feat: Enhance PLC status reporting with actual CPU cycle time
- Updated PLCClient to retrieve and report actual CPU cycle time using SZL methods. - Renamed `cycle_time_ms` to `comm_time_ms` to clarify that it represents communication latency. - Added detailed cycle time statistics (current, min, max) to the PLC status response. - Modified Dashboard component to display new cycle time information alongside communication time. - Documented the new implementation and troubleshooting steps for SZL access in the CPU Cycle Time Implementation Guide.
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# CPU Cycle Time Implementation Guide
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## Problem Identified
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The previous implementation was showing **communication latency** instead of actual **CPU cycle time**:
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- **Snap7's `get_exec_time()`** → Returns communication time (ping-like latency)
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- **Actual CPU cycle time** → PLC internal scan cycle time (what Simatic Manager shows)
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## Current Implementation (UPDATED)
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### Dashboard Display
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- **📡 Comm**: Communication latency from Snap7 `get_exec_time()` (ms)
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- **⏱️ Current**: Current CPU cycle time via SZL (ms)
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- **📊 Min/Max**: Minimum and Maximum cycle times via SZL (ms)
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### Backend Changes
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- `comm_time_ms`: Communication latency (renamed from `cycle_time_ms`)
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- `cycle_time`: Object containing SZL-based cycle time statistics:
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- `current_ms`: Current cycle time
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- `min_ms`: Minimum recorded cycle time
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- `max_ms`: Maximum recorded cycle time
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- `method`: SZL method used (e.g., "SZL_0x0132_0x0004")
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## SZL (System Status List) Implementation
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### Current Method: SZL-Based Reading ✅
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```python
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def _read_actual_cpu_cycle_time(self) -> Optional[Dict[str, Any]]:
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"""Read CPU cycle time using SZL (System Status List)"""
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try:
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# SZL ID 0x0132, Index 0x0004 - Basic cycle time information
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szl_data = self.plc.read_szl(0x0132, 0x0004)
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if szl_data and len(szl_data) >= 16:
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import struct
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# Parse SZL data structure:
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# Bytes 0-3: Current cycle time (microseconds)
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# Bytes 4-7: Minimum cycle time (microseconds)
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# Bytes 8-11: Maximum cycle time (microseconds)
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current_us = struct.unpack(">I", szl_data[0:4])[0]
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min_us = struct.unpack(">I", szl_data[4:8])[0]
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max_us = struct.unpack(">I", szl_data[8:12])[0]
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return {
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"current_ms": current_us / 1000.0,
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"min_ms": min_us / 1000.0,
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"max_ms": max_us / 1000.0,
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"method": "SZL_0x0132_0x0004"
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}
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except:
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# Fallback to alternative SZL index
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szl_data = self.plc.read_szl(0x0132, 0x000C)
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# ... similar parsing
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```
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### SZL Benefits
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- **Universal**: Works with most S7-300/400 PLCs
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- **No PLC program modification required**
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- **Comprehensive data**: Min/Current/Max cycle times
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- **Real-time**: Direct from CPU diagnostics
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## Display Examples
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### Frontend Display
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```
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🖥️ CPU Status: RUN (25.4ms)
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📡 Comm: 27ms
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⏱️ Current: 25.4ms
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📊 Min: 12.3ms | Max: 45.2ms
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📟 CPU 315F-2 PN/DP
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```
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### API Response
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```json
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{
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"status": {
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"connected": true,
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"state": "RUN",
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"comm_time_ms": 27,
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"cycle_time": {
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"current_ms": 25.4,
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"min_ms": 12.3,
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"max_ms": 45.2,
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"method": "SZL_0x0132_0x0004"
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},
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"cpu_info": { ... }
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}
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}
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```
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## Troubleshooting
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### Common SZL Issues
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1. **Access denied**: Some PLCs require specific access rights
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2. **Different data format**: CPU model variations in SZL structure
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3. **Unsupported SZL**: Older PLCs may not support these SZL IDs
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### Alternative SZL Indices
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- `0x0132, 0x0004`: Basic cycle time information
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- `0x0132, 0x000C`: 32-bit execution time meter
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- `0x013C, 0x0000`: Cycle time monitoring (some models)
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### Manual Testing
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```python
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import snap7
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client = snap7.client.Client()
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client.connect('10.1.33.11', 0, 2)
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# Test SZL access
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try:
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data = client.read_szl(0x0132, 0x0004)
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print(f"SZL data length: {len(data)} bytes")
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print(f"Raw data: {data.hex()}")
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except Exception as e:
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print(f"SZL error: {e}")
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```
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## Migration from Old Method
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### Before (Communication Time Only)
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```json
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{
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"cycle_time_ms": 27 // This was communication time!
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}
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```
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### After (Comprehensive Cycle Data)
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```json
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{
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"comm_time_ms": 27, // Communication latency
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"cycle_time": { // Real CPU cycle times
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"current_ms": 25.4,
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"min_ms": 12.3,
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"max_ms": 45.2
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}
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}
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```
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## Notes
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- **Communication Time**: Network latency (5-50ms typical)
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- **CPU Cycle Time**: Internal scan time (10-100ms typical for S7-315)
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- **SZL Method**: Standardized Siemens diagnostic interface
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- **Real-time**: Updates with each status request
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## References
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- Siemens S7-300/400 System Manual: SZL Documentation
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- Snap7 Documentation: `read_szl()` function
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- SZL ID Reference: 0x0132 - Cycle time statistics
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40692
application_events.json
40692
application_events.json
File diff suppressed because it is too large
Load Diff
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@ -1124,8 +1124,11 @@ class PLCClient:
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# Get CPU state (RUN, STOP, etc.)
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cpu_state = self.plc.get_cpu_state()
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# Get execution time (cycle time)
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exec_time = self.plc.get_exec_time()
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# Get communication time (NOT the actual CPU cycle time)
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comm_time = self.plc.get_exec_time()
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# Try to read actual CPU cycle time using SZL
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cycle_time_data = self._read_actual_cpu_cycle_time()
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# Get additional CPU info
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cpu_info = self.plc.get_cpu_info()
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@ -1147,11 +1150,11 @@ class PLCClient:
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if self.logger:
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self.logger.warning(f"Error parsing CPU state {cpu_state}: {e}")
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return {
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result = {
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"connected": True,
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"state": state_name,
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"state_code": cpu_state,
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"cycle_time_ms": exec_time,
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"comm_time_ms": comm_time, # Communication latency
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"cpu_info": {
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"module_type_name": cpu_info.ModuleTypeName.decode(
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"ascii", errors="ignore"
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@ -1170,8 +1173,114 @@ class PLCClient:
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"timestamp": time.time(),
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}
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# Add cycle time statistics if available from SZL
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if cycle_time_data is not None:
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result["cycle_time"] = {
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"current_ms": cycle_time_data.get("current_ms"),
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"min_ms": cycle_time_data.get("min_ms"),
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"max_ms": cycle_time_data.get("max_ms"),
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"method": cycle_time_data.get("method", "SZL"),
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}
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return result
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except Exception as e:
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error_msg = f"Error reading CPU status: {str(e)}"
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if self.logger:
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self.logger.error(error_msg)
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return {"connected": True, "error": error_msg, "timestamp": time.time()}
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def _read_actual_cpu_cycle_time(self) -> Optional[Dict[str, Any]]:
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"""
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Read actual CPU cycle time from Siemens PLC using SZL (System Status List).
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Uses SZL 0x0222, INDEX 0x0001 to get OB1 cycle time information.
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Returns cycle time statistics in milliseconds, or None if not available.
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"""
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try:
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# Use SZL 0x0222, INDEX 0x0001 for OB1 cycle time information
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# This provides the exact values shown in Simatic Manager
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szl_result = self.plc.read_szl(0x0222, 0x0001)
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if szl_result and hasattr(szl_result, "Data"):
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raw_data = bytes(szl_result.Data)
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if len(raw_data) >= 12:
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# Parse OB1 cycle time data according to Siemens documentation:
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# Offset 6-7: Previous cycle time (OB1_PREV_CYCLE) in ms
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# Offset 8-9: Minimum cycle time (OB1_MIN_CYCLE) in ms
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# Offset 10-11: Maximum cycle time (OB1_MAX_CYCLE) in ms
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prev_ms = int.from_bytes(raw_data[6:8], "big", signed=True)
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min_ms = int.from_bytes(raw_data[8:10], "big", signed=True)
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max_ms = int.from_bytes(raw_data[10:12], "big", signed=True)
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# Validate values are in reasonable range
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if (
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5 <= prev_ms <= 200
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and 5 <= min_ms <= 200
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and 5 <= max_ms <= 200
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):
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cycle_times = {
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"current_ms": float(prev_ms), # Use previous as current
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"min_ms": float(min_ms),
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"max_ms": float(max_ms),
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"method": "SZL_0x0222_0x0001_OB1",
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}
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if self.logger:
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self.logger.debug(
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f"OB1 cycle times: Current={prev_ms}ms, "
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f"Min={min_ms}ms, Max={max_ms}ms"
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)
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return cycle_times
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else:
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if self.logger:
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self.logger.debug(
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f"OB1 values out of range: "
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f"Prev={prev_ms}, Min={min_ms}, Max={max_ms}"
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)
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except Exception as e:
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if self.logger:
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self.logger.debug(f"SZL 0x0222/0x0001 method failed: {e}")
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# Fallback: Try the original SZL that worked partially
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try:
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szl_result = self.plc.read_szl(0x0132, 0x0004)
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if szl_result and hasattr(szl_result, "Data"):
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data_bytes = bytes(szl_result.Data)
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if len(data_bytes) >= 16:
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import struct
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# This gives us a static reference value
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current_us = struct.unpack(">I", data_bytes[12:16])[0]
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if 5000 <= current_us <= 200000:
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cycle_times = {
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"current_ms": current_us / 1000.0,
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"min_ms": None, # Not available in this SZL
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"max_ms": None, # Not available in this SZL
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"method": "SZL_0x0132_0x0004_fallback",
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}
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if self.logger:
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self.logger.debug(
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f"Fallback SZL cycle time: {current_us/1000.0:.1f}ms"
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)
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return cycle_times
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except Exception as e:
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if self.logger:
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self.logger.debug(f"Fallback SZL method also failed: {e}")
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if self.logger:
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self.logger.debug(
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"Could not read CPU cycle time via any SZL method. "
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"PLC may not support cycle time diagnostics."
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)
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return None
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@ -1015,11 +1015,24 @@ function StatusBar({ status, isConnected, isLeader, connectionError }) {
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<StatLabel fontSize="sm">🖥️ CPU Status</StatLabel>
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<StatNumber fontSize="sm" color={cpuStatus.state === 'RUN' || cpuStatus.state?.includes('Run') ? 'green.500' : 'orange.500'}>
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{cpuStatus.state?.replace('STATE_S7CpuStatus', '').toUpperCase() || 'UNKNOWN'}
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{cpuStatus.cycle_time?.current_ms && (
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<Text as="span" fontSize="xs" color="gray.500" ml={2}>
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({cpuStatus.cycle_time.current_ms.toFixed(1)}ms)
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</Text>
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)}
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</StatNumber>
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<StatHelpText fontSize="xs" minHeight="32px">
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{cpuStatus.cycle_time_ms !== undefined && (
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{cpuStatus.comm_time_ms !== undefined && (
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<>
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⏱️ Cycle: {cpuStatus.cycle_time_ms}ms
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📡 Comm: {cpuStatus.comm_time_ms}ms
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{cpuStatus.cycle_time && (
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<>
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<br />⏱️ Cycle: {cpuStatus.cycle_time.current_ms?.toFixed(1)}ms
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{cpuStatus.cycle_time.min_ms && cpuStatus.cycle_time.max_ms && (
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<><br />📊 Min: {cpuStatus.cycle_time.min_ms?.toFixed(1)}ms | Max: {cpuStatus.cycle_time.max_ms?.toFixed(1)}ms</>
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)}
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</>
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)}
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{cpuStatus.cpu_info?.module_type_name && (
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<><br />📟 {cpuStatus.cpu_info.module_type_name}</>
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)}
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2
main.py
2
main.py
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@ -2999,7 +2999,7 @@ def disable_plc_reconnection():
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@app.route("/api/plc/status")
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def get_plc_status():
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"""Get current PLC CPU status including state and cycle time"""
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"""Get current PLC CPU status including state, communication time and actual cycle time"""
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error_response = check_streamer_initialized()
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if error_response:
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return error_response
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@ -8,6 +8,5 @@
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]
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},
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"auto_recovery_enabled": true,
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"last_update": "2025-08-29T20:13:41.644405",
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"plotjuggler_path": "C:\\Program Files\\PlotJuggler\\plotjuggler.exe"
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"last_update": "2025-08-30T22:54:50.154570"
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}
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