How RS485 Wiring Works for Level Sensors
HPzenAi LS-PRE level sensors use RS485 Modbus RTU for communication. The sensor cable is a 3-wire vented cable: 12V DC power, RS485 A (positive differential signal), and RS485 B (negative differential signal). The gateway provides the RS485 bus master function, polling each sensor by its Modbus address. Sensors can be connected individually or daisy-chained up to 32 devices on a single RS485 bus port.
RS485 Terminal Identification
Identify the correct terminals on both the sensor cable and the gateway terminal block before wiring. Incorrect polarity on RS485 A/B prevents all communication.
| Wire / Terminal | Label | Function |
|---|---|---|
| Red or Brown | +12V | 12V DC power supply to the sensor |
| Black | GND | Power supply ground / negative return |
| Yellow or White | RS485 A | RS485 differential signal — positive |
| Green or Blue | RS485 B | RS485 differential signal — negative |
| Shield / Drain | Shield | Cable shield — connect to earth at gateway end only |
Cable Selection
Use the correct cable type for reliable RS485 communication over the full sensor cable run.
- 1Use shielded twisted-pair (STP) cable with a minimum 0.5 mm² conductor cross-section.
- 2For runs up to 10 m, the sensor's pre-attached cable is sufficient — no extension needed.
- 3For runs from 10 m to 100 m, extend using shielded RS485 cable (e.g. Belden 9841 or equivalent).
- 4Maximum recommended cable run: 100 m for LS-PRE sensors at 9600 baud.
- 5For longer runs, reduce the bus baud rate to 4800 or 2400 baud in the gateway RS485 settings.
- 6Do not use unshielded cable (standard twin-and-earth or speaker wire) for RS485 runs in environments with electrical machinery.
Single Sensor Wiring
For a single level sensor connected directly to the gateway, make the following connections at the gateway terminal block.
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- 1Sensor +12V wire → Gateway +12V output terminal (or separate 12V PSU positive).
- 2Sensor GND wire → Gateway GND terminal (or separate 12V PSU negative).
- 3Sensor RS485 A wire → Gateway RS485 A terminal.
- 4Sensor RS485 B wire → Gateway RS485 B terminal.
- 5Connect the cable shield to the gateway earth terminal — leave the far (sensor) end of the shield unconnected.
- 6Tighten all terminal screws to 0.5–0.6 Nm — overtightening can damage terminal spring contacts.
Multiple Sensors on One Bus (Daisy-Chain)
Up to 32 LS-PRE sensors can share a single RS485 bus on one gateway port. Use a daisy-chain topology — run the bus from the gateway to the first sensor, then on to the second, and so on. Do not use a star topology (individual cables from each sensor back to the gateway).
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- 1Run the RS485 bus cable from the gateway terminal block to the first sensor, then on to the second sensor, and continue to the furthest device.
- 2At each sensor, break out the A, B, and power wires to the sensor's terminals and continue the bus cable to the next sensor.
- 3All RS485 A terminals are connected together on the shared bus; all RS485 B terminals are connected together.
- 4Each sensor must be configured with a unique Modbus address (1–32) before connecting to the shared bus.
- 5Install a 120 Ω termination resistor between RS485 A and B at the terminals of the furthest sensor only.
- 6Do not install termination resistors at intermediate sensors — only at the end of the bus.
Powering Multiple Sensors
The 12V DC power and RS485 signal are carried in the same cable for LS-PRE sensors. However, the gateway's onboard 12V output has a limited current capacity. For installations with many sensors, use a separate DIN rail 12V power supply.
- 1Each LS-PRE sensor draws approximately 50–80 mA at 12V DC during normal operation.
- 2The gateway's onboard 12V output supports up to 500 mA total — suitable for up to 6 sensors from the gateway supply.
- 3For 7 or more sensors, use a dedicated DIN rail 12V power supply (rated 2A or higher) for sensor power.
- 4Connect the power supply negative (GND) to the gateway GND terminal to ensure a common reference for RS485 communication.
- 5Use an HPzenAi SPD (surge protection device) on the power supply mains input for outdoor or lightning-exposed installations.
RS485 Wiring Checklist
Verify all wiring before powering on and scanning for devices.
- 1RS485 A and B wires connected to correct terminals (not swapped).
- 2All terminal screws tightened firmly.
- 3Cable shield connected to earth at gateway end only.
- 4Bus topology is daisy-chain — no star wiring.
- 5120 Ω termination resistor installed at the furthest sensor only.
- 6Each sensor has a unique Modbus address (1–32).
- 7Total cable run does not exceed 100 m.
- 8Power supply capacity is sufficient for the number of sensors installed.
- 9Cable routed away from high-voltage and motor wiring by at least 200 mm.