Modbus PLC Simulator

Overview

This project demonstrates Serial Studio's Modbus TCP/RTU connectivity using a physics-based hydraulic test stand simulator. The simulator acts as a Modbus TCP server, providing realistic industrial telemetry data with automatic failure modes and recovery sequences.

This example showcases Serial Studio's Pro feature for Modbus protocol support, including real-time data acquisition, custom frame parsing, and industrial dashboard visualization.

Note: Modbus support requires a Serial Studio Pro license. Visit serial-studio.com for more details.

Simulated System

The simulator models a 50HP hydraulic power unit with the following components:

Hardware Model

Motor: 50HP VFD-controlled induction motor (0-3600 RPM)
Pump: 45cc/rev 9-piston axial pump with realistic efficiency curves
Control Valve: Proportional valve with first-order lag response
Sensors: Temperature, pressure, flow, vibration, and motor load monitoring

Physics Simulation

VFD Soft-Start: S-curve ramp from 0 to 1800 RPM over 10 seconds
PID Pressure Control: Maintains target pressure of 1500 PSI
Thermodynamic Model: Oil heating from pump inefficiency and valve throttling
Vibration Analysis: ISO 10816-compliant vibration monitoring
Realistic Noise: Ornstein-Uhlenbeck sensor noise on all measurements

Operational Phases

The simulator automatically cycles through realistic test sequences:

STARTUP: VFD soft-start with smooth S-curve acceleration
RUNNING: Normal operation with PID pressure control and load cycling
PRESSURE_TEST: High-pressure stress test every 60 seconds
FAILURE: Random pump cavitation with RPM/pressure instability (~0.02% probability)
SHUTDOWN: Emergency stop with controlled motor coast-down
RESTART_WAIT: E-STOP hold period before automatic restart

Modbus Register Map

The simulator provides 9 holding registers (function code 0x03) starting at address 0:

Address	Name	Range	Units	Description
HR[0]	E-Stop	0-1	-	Emergency stop status (0=Normal, 1=E-Stop)
HR[1]	Start LED	0-1	-	Motor running indicator (0=Off, 1=Running)
HR[2]	Temperature	72-180	°F	Hydraulic oil temperature
HR[3]	Pressure	0-3000	PSI	System pressure (alarm at 2800 PSI)
HR[4]	Motor RPM	0-3600	RPM	Motor speed
HR[5]	Valve Position	0-100	%	Control valve opening percentage
HR[6]	Flow Rate	0-500	GPM×10	Pump flow rate (divide by 10)
HR[7]	Motor Load	0-100	%	Motor load percentage
HR[8]	Vibration	0-150	mm/s×10	Vibration velocity RMS (divide by 10)

Requirements

Python Dependencies

bash

pip install pymodbus

The simulator supports both pymodbus 3.x and 4.x automatically.

Serial Studio

Serial Studio Pro (for Modbus support)
Compatible with both Modbus TCP and Modbus RTU modes

How to Run

1. Start the PLC Simulator

Run the Python script to start the Modbus TCP server:

bash

python3 plc_simulator.py

Expected output:

======================================================================
  HYDRAULIC TEST STAND SIMULATOR
======================================================================
  Server: 0.0.0.0:5020  |  Update: 50ms (20Hz)

  Registers (FC03 Holding):
    0:E-Stop  1:Start  2:Temp(°F)  3:PSI  4:RPM  5:Valve(%)
    6:GPM(÷10)  7:Load(%)  8:Vibration(÷10 mm/s)

  Phases: STARTUP → RUNNING → PRESSURE_TEST → (FAILURE → SHUTDOWN)
  Flags:  E=E-Stop  A=Alarm(>2800PSI)  F=Failure
======================================================================

[STARTUP ] -   R:   0 P:  14 F: 0.0 L: 0 V:0.1 T: 72
[STARTUP ] -   R: 180 P: 110 F: 4.8 L:17 V:1.8 T: 72
...

2. Configure Serial Studio

Load the project file:
- Open Serial Studio
- File → Open → Select Modbus PLC Simulator.ssproj
Configure Modbus connection:
- I/O Interface: Modbus
- Protocol: Modbus TCP
- Host: 127.0.0.1 (or the IP address of the machine running the simulator)
- Port: 5020
- Slave Address: 1
- Register Type: Holding Registers (0x03)
- Start Address: 0
- Register Count: 9
- Poll Interval: 100 ms (recommended)
Connect:
- Click Connect in Serial Studio
- The dashboard will begin displaying live telemetry

Serial Studio Visualizations

The included project file (Modbus PLC Simulator.ssproj) provides:

Status LEDs: Emergency Stop and Running indicators
Temperature Bar: Oil temperature with alarm thresholds
Pressure Gauge: System pressure with high alarm at 2800 PSI
RPM Gauge: Motor speed with color-coded zones
Valve Position Bar: Control valve opening percentage
Flow Rate Graph: Real-time pump flow monitoring
Motor Load Gauge: Motor load percentage
Vibration Graph: ISO 10816 vibration monitoring
Time-Series Plots: Trend visualization for all parameters

Frame Parser

The project includes a comprehensive JavaScript frame parser that:

Handles all Modbus function codes (0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x10)
Parses both register and coil/discrete input responses
Applies scaling factors (Flow Rate and Vibration are divided by 10)
Handles Modbus exception responses (0x80+)
Maintains last known good values on errors

The parser is embedded in the .ssproj file and requires Binary (Direct) decoder mode.

Files Included

plc_simulator.py: Physics-based hydraulic test stand simulator with Modbus TCP server
Modbus PLC Simulator.ssproj: Serial Studio project file with dashboard configuration
README.md: This documentation
doc/screenshot.png: Dashboard screenshot (to be added)

Observing Failure Modes

The simulator includes realistic failure sequences that occur randomly (~0.02% probability per update):

Cavitation Detection: Watch for rapid RPM oscillations (±30-50 RPM)
Pressure Spike: Pressure rises toward relief valve setting (3000 PSI)
High Pressure Alarm: Triggers at 2800 PSI
Vibration Spike: Cavitation causes vibration to exceed 11 mm/s (unacceptable zone)
E-Stop Activation: Automatic emergency stop after 3 seconds of instability
Controlled Shutdown: Motor coast-down and pressure bleed
Auto-Restart: System restarts after 3-second safety delay

To force a failure for testing, modify the FAILURE_PROBABILITY constant in plc_simulator.py (set to 0.1 for ~10% chance).

Technical Details

Update Rate

Simulator runs at 20 Hz (50ms update interval)
Recommended Serial Studio poll interval: 100ms to avoid overwhelming the interface

Network Configuration

Default Port: 5020 (can be changed in SERVER_PORT constant)
Binding: 0.0.0.0 (accepts connections from any network interface)
Protocol: Modbus TCP (application protocol)

Modbus Compatibility

Fully compliant with Modbus TCP specification
Compatible with any Modbus client (not limited to Serial Studio)
Supports concurrent connections (standard Modbus TCP server)

Advanced Usage

Custom Register Groups

Serial Studio supports multi-group mode for polling non-contiguous register ranges. To poll specific registers:

Enable Multi-Group Mode in the Modbus setup pane
Add register groups with specific start addresses and counts
Configure custom frame parsing for mixed data types

Integration with Real PLCs

This simulator can be used as a template for connecting to real industrial PLCs:

Modify the register map to match your PLC's register layout
Update the frame parser scaling factors as needed
Adjust polling intervals based on PLC response times
Configure Modbus RTU for serial-connected PLCs (RS-485)

Troubleshooting

Problem: Serial Studio shows "Connection Failed"

Verify the simulator is running (python3 plc_simulator.py)
Check firewall settings allow TCP port 5020
Confirm the host IP address is correct

Problem: No data updates in Serial Studio

Verify poll interval is set (recommended: 100ms)
Check that register count is 9 and start address is 0
Review frame parser for errors in the JavaScript console

Problem: Erratic readings or NaN values

Ensure decoder mode is set to Binary (Direct)
Verify the frame parser is properly embedded in the project file
Check for Modbus exception responses in the simulator log

License

This example is dual-licensed under GPL-3.0 and the Serial Studio Commercial License.

For more information about Serial Studio and Modbus integration, visit the Serial Studio documentation.