The replacement procedure for the hot runner communication terminal resistor must follow a four-step process: "Safe Power Off → Precise Removal → Standard Installation → Comprehensive Verification" to ensure stable restoration of the communication link and avoid secondary failures caused by improper operation.
1. Safety Preparation and Power Off Confirmation
Power Off: Turn off the main power supply to the injection molding machine and hot runner control system, and execute the LOTO (Lockout and Tag) procedure to prevent accidental power restoration.
Discharge Process: Wait 5 minutes to ensure that the residual voltage in the communication line and controller has been completely released.
Tools Preparation:
Soldering iron (temperature setting 300℃±20℃)
Desoldering pump or desoldering tape
Multimeter (for resistance measurement)
Anti-static wrist strap (to be worn and grounded)
New Resistor: 120Ω ±1% accuracy, 0.25W and above power (1210 package metal film resistor recommended)
Safety Note: Never operate while the circuit is energized to avoid damaging the communication interface or causing electric shock.
2. Removing the Burnt Resistor
Locating the Terminating Resistor: Typically located at the communication interfaces of the two devices at the farthest ends of the bus (first and last stations).
Heating and Desoldering: Evenly heat the solder joints at both ends of the resistor with a soldering iron, using a desoldering pump to remove the solder.
Gently remove the old resistor, avoiding pulling force that could cause the pads to detach.
Cleaning the Pads: Clean the pads with isopropyl alcohol swabs to remove any residue.
Check for carbonization, cracking, or signs of poor soldering; repair the pads if necessary.
Note: If the resistor is surface mount, it is recommended to use a temperature-controlled soldering iron with a hot air gun to prevent PCB deformation due to heat.
3. Installing the New Resistor
Resistor Selection Confirmation:
Resistance: 120Ω
Power: ≥0.25W
Accuracy: ±1%
Package: 1210 or larger is preferred for improved heat dissipation.
Soldering and Installation:
Align the two ends of the new resistor with the pads, ensuring a flat and straight surface.
Use low-temperature solder (e.g., Sn63/Pb37) for soldering; single-point soldering time should not exceed 3 seconds. Solder joints should be smooth and full, without any cold solder joints, bridging, or poor soldering.
Wiring Verification:
Ensure that terminating resistors are only installed at both ends of the bus. Terminating resistors are strictly prohibited from being connected to any slave devices in the middle. The shielding layer should remain grounded at one end (control room side).
Practical Recommendation: Test leads with clamps can be used to temporarily fix the resistor for easier soldering positioning.
4. Three-Step Verification After Replacement
① Resistance Measurement: Power-off test of physical connection
Measure the equivalent resistance between lines A and B using a multimeter. The normal value should be approximately 60Ω (two 120Ω parallel lines).
② Waveform Observation: Observe signal quality with an oscilloscope
Connect the oscilloscope to the A/B differential lines after power-on.
Observe whether the waveform is a clear square wave, without ringing, overshoot, or edge distortion.
③ Log Monitoring: Monitor communication stability
Run the system for at least 4 hours. Check the PLC/SCADA logs for abnormalities such as "communication timeout" or "CRC check failure".
Acceptance Criteria: All three criteria must be met before continuous production can begin
