v1.9.14: Senseair S88 - implémentation lecture Modbus RTU

read_co2() lit IR1..IR4 en une trame (status + CO2) à 9600 8N1,
adresse 0xFE 'any address', avec vérification CRC16-Modbus et rejet
de la mesure si status non-nul (warm-up ou erreur).

CRC requête/réponse validés contre les exemples du datasheet TDE14367.
Doc protocole consolidée dans S88/README.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

S88/write_data.py

@@ -2,8 +2,8 @@
 Script to get CO2 values from Senseair S88 sensor and write to database
 /usr/bin/python3 /var/www/nebuleair_pro_4g/S88/write_data.py
 
-Port and protocol details come from config_table (key S88_port).
-The actual sensor read protocol is implemented in read_co2() below.
+Modbus RTU 9600 8N1. Reads IR1..IR4 in one frame to get status + CO2.
+If status (IR1) is non-zero the reading is skipped (warm-up or error).
 '''
 
 import serial
@@ -14,6 +14,25 @@ DB_PATH = "/var/www/nebuleair_pro_4g/sqlite/sensors.db"
 DEFAULT_PORT = "/dev/ttyAMA5"
 BAUDRATE = 9600
 
+# Modbus slave address: 0xFE = "any address", any S88 responds regardless of
+# its configured individual address. Fine for single-sensor setups.
+SLAVE_ADDR = 0xFE
+# Read IR1..IR4 in one frame: function 0x04, start 0x0000, qty 0x0004
+READ_REQUEST = bytes([SLAVE_ADDR, 0x04, 0x00, 0x00, 0x00, 0x04, 0xE5, 0xC6])
+EXPECTED_RESPONSE_LEN = 13  # 1 addr + 1 fn + 1 count + 8 data + 2 CRC
+
+
+def crc16_modbus(data):
+    crc = 0xFFFF
+    for byte in data:
+        crc ^= byte
+        for _ in range(8):
+            if crc & 1:
+                crc = (crc >> 1) ^ 0xA001
+            else:
+                crc >>= 1
+    return crc
+
 
 def get_config(cursor, key, default):
     cursor.execute("SELECT value FROM config_table WHERE key = ?", (key,))
@@ -22,9 +41,42 @@ def get_config(cursor, key, default):
 
 
 def read_co2(ser):
-    # TODO: implement the Senseair S88 read protocol once the datasheet is provided.
-    # Expected return: integer CO2 concentration in ppm, or None on failure.
-    raise NotImplementedError("Senseair S88 read protocol not implemented yet")
+    ser.reset_input_buffer()
+    ser.write(READ_REQUEST)
+    response = ser.read(EXPECTED_RESPONSE_LEN)
+
+    if len(response) < 5:
+        print(f"S88: short response ({len(response)} bytes)")
+        return None
+
+    # Modbus exception response: function code has high bit set (0x84 instead of 0x04)
+    if response[1] & 0x80:
+        print(f"S88 Modbus exception: code={response[2]:#04x}")
+        return None
+
+    if len(response) != EXPECTED_RESPONSE_LEN:
+        print(f"S88: unexpected response length {len(response)} (expected {EXPECTED_RESPONSE_LEN})")
+        return None
+
+    # Verify CRC (last two bytes, low byte first)
+    received_crc = response[-2] | (response[-1] << 8)
+    if crc16_modbus(response[:-2]) != received_crc:
+        print("S88: CRC mismatch")
+        return None
+
+    if response[0] != SLAVE_ADDR or response[1] != 0x04 or response[2] != 0x08:
+        print(f"S88: unexpected header {response[:3].hex()}")
+        return None
+
+    status = (response[3] << 8) | response[4]
+    co2 = (response[9] << 8) | response[10]
+
+    if status != 0:
+        # DI8 = Warm Up (bit 7 of low byte). Other bits = errors.
+        print(f"S88: sensor not ready, status={status:#06x}")
+        return None
+
+    return co2
 
 
 def main():
@@ -65,8 +117,6 @@ def main():
         )
         conn.commit()
         print(f"CO2: {co2_ppm} ppm (saved at {rtc_time_str})")
-    except NotImplementedError as e:
-        print(f"S88 not ready: {e}")
     except Exception as e:
         print(f"S88 error: {e}")
     finally: