RS485 | GUILCOR SENSORS

RS485
Temperature sensors

Robust serial communication standard designed for long-distance and multi-device industrial networks with high noise immunity.

Maximum precision
+/- 0,1 / 0,5°K

Minimum temperature
-200°C

Maximum temperature
+1200°C

Resolution
0,01°C

Power supply voltage

9,0→30,0V

Speed (Bauds)

4800/115200

Supported protocole
Modbus RTU

Price
High

What is a RS485 sensor ?

The RS485 is a robust differential serial interface that allows for the connection of up to 32 sensors on the same bus, over distances of up to 1,200 meters.

RS485 sensors are often intelligent transmitters that combine:

a temperature sensor (PT100, thermocouple, NTC, etc.),
an A/D converter,
and a Modbus RTU communication module.

Each sensor has a unique address on the bus.

Operating principle

The sensor measures the temperature through its internal or external probe, converts the signal, and then sends the data over the differential RS485 line (A/B).

The master (PC, PLC, or data logger) queries the slaves via Modbus RTU.

Example of a Modbus frame:

[ID] [03] [00 01] [00 01] [CRC_L] [CRC_H]

→ Typical response:

[ID] [03] [02] [00 19] [CRC_L] [CRC_H]

💡 0x0019 = 25 °C in binary coding (resolution 0.1 °C).

Example of digital conversion (Python - ModBus RTU)

from pymodbus.client import ModbusSerialClient

client = ModbusSerialClient(port='/dev/ttyUSB0', baudrate=9600, parity='N', stopbits=1)
client.connect()

result = client.read_holding_registers(1, 1, unit=2)  # Adresse 2, registre 1
temp = result.registers[0] / 10.0
print(f"Température : {temp:.1f} °C")

client.close()

✅ Example output:

Temperature: 24.7 °C

Principle diagram

[PC / Automate]───[USB–RS485 Adapter]───(A/B)───[Sensor 1]───[Sensor 2]───...

💡 The RS485 bus can exceed 1 km with shielded twisted cable (e.g., Cat. 6 STP).

Application areas

🏭 Industrial automation and thermal processes

⚙️ HVAC (heating, ventilation, air conditioning)

🌿 Greenhouses, controlled environment agriculture

🧪 Laboratories, test benches, data centers

🚧 Long-distance multipoint networks

Should I choose a RS485 sensor ?

Strengths points

📡 Long-distance and robust communication
→ Up to 1,200 m without signal loss — immune to interference.
🧠 Modbus RTU multipoint network
→ Up to 32 sensors on a single line, ideal for monitoring.
🎯 High precision and stability
→ Accuracy of 0.1 °C possible according to the built-in sensor.
⚙️ Universal compatibility
→ Simple reading from PC, Siemens, Schneider, Wago, etc.

Weaknesses points

🪛 Requires initial setup
→ Address, baud rate, and parity must be defined before integration.
📚 Requires a Modbus master
→ Not directly compatible with Arduino without a specific library.
💰 Higher cost than I²C or analog sensors
→But compensated by its robustness and versatility.

Useful information

Here is some useful information regarding the RS485 sensors.

Typical broaching

Brooch	Name	Description
1	A / D+	Positive differential line
2	B / D−	Negative differential line
3	GND	Mass
4	VCC	9–30 V DC

💡 Some models combine power and signal on M12 (A+, B−, GND, VCC).

Example of Modbus register

Adress (déc)	Description	Format	Unity
0x0000	Instantaneous temperature	int16	0,1 °C
0x0001	Moving average	int16	0,1 °C
0x0002	Max temperature	int16	0,1 °C
0x0003	Min temperature	int16	0,1 °C
0x0004	Sensor address	int8	—

💡 Each manufacturer provides its own detailed Modbus table.

Typical integration diagram

+24V ───────────────┐ │ [Sensor RS485] │ A ───────┬── Bus RS485 ───┬── [Sensor 2] │ B ───────┴────────────────┘ │ GND

💡 Termination resistors of 120 Ω are required at both ends of the bus.

Example of Modbus configuration (via serial terminal)

Order	Function
ADDR=2	Defines the sensor's address
BAUD=9600	Defines the speed of communication
PARITY=N	Defines the parity (None, Even, Odd)
SAVE	Internal memory backup

💡 Most RS485 models include a small serial menu for local configuration.