Replacing a hot runner communication termination resistor requires following a four-step process: "Safe Power-off → Precise Removal → Standardized Installation → Comprehensive Verification." This ensures the stable restoration of the communication link and prevents secondary failures caused by improper operation.
1. Safety Preparation and Power-off Verification
System Shutdown & Power-off: Turn off the main power supply for both the injection molding machine and the hot runner control system. Implement LOTO (Lockout/Tagout) procedures to prevent accidental power restoration.
Discharge Process: Wait 5 minutes to ensure that any residual voltage within the communication lines and controller has fully dissipated.
Tool Preparation:
Soldering iron (temperature set to 300°C ± 20°C)
Solder sucker or desoldering wick
Multimeter (for measuring resistance values)
Anti-static wrist strap (must be worn and grounded)
New Resistor: 120Ω ± 1% tolerance, 0.25W or higher power rating (a 1210-package metal film resistor is recommended).
Safety Note: Never attempt to perform operations while the system is powered on; doing so risks damaging the communication interface or causing electric shock.
2. Removing the Burnt-out Resistor
Locating the Termination Resistor: These are typically situated at the communication interfaces of the two devices located at the extreme ends of the bus (the first station and the last station).
Heating and Desoldering:
Use the soldering iron to evenly heat the solder joints at both ends of the resistor, then use the solder sucker to remove the molten solder.
Gently lift and remove the old resistor; avoid pulling forcefully, as this may cause the solder pads to detach from the PCB.
Cleaning the Solder Pads:
Use an isopropyl alcohol-soaked cotton swab to clean any residue remaining on the solder pads.
Inspect the pads for any signs of carbonization, cracking, or poor solder adhesion; repair the pads if necessary.
Note: If the resistor is a surface-mount device (SMD), it is recommended to use a temperature-controlled soldering iron in conjunction with a hot air gun to prevent thermal deformation of the PCB.
3. Installing the New Resistor
Resistor Selection Verification:
Resistance Value: 120Ω
Power Rating: ≥0.25W
Tolerance: ±1%
Package Size: Prioritize 1210 or larger sizes to enhance heat dissipation capabilities.
Soldering Installation:
Align the two ends of the new resistor with the solder pads, ensuring it sits flat and is not tilted.
Solder using low-temperature solder (e.g., Sn63/Pb37); limit the soldering time per joint to no more than 3 seconds.
The solder joints should be smooth and well-formed, free from open circuits, solder bridges, or cold solder joints.
Wiring Verification:
Ensure that terminating resistors are installed only at the two ends of the bus line.
It is strictly prohibited to connect terminating resistors to any intermediate slave devices.
The shielding layer must remain single-ended grounded (at the control room side).
Practical Tip: Use test leads equipped with clips to temporarily hold the resistor in place, facilitating precise positioning during soldering.
4. Three-Step Verification After Replacement
① Measure Resistance: Check Physical Connections (Power Off)
Use a multimeter to measure the equivalent resistance between the A and B lines → The normal value should be ≈60Ω (representing two 120Ω resistors in parallel).
② Observe Waveforms: Monitor Signal Quality (Oscilloscope)
After powering on the system, connect an oscilloscope to the A/B differential lines.
Verify that the waveform appears as a clean square wave, free from ringing, overshoot, or edge distortion.
③ Monitor Logs: Track Communication Stability
Run the system for at least 4 hours.
Check the PLC/SCADA logs for any anomalies, such as "Communication Timeout," "CRC Check Failure," etc.
Acceptance Criteria: Continuous production may commence only after all three verification steps have been successfully met.
