diff --git a/app/routers.py b/app/routers.py
index c9f17db..abbb1cf 100644
--- a/app/routers.py
+++ b/app/routers.py
@@ -318,6 +318,30 @@ async def reset_measurement(unit_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=502, detail=str(e))
 
 
+@router.get("/{unit_id}/measurement-state")
+async def get_measurement_state(unit_id: str, db: Session = Depends(get_db)):
+    """Get current measurement state (Start/Stop)."""
+    cfg = db.query(NL43Config).filter_by(unit_id=unit_id).first()
+    if not cfg:
+        raise HTTPException(status_code=404, detail="NL43 config not found")
+
+    if not cfg.tcp_enabled:
+        raise HTTPException(status_code=403, detail="TCP communication is disabled for this device")
+
+    client = NL43Client(cfg.host, cfg.tcp_port, ftp_username=cfg.ftp_username, ftp_password=cfg.ftp_password)
+    try:
+        state = await client.get_measurement_state()
+        is_measuring = state == "Start"
+        return {
+            "status": "ok",
+            "measurement_state": state,
+            "is_measuring": is_measuring
+        }
+    except Exception as e:
+        logger.error(f"Failed to get measurement state for {unit_id}: {e}")
+        raise HTTPException(status_code=502, detail=str(e))
+
+
 @router.post("/{unit_id}/sleep")
 async def sleep_device(unit_id: str, db: Session = Depends(get_db)):
     """Put the device into sleep mode for battery conservation."""
diff --git a/app/services.py b/app/services.py
index 8c9ad80..636ce3d 100644
--- a/app/services.py
+++ b/app/services.py
@@ -253,6 +253,16 @@ class NL43Client:
         await self._send_command("Reset\r\n")
         logger.info(f"Measurement data reset on {self.device_key}")
 
+    async def get_measurement_state(self) -> str:
+        """Get the current measurement state.
+
+        Returns: "Start" if measuring, "Stop" if stopped
+        """
+        resp = await self._send_command("Measure?\r\n")
+        state = resp.strip()
+        logger.info(f"Measurement state on {self.device_key}: {state}")
+        return state
+
     async def get_battery_level(self) -> str:
         """Get the battery level."""
         resp = await self._send_command("Battery Level?\r\n")
diff --git a/docs/API.md b/docs/API.md
index 0ed5ca1..710cdfd 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -120,6 +120,27 @@ POST /{unit_id}/reset
 ```
 Resets the measurement data.
 
+### Get Measurement State
+```
+GET /{unit_id}/measurement-state
+```
+Gets the current measurement state to determine if the device is actively measuring.
+
+**Response:**
+```json
+{
+  "status": "ok",
+  "measurement_state": "Start",
+  "is_measuring": true
+}
+```
+
+**Values:**
+- `measurement_state`: "Start" (measuring) or "Stop" (not measuring)
+- `is_measuring`: Boolean convenience field (true if measuring, false otherwise)
+
+**Note:** Elapsed time and interval counts are displayed on the device OSD but not available via command. Track elapsed time in your application by recording the start time when you call the `/start` endpoint.
+
 ### Manual Store
 ```
 POST /{unit_id}/store
diff --git a/docs/nl43_Command_ref.md b/docs/nl43_Command_ref.md
index e69de29..e5f32cb 100644
--- a/docs/nl43_Command_ref.md
+++ b/docs/nl43_Command_ref.md
@@ -0,0 +1,770 @@
+# NL-43/NL-53 Sound Level Meter - AI Command Reference
+## Quick Reference for Programming & Development
+
+---
+
+## 🚀 QUICK START - COMMAND SYNTAX
+
+### Setting Command (Change device settings)
+```
+Format: $Command Name,Parameter[CR][LF]
+Example: $LCD Auto Off,Short[CR][LF]
+         $Backlight,On[CR][LF]
+```
+
+**Critical Rules:**
+- **Prefix:** `$` (auto-added by device, shows command processing status)
+- **Separator:** `,` (comma required after command name)
+- **Spaces:** Must preserve exact spacing in command names
+- **Terminator:** `[CR][LF]` (0x0D 0x0A)
+- **Case:** Insensitive (LCD = lcd = Lcd)
+- **Wait:** Minimum 1 second after receiving `$` response before next command
+
+### Request Command (Get device status/data)
+```
+Format: Command Name?[CR][LF]
+Example: Battery Level?[CR][LF]
+         System Version?[CR][LF]
+```
+
+**Critical Rules:**
+- **Suffix:** `?` after command name
+- **No prefix:** No `$` for requests
+- **Terminator:** `[CR][LF]`
+- **Case:** Insensitive
+
+---
+
+## 📋 RESULT CODES
+
+Every command returns a result code:
+
+```
+Format: R+####
+```
+
+| Code | Status | Meaning |
+|------|--------|---------|
+| `R+0000` | ✅ Success | Command executed successfully |
+| `R+0001` | ❌ Command Error | Command not recognized |
+| `R+0002` | ❌ Parameter Error | Wrong parameter format or count |
+| `R+0003` | ❌ Specification Error | Used set when should request (or vice versa) |
+| `R+0004` | ❌ Status Error | Device not in correct state for this command |
+
+---
+
+## ⏱️ TIMING & CONTROL VALUES
+
+| Operation | Time | Notes |
+|-----------|------|-------|
+| Device response time | < 3 seconds | Max time for device to respond |
+| Between characters | < 100 ms | When sending multi-char strings |
+| Device idle time | < 200 ms | After device sends data |
+| **Recommended wait** | **≥ 1 second** | **After `$` before next command** |
+
+---
+
+## 🔧 COMMUNICATION CODES
+
+| Code | Hex | Purpose |
+|------|-----|---------|
+| `[CR]` | 0x0D | First terminator (Carriage Return) |
+| `[LF]` | 0x0A | Second terminator (Line Feed) |
+| `[SUB]` | 0x1A | Stop request |
+
+---
+
+## 📚 COMMAND CATEGORIES & FULL LIST
+
+### Communication
+| Command | Type | Function |
+|---------|------|----------|
+| `Echo` | S/R | Communication echo ON/OFF |
+
+### System
+| Command | Type | Function |
+|---------|------|----------|
+| `System Version` | R | Get firmware version |
+| `Type` | R | Get device type (NL-43/NL-53) |
+| `Serial Number` | R | Get device serial number |
+| `Clock` | S/R | Set/get current time |
+| `Language` | S/R | Set/get display language |
+| `Index Number` | S/R | Set/get index number |
+| `Key Lock` | S/R | Enable/disable key lock |
+
+### Display
+| Command | Type | Function |
+|---------|------|----------|
+| `Backlight` | S/R | Control backlight ON/OFF |
+| `Backlight Auto Off` | S/R | Set auto-off time for backlight |
+| `LCD` | S/R | Control LCD ON/OFF |
+| `LCD Auto Off` | S/R | Set auto-off time for LCD |
+| `Backlight Brightness` | S/R | Set backlight brightness |
+| `Battery Type` | S/R | Set battery type |
+| `Battery Level` | R | Get current battery level |
+| `Output Level Range Upper` | S/R | Set upper limit of bar graph |
+| `Output Level Range Lower` | S/R | Set lower limit of bar graph |
+| `Display Leq` | S/R | Enable/disable Leq display |
+| `Display LE` | S/R | Enable/disable LE display |
+| `Display Lpeak` | S/R | Enable/disable Lpeak display |
+| `Display Lmax` | S/R | Enable/disable Lmax display |
+| `Display Lmin` | S/R | Enable/disable Lmin display |
+| `Display LN1` | S/R | Enable/disable L5 display |
+| `Display LN2` | S/R | Enable/disable L10 display |
+| `Display LN3` | S/R | Enable/disable L50 display |
+| `Display LN4` | S/R | Enable/disable L90 display |
+| `Display LN5` | S/R | Enable/disable L95 display |
+| `Time Level Time Scale` | S/R | Set time-level time scale |
+
+### SD Card
+| Command | Type | Function |
+|---------|------|----------|
+| `SD Card Total Size` | R | Get total SD card capacity |
+| `SD Card Free Size` | R | Get SD card free space |
+| `SD Card Percentage` | R | Get SD card free space ratio |
+
+### Measurement
+| Command | Type | Function |
+|---------|------|----------|
+| `Frequency Weighting (Main)` | S/R | Set main frequency weighting (A/C/Z) |
+| `Frequency Weighting (Sub1/2/3)` | S/R | Set sub frequency weighting |
+| `Time Weighting (Main)` | S/R | Set main time weighting (F/S/I) |
+| `Time Weighting (Sub1/2/3)` | S/R | Set sub time weighting |
+| `Measure` | S/R | Start/stop measurement |
+| `Pause` | S/R | Pause measurement |
+| `Windscreen Correction` | S/R | Enable/disable windscreen correction |
+
+### Storage
+| Command | Type | Function |
+|---------|------|----------|
+| `Store Mode` | S/R | Set storage mode (Manual/Auto) |
+| `Store Name` | S/R | Set storage name |
+| `Manual Address` | S/R | Set manual storage address |
+| `Manual Store` | S | Execute manual storage |
+| `Overwrite` | S/R | Enable/disable storage overwriting |
+| `Measurement Time Preset Manual` | S/R | Set manual measurement time |
+| `Measurement Time Preset Auto` | S/R | Set auto measurement time |
+
+### I/O
+| Command | Type | Function |
+|---------|------|----------|
+| `AC OUT` | S/R | Set AC output ON/OFF |
+| `DC OUT` | S/R | Set DC output ON/OFF |
+| `Baud Rate` | S/R | Set RS-232C baud rate |
+| `USB Class` | S/R | Set USB communication mode |
+| `Ethernet` | S/R | Enable/disable LAN function |
+| `Ethernet DHCP` | S/R | Enable/disable DHCP |
+| `Ethernet IP` | S/R | Set IP address |
+| `Ethernet Subnet` | S/R | Set subnet mask |
+| `Ethernet Gateway` | S/R | Set default gateway |
+| `Web` | S/R | Enable/disable web application |
+| `FTP` | S/R | Enable/disable FTP |
+| `TCP` | S/R | Enable/disable TCP communication |
+
+### Data Output
+| Command | Type | Function |
+|---------|------|----------|
+| `DOD` | R | Output current displayed value |
+| `DRD` | R | Continuous data output |
+| `DRD?status` | R | Continuous output with status |
+| `DLC` | R | Output final calculation result |
+
+---
+
+## 💡 DETAILED COMMAND SYNTAX
+
+### Echo (Communication Echo)
+**Purpose:** Enable/disable echo of sent commands
+
+**Setting:**
+```
+$Echo,Off[CR][LF]     # Disable echo
+$Echo,On[CR][LF]      # Enable echo
+```
+
+**Request:**
+```
+Echo?[CR][LF]
+Response: Off | On
+```
+
+---
+
+### System Version
+**Purpose:** Get firmware version
+
+**Request:**
+```
+System Version?[CR][LF]
+Response: "xx.xx.xxxx"  # Example: "01.02.0034"
+```
+
+**Note:** No setting command available
+
+---
+
+### Type
+**Purpose:** Get device type
+
+**Request:**
+```
+Type?[CR][LF]
+Response: "NL-43" | "NL-53"
+```
+
+**Note:** No setting command available
+
+---
+
+### Serial Number
+**Purpose:** Get device serial number
+
+**Request:**
+```
+Serial Number?[CR][LF]
+Response: "xxxxxxxx"  # 8-digit number (00000000-99999999)
+```
+
+---
+
+### Clock
+**Purpose:** Set/get current date and time
+
+**Setting:**
+```
+$Clock,2025/12/24 14:30:00[CR][LF]
+
+Format: $Clock,YYYY/MM/DD HH:MM:SS[CR][LF]
+- Year: 2023-2079
+- Month: 1-12
+- Day: 1-31
+- Hour: 0-23
+- Minute: 0-59
+- Second: 0-59
+```
+
+**Request:**
+```
+Clock?[CR][LF]
+Response: 2025/12/24 14:30:00
+```
+
+---
+
+### Language
+**Purpose:** Set/get display language
+
+**Setting:**
+```
+$Language,English[CR][LF]
+
+Options:
+- Japanese
+- English
+- Germany
+- Spanish
+- French
+- Simplified Chinese
+- Korean
+```
+
+**Request:**
+```
+Language?[CR][LF]
+Response: English
+```
+
+---
+
+### Index Number
+**Purpose:** Set/get index number
+
+**Setting:**
+```
+$Index Number,0042[CR][LF]
+
+Format: 0000-9999 (4 digits)
+```
+
+**Request:**
+```
+Index Number?[CR][LF]
+Response: 0042
+```
+
+---
+
+### Key Lock
+**Purpose:** Enable/disable key lock
+
+**Setting:**
+```
+$Key Lock,Off[CR][LF]
+$Key Lock,On[CR][LF]
+```
+
+**Request:**
+```
+Key Lock?[CR][LF]
+Response: Off | On
+```
+
+---
+
+### Backlight
+**Purpose:** Control backlight ON/OFF
+
+**Setting:**
+```
+$Backlight,Off[CR][LF]
+$Backlight,On[CR][LF]
+```
+
+**Request:**
+```
+Backlight?[CR][LF]
+Response: Off | On
+```
+
+---
+
+### Backlight Auto Off
+**Purpose:** Set backlight auto-off timer
+
+**Setting:**
+```
+$Backlight Auto Off,Short[CR][LF]
+$Backlight Auto Off,Long[CR][LF]
+$Backlight Auto Off,Off[CR][LF]
+
+Options:
+- Short: 30 seconds
+- Long: 60 seconds
+- Off: Always on
+```
+
+**Request:**
+```
+Backlight Auto Off?[CR][LF]
+Response: Short | Long | Off
+```
+
+---
+
+### LCD Auto Off
+**Purpose:** Set LCD auto-off timer
+
+**Setting:**
+```
+$LCD Auto Off,Short[CR][LF]
+$LCD Auto Off,Long[CR][LF]
+$LCD Auto Off,Off[CR][LF]
+
+Options:
+- Short: 5 minutes
+- Long: 10 minutes
+- Off: Always on
+```
+
+**Request:**
+```
+LCD Auto Off?[CR][LF]
+Response: Short | Long | Off
+```
+
+---
+
+### Backlight Brightness
+**Purpose:** Set backlight brightness
+
+**Setting:**
+```
+$Backlight Brightness,1[CR][LF]
+
+Range: 1-4 (1=dimmest, 4=brightest)
+```
+
+**Request:**
+```
+Backlight Brightness?[CR][LF]
+Response: 1 | 2 | 3 | 4
+```
+
+---
+
+### Battery Type
+**Purpose:** Set battery type
+
+**Setting:**
+```
+$Battery Type,Alkaline[CR][LF]
+$Battery Type,NiMH[CR][LF]
+
+Options:
+- Alkaline: Alkaline batteries
+- NiMH: Nickel-metal hydride batteries
+```
+
+**Request:**
+```
+Battery Type?[CR][LF]
+Response: Alkaline | NiMH
+```
+
+---
+
+### Battery Level
+**Purpose:** Get current battery level
+
+**Request:**
+```
+Battery Level?[CR][LF]
+Response: 0-100  # Percentage (0=empty, 100=full)
+```
+
+**Note:** Read-only command
+
+---
+
+### Measure
+**Purpose:** Start/stop measurement
+
+**Setting:**
+```
+$Measure,Run[CR][LF]     # Start measurement
+$Measure,Stop[CR][LF]    # Stop measurement
+```
+
+**Request:**
+```
+Measure?[CR][LF]
+Response: Run | Stop
+```
+
+---
+
+### Pause
+**Purpose:** Pause/resume measurement
+
+**Setting:**
+```
+$Pause,On[CR][LF]      # Pause measurement
+$Pause,Off[CR][LF]     # Resume measurement
+```
+
+**Request:**
+```
+Pause?[CR][LF]
+Response: On | Off
+```
+
+---
+
+### Frequency Weighting (Main)
+**Purpose:** Set main frequency weighting
+
+**Setting:**
+```
+$Frequency Weighting,A[CR][LF]
+$Frequency Weighting,C[CR][LF]
+$Frequency Weighting,Z[CR][LF]
+
+Options:
+- A: A-weighting (most common)
+- C: C-weighting
+- Z: Z-weighting (flat response)
+```
+
+**Request:**
+```
+Frequency Weighting?[CR][LF]
+Response: A | C | Z
+```
+
+---
+
+### Time Weighting (Main)
+**Purpose:** Set main time weighting
+
+**Setting:**
+```
+$Time Weighting,F[CR][LF]
+$Time Weighting,S[CR][LF]
+$Time Weighting,I[CR][LF]
+
+Options:
+- F: Fast (125ms)
+- S: Slow (1000ms)
+- I: Impulse
+```
+
+**Request:**
+```
+Time Weighting?[CR][LF]
+Response: F | S | I
+```
+
+---
+
+### Baud Rate
+**Purpose:** Set RS-232C communication speed
+
+**Setting:**
+```
+$Baud Rate,9600[CR][LF]
+$Baud Rate,19200[CR][LF]
+$Baud Rate,38400[CR][LF]
+$Baud Rate,57600[CR][LF]
+$Baud Rate,115200[CR][LF]
+```
+
+**Request:**
+```
+Baud Rate?[CR][LF]
+Response: 9600 | 19200 | 38400 | 57600 | 115200
+```
+
+**Note:** Default is usually 38400
+
+---
+
+### Ethernet IP
+**Purpose:** Set IP address for LAN connection
+
+**Setting:**
+```
+$Ethernet IP,192.168.1.100[CR][LF]
+
+Format: xxx.xxx.xxx.xxx (0-255 for each octet)
+```
+
+**Request:**
+```
+Ethernet IP?[CR][LF]
+Response: 192.168.1.100
+```
+
+---
+
+### Ethernet DHCP
+**Purpose:** Enable/disable automatic IP address assignment
+
+**Setting:**
+```
+$Ethernet DHCP,Off[CR][LF]   # Manual IP
+$Ethernet DHCP,On[CR][LF]    # Automatic IP
+```
+
+**Request:**
+```
+Ethernet DHCP?[CR][LF]
+Response: Off | On
+```
+
+---
+
+### DOD (Display Output Data)
+**Purpose:** Get current displayed measurement value
+
+**Request:**
+```
+DOD?[CR][LF]
+
+Response format:
+ddd.d,cc.c,aa.a
+- ddd.d: Main display value
+- cc.c: Sub display value  
+- aa.a: Additional value
+```
+
+**Example Response:**
+```
+075.2,072.1,080.5
+# 75.2 dB main, 72.1 dB sub, 80.5 dB peak
+```
+
+---
+
+### DRD (Data Real-time Display)
+**Purpose:** Continuous output of measurement data
+
+**Request:**
+```
+DRD?[CR][LF]
+```
+
+**Stop continuous output:**
+```
+[SUB] (0x1A)
+```
+
+**Response:** Continuous stream of measurement values until stop requested
+
+---
+
+### USB Class
+**Purpose:** Set USB communication mode
+
+**Setting:**
+```
+$USB Class,Serial[CR][LF]         # Serial communication mode
+$USB Class,Mass Storage[CR][LF]   # Mass storage mode (file access)
+```
+
+**Request:**
+```
+USB Class?[CR][LF]
+Response: Serial | Mass Storage
+```
+
+**Important:** When in Mass Storage mode, communication commands are blocked
+
+---
+
+## 🎯 COMMON PROGRAMMING PATTERNS
+
+### Basic Connection Test
+```python
+# 1. Check device type
+send("Type?[CR][LF]")
+wait_for_response()  # Should get "NL-43" or "NL-53"
+
+# 2. Get version
+send("System Version?[CR][LF]")
+wait_for_response()  # e.g., "01.02.0034"
+
+# 3. Enable echo for debugging
+send("$Echo,On[CR][LF]")
+wait_for_response()  # Should get "R+0000"
+```
+
+### Start Basic Measurement
+```python
+# 1. Set frequency weighting to A
+send("$Frequency Weighting,A[CR][LF]")
+wait_1_second()
+
+# 2. Set time weighting to Fast
+send("$Time Weighting,F[CR][LF]")
+wait_1_second()
+
+# 3. Start measurement
+send("$Measure,Run[CR][LF]")
+wait_1_second()
+
+# 4. Get current reading
+send("DOD?[CR][LF]")
+reading = wait_for_response()
+```
+
+### Configure Network
+```python
+# 1. Disable DHCP for static IP
+send("$Ethernet DHCP,Off[CR][LF]")
+wait_1_second()
+
+# 2. Set IP address
+send("$Ethernet IP,192.168.1.100[CR][LF]")
+wait_1_second()
+
+# 3. Set subnet
+send("$Ethernet Subnet,255.255.255.0[CR][LF]")
+wait_1_second()
+
+# 4. Set gateway
+send("$Ethernet Gateway,192.168.1.1[CR][LF]")
+wait_1_second()
+
+# 5. Enable Ethernet
+send("$Ethernet,On[CR][LF]")
+wait_1_second()
+```
+
+### Continuous Data Acquisition
+```python
+# Start continuous output
+send("DRD?[CR][LF]")
+
+# Read data in loop
+while acquiring:
+    data = read_line()
+    process(data)
+
+# Stop when done
+send("[SUB]")  # 0x1A
+```
+
+---
+
+## ⚠️ COMMON ERRORS & TROUBLESHOOTING
+
+### R+0001 - Command Not Recognized
+- Check spelling (spaces matter!)
+- Verify command exists for your model
+- Check for extra/missing spaces in command name
+
+### R+0002 - Parameter Error
+- Verify parameter format matches spec
+- Check parameter value is in valid range
+- Ensure comma separator is present
+
+### R+0003 - Specification Error
+- Using `$` prefix on request command
+- Using `?` on setting command
+- Command type mismatch
+
+### R+0004 - Status Error
+- Device busy (measuring, storing, etc.)
+- Stop measurement before changing settings
+- Check device is powered on and ready
+- May need to wait longer between commands
+
+### No Response
+- Check baud rate matches (default 38400)
+- Verify `[CR][LF]` line endings
+- Wait minimum 1 second between commands
+- Check cable connection
+- Verify USB Class mode is "Serial" not "Mass Storage"
+
+---
+
+## 📝 NOTES FOR AI ASSISTANTS
+
+**When helping with code:**
+1. Always include the `$` prefix for setting commands
+2. Always include `?` suffix for request commands
+3. Always include `[CR][LF]` terminators
+4. Always wait ≥1 second between commands
+5. Check for `R+0000` success code after settings
+6. Handle error codes appropriately
+
+**For serial communication:**
+- Use 8 data bits, no parity, 1 stop bit (8N1)
+- Default baud: 38400
+- Hardware flow control: None
+- Hex codes: CR=0x0D, LF=0x0A, SUB=0x1A
+
+**For network communication:**
+- TCP port: Varies by configuration
+- Ensure device is in correct USB/Ethernet mode
+- Commands are identical over serial/network
+
+---
+
+## 📖 DOCUMENT SOURCE
+
+This reference was extracted from:
+- **Document:** NL-43/NL-53 Communication Guide
+- **Filename:** NL-43_NL-53_Communication_Guide_66132.pdf
+- **Manufacturer:** RION Co., Ltd.
+- **Full manual available at:** https://rion-sv.com/nl-43_53_63/manual/
+
+**Document Coverage:**
+- Full communication protocol specification
+- RS-232C and USB serial connection
+- Ethernet/LAN configuration
+- Complete command reference
+- Data output formats
+- Timing specifications
+
+For complete technical details, measurement specifications, and advanced features, refer to the full PDF documentation.