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Pi-----------------------------------RS485 transceiver
GPIO 14 (TX)----------------- DI
GPIO 15 (RX)-----------------RO
GPIO 18-------------------------RSE
3.3V------------------------------VCC
GND------------------------------GND (was VCC, possibly a typo)

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import serial
import time
from pymodbus.client import ModbusSerialClient
client = ModbusSerialClient(port='/dev/serial0', baudrate=9600, timeout=1)
if client.connect():
 print("Connection successful.")
 
 try:
 response = client.read_holding_registers(address=0, count=1, device_id=1)
 if not response.isError():
 raw_value = response.registers[0]
 turbidity_value = raw_value / 100.0 
 print(f"Turbidity value: {turbidity_value} UNT")
 else:
 print(f"MODBUS Error: {response}")
 except Exception as e:
 print(f"An error occurred: {e}")
 finally:
 client.close()
 print("Connection closed.")
else:
 print("Failed to connect to the MODBUS device.")

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Connection successful.
No response received after 3 retries, continue with next request
An error occurred: Modbus Error: [Input/Output] No response received after 3 retries, continue with next request
Connection closed.

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from pymodbus.client import ModbusSerialClient
baudrates = [9600, 19200, 38400, 115200]
slave_ids = list(range(1, 11)) + [250]
for baud in baudrates:
 print(f"\n=== Testing baudrate {baud} ===")
 client = ModbusSerialClient(
 port="/dev/ttyUSB0",
 baudrate=baud,
 parity="N",
 stopbits=1,
 bytesize=8,
 timeout=1
 )
 if not client.connect():
 print(f"Could not open port at {baud}")
 continue
 for sid in slave_ids:
 try:
 print(f" -> Trying device_id {sid}...")
 # Turbidity register is at 0x0000
 resp = client.read_holding_registers(address=0, count=1, device_id=sid)
 if resp and not resp.isError():
 raw = resp.registers[0]
 turbidity = raw / 100.0
 print(f"Found device at baud {baud}, ID {sid}: raw={raw}, turbidity={turbidity:.2f} NTU")
 except Exception as e:
 print(f" [ID {sid}] Error: {e}")
 client.close()

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Found device at baud 9600, ID 1: raw=0, turbidity=0.00 NTU

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from pymodbus.client import ModbusSerialClient
import time
client = ModbusSerialClient(
 port="/dev/ttyUSB0",
 baudrate=9600,
 parity="N",
 stopbits=1,
 bytesize=8,
 timeout=1
)
if client.connect():
 try:
 while True:
 resp = client.read_holding_registers(address=0, count=1, device_id=1)
 if resp and not resp.isError():
 raw = resp.registers[0]
 turbidity = raw / 100.0
 print(f"Turbidity: {turbidity:.2f} NTU (raw={raw})")
 else:
 print("Read error:", resp)
 time.sleep(0.5)
 except KeyboardInterrupt:
 print("Stopped.")
 finally:
 client.close()
else:
 print("Failed to connect to sensor")

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import RPi.GPIO as GPIO
from pymodbus.client import ModbusSerialClient
RS485_RSE_PIN = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(RS485_RSE_PIN, GPIO.OUT)
GPIO.output(RS485_RSE_PIN, GPIO.LOW) # Set to LOW for receive mode by default
def set_rse_transmit():
 GPIO.output(RS485_RSE_PIN, GPIO.HIGH)
def set_rse_receive():
 GPIO.output(RS485_RSE_PIN, GPIO.LOW)
class CustomModbusClient(ModbusSerialClient):
 def connect(self):
 result = super().connect()
 # Attach the callback functions to the client
 self.rse_callbacks = {
 'before_send': set_rse_transmit,
 'after_send': set_rse_receive
 }
 return result
baudrates = [9600, 19200, 38400, 115200]
slave_ids = list(range(1, 11)) + [250]
try:
 for baud in baudrates:
 print(f"\n=== Testing baudrate {baud} ===")
 client = CustomModbusClient(
 port="/dev/serial0",
 baudrate=baud,
 parity="N",
 stopbits=1,
 bytesize=8,
 timeout=1
 )
 if not client.connect():
 print(f"Could not open port at {baud}")
 continue
 for sid in slave_ids:
 try:
 print(f" -> Trying device_id {sid}...")
 resp = client.read_holding_registers(address=0, count=1, device_id=sid)
 if resp and not resp.isError():
 raw = resp.registers[0]
 turbidity = raw / 100.0
 print(f"Found device at baud {baud}, ID {sid}: raw={raw}, turbidity={turbidity:.2f} NTU")
 except Exception as e:
 print(f" [ID {sid}] Error: {e}")
 client.close()
finally:
 GPIO.cleanup() # Clean up GPIO settings on exit

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import serial
import RPi.GPIO as GPIO, time
import struct
RSE_PIN = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(RSE_PIN, GPIO.OUT)
GPIO.output(RSE_PIN, 0) # start in receive mode
def tx():
 GPIO.output(RSE_PIN, 1)
 time.sleep(0.002)
def rx():
 time.sleep(0.002)
 GPIO.output(RSE_PIN, 0)
# Precomputed Modbus RTU frame: device 1, read holding register 0x0000, count=1
QUERY_FRAME = bytes.fromhex("01 03 00 00 00 01 84 0A")
ser = serial.Serial(
 port="/dev/serial0",
 baudrate=9600,
 parity=serial.PARITY_NONE,
 stopbits=serial.STOPBITS_ONE,
 bytesize=serial.EIGHTBITS,
 timeout=1
)
print("Connected to XM8518 sensor on /dev/serial0")
try:
 while True:
 tx()
 ser.write(QUERY_FRAME)
 ser.flush()
 rx()
 resp = ser.read(7)
 print(f"Raw response: {resp}")
 if len(resp) == 7 and resp[0] == 1 and resp[1] == 3:
 raw = struct.unpack(">H", resp[3:5])[0]
 turbidity = raw / 100.0
 print(f"Turbidity: {turbidity:.2f} NTU (raw={raw})")
 else:
 print("No response:", resp.hex(" "))
 time.sleep(2)
except KeyboardInterrupt:
 print("\nStopped by user.")
finally:
 ser.close()
 GPIO.cleanup()

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Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x00\x00\xb8D'
Turbidity: 0.00 NTU (raw=0)
Raw response: b'\x01\x03\x02\x0329a'
Turbidity: 8.18 NTU (raw=818)
Raw response: b'\x01\x03\x02\x03\xd9y.'
Turbidity: 9.85 NTU (raw=985)
Raw response: b'\x01\x03\x02\x03\xe8\xb8\xfa'
Turbidity: 10.00 NTU (raw=1000)
Raw response: b'\x01\x03\x02\x03\xe8\xb8\xfa'
Turbidity: 10.00 NTU (raw=1000)