After replacing the termination resistor in a hot runner communication system, no additional "configuration" is required; it functions as an impedance-matching component at the physical layer and becomes effective automatically once correctly installed. The critical point is to ensure that 120Ω resistors are installed only at the two ends of the bus; connecting them to intermediate nodes is strictly prohibited, as this will lead to communication anomalies.
1. No Software or Parameter Configuration Required
Termination resistors are purely hardware components and do not involve the configuration of parameters such as communication protocols, baud rates, or device addresses.
After replacement, no menu settings need to be adjusted within the PLC, HMI, or hot runner controller.
As long as the physical connections are correct (one 120Ω resistor at each end), the system will automatically match impedance upon power-up.
Tip: If communication remains abnormal after replacement, prioritize troubleshooting the installation location and resistance value rather than assuming that "parameters have not been set."
2. Physical Connection Items That Must Be Verified
|
Check Item |
Correct Practice |
Incorrect Examples |
|
Installation Location |
Install only on the two devices located at the furthest ends of the bus |
Install termination resistors on intermediate slave devices as well |
|
Resistance Value Verification |
Use a multimeter to measure the equivalent resistance between points A and B (should be ≈60Ω) |
Measured value is 120Ω (only one end installed) or ∞ (no resistors installed) |
|
Wiring Method |
Use a "daisy-chain" (hand-in-hand) connection for communication cables |
Use star-topology or T-branch wiring |
|
Shielding Grounding |
Ground the cable shield at only one end* (typically at the control room side) |
Ground both ends simultaneously, creating a ground loop that causes interference |
Special Note: Some intelligent modules (e.g., RSM485PHT) feature built-in, switchable termination resistors. You must verify whether these have already been enabled via jumpers or DIP switches to avoid creating redundant parallel connections externally.
3. Three-Step Verification Required After Replacement
① Measure Resistance (Power Off)
Shut down the system and cut power; measure the resistance between lines A and B.
Normal Value: ≈60Ω (Two 120Ω resistors in parallel)
② Observe Waveforms (Power On)
Connect an oscilloscope to observe the differential signals.
Normal Waveform: A clear square wave, free of ringing or overshoot.
③ Monitor Logs (System Running)
Monitor the PLC/SCADA communication logs for a minimum of 4 hours.
Normal Behavior: No anomalies such as "Communication Timeout," "CRC Error," etc.
Acceptance Criteria: All three steps must be successfully passed to confirm that the replacement was successful.
