Normally, and with the sensors in standard AWS810 configuration, the RS‑485 line does not need external termination resistors. DMU801 serial plug‑in modules have built‑in resistors to reduce signal reflections.
| Serial module | Port | Termination type |
|---|---|---|
| DMS801 | 1–3 | Built-in AC termination (120 Ω resistor in series with 1 nF capacitor) |
| DMS801 and DMS802 | 4 | Software‑controllable 120 Ω termination |
In addition, the RS‑485 transceivers have fail‑safe functionality and controlled transmit slew rate to eliminate the need for external resistors in most cases.
However, in case of a communication issue on the RS‑485 line, you can try to fix it as follows:
- Add a 120 Ω external resistor at the end of the line (in DMS801 ports 1–3).
- Switch on the software‑controllable termination using DMU801 Configuration Tool (in DMS801/DMS802 port 4). For instructions, see DMU801 Configuration Tool User Guide (M212407EN).
The communication problem may be caused by signal reflections if there is an extra long sensor cable or high‑speed slew rate at the sensor end.