After replacing an Omron SRS1-T module, there is no need to reconfigure any parameters. It is a purely hardware-based terminal resistor module designed for impedance matching at the physical layer; it involves no communication protocols or software configurations. Once correctly installed, communication will resume automatically.
1. Why is Parameter Configuration Unnecessary?
The SRS1-T is passive hardware: It does not store data or execute programs; it merely provides a 121Ω impedance at both ends of the bus to absorb signal reflections.
Functionality takes effect immediately: Provided the physical connections are correct (one module at the first station and one at the last), impedance matching occurs automatically upon power-up-no enabling, initialization, or "parameter writing" is required.
Non-intelligent module: Unlike PLCs or servo drives, the SRS1-T features no DIP switches, rotary selectors, or communication addresses; there are absolutely no configurable settings.
Tip: If communication anomalies occur after replacement, prioritize troubleshooting the installation location and resistance value rather than assuming that "parameters were not set."
2. Three Physical Connection Checks You Must Verify
|
Check Item |
Correct Practice |
Incorrect Example |
|
Installation Location |
Only at the two outermost nodes of the bus (the first and last stations) |
Installed at intermediate slave stations as well |
|
Resistance Verification |
Equivalent resistance between A and B lines ≈ 60Ω (121Ω // 121Ω) |
Measured value is 121Ω (only one end installed) or ∞ (no modules installed) |
|
Shield Grounding |
Shielding layer grounded at a single end (at the control room side) |
Grounded at both ends, creating a ground loop that causes interference |
Special Note: Some DeviceNet branch units (e.g., V624-DRT1) have built-in terminal resistors. If such a unit is already in use, there is no need to connect an external SRS1-T module, as doing so would result in redundant parallel connections and an excessively low resistance value.
3. Post-Replacement Verification Procedure
① Resistance Measurement (Power Off)
Shut down the system and cut power; use a multimeter to measure the resistance between lines A and B.
Normal Value: ≈60Ω (a fluctuation range of 58–63Ω is permissible).
② Waveform Analysis (Power On)
Connect an oscilloscope to observe the differential signal.
Normal Waveform: A clear square wave, free of ringing or overshoot.
③ Log Monitoring (System Operation)
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.
