorates after running for a period of time, it is very likely caused by the resistor burning out due to heat.
③ Check the log: Trace the fault mode
Retrieve the PLC/SCADA communication log and pay attention to:
Whether the fault is concentrated during high-temperature periods or when high-power equipment is started
Whether it reports "communication timeout," "CRC check failure," or "device offline"
Whether it temporarily recovers after restarting and reappears after several hours
Conclusion: If any two of the three conditions are met, it can be determined that the terminating resistor is burned out and needs to be replaced.
2. Replacement Procedure: Safe, Standardized, and Verifiable
① Preparation
Tools: Soldering iron, desoldering pump, multimeter, anti-static wrist strap
Spare Parts: 120Ω ±1% accuracy, 0.25W (1/4W) and above power rating metal film resistor (1210 package recommended)
Safety: LOTO (Lock-Off Tag) to prevent accidental power supply
② Removing the Old Resistor
Locate the burnt resistor (usually located at the communication interface of the first and last devices on the bus).
Use a soldering iron and desoldering pump to thoroughly remove solder and remove the old resistor.
Clean the pads to ensure no residual carbides or cold solder joints.
③ Installing the New Resistor
Insert both ends of the new resistor into the pads, ensuring correct positioning regardless of polarity.
Use low-temperature solder (e.g., Sn63/Pb37) to avoid heat damage.
The solder joints should be smooth and full, without cold solder joints or bridging.
④ Wiring Verification
Confirm that terminating resistors are only installed at both ends of the bus.
Intermediate nodes are strictly prohibited from connection to avoid impedance mismatch.
The shielding layer remains single-ended grounded (control room side).
3. After Replacement: Three-Step Verification to Ensure Recovery
① Measure Resistance Again
Measure the equivalent resistance between A and B after power-off, confirming recovery to approximately 60Ω.
② Re-examine the Waveform
Observe the signal with an oscilloscope after power-on, confirming the waveform is a clear square wave without ringing or overshoot.
③ Continuously Check Logs
Run the system for more than 4 hours, confirming no "communication timeout" or "CRC error" logs.
Record the communication error rate, which should be below 10⁻⁶.
Acceptance Criteria: Only after all three verification steps are passed can continuous production begin.
Summary: Although small, terminating resistors are the "ballast" of the communication link. Resistors of 120Ω/0.25W or higher specifications must be selected, and the "diagnosis → replacement → verification" process must be strictly followed to avoid significant losses due to minor issues.
