Modbus Simulator — User Guide

Version 1.1.2 · Windows 10/11 (64-bit)

Modbus Simulator runs as both a Modbus Master (polls devices) and a Modbus Slave (acts as a device) on the same PC. It supports Modbus TCP, RTU, ASCII, and UDP — all included in one license.

Installation

1. Download the installer from modbussimulator.com/download
2. Run ModbusSimulator-Setup.exe — no admin rights required for most installations
3. Follow the installation wizard (default path: C:\Program Files\ModbusSimulator\)
4. Launch from the Start Menu or desktop shortcut

First Launch

On first launch you'll see the main window with two tabs: Master and Slave. The 30-day trial starts automatically — no registration required.

1. Select the tab for the mode you need (Master or Slave)
2. Choose protocol: TCP or RTU
3. Configure the connection (IP + port for TCP, COM port + baud rate for RTU)
4. Click Start / Connect

Trial vs Licensed

Slave Mode — Overview

Slave mode makes your PC act as a Modbus device. SCADA, HMI, PLCs, and custom applications connect to it and read/write registers — exactly as they would with a real device.

When to use it:

• Test SCADA screens before hardware arrives
• Develop Modbus drivers without needing the physical device
• Simulate energy meters, drives, sensors, or I/O modules
• Test how your master handles exceptions and timeouts

TCP Slave Setup

RTU Slave Setup

Configure Registers

The register table shows all four register types. Click any cell to edit the value directly.

Simulate Exception Responses

To test how your master handles errors: right-click any register row → Set Exception Response → choose exception code:

• 01 — Illegal Function
• 02 — Illegal Data Address
• 03 — Illegal Data Value
• 04 — Slave Device Failure

The slave will return that exception code for all requests to that register until you clear it.

Master Mode — Overview

Master mode polls a connected Modbus slave device (PLC, energy meter, drive, I/O module, or another instance of this simulator) and lets you read and write registers.

TCP Master Setup

RTU Master Setup

1. Select RTU protocol → choose COM port
2. Set baud rate, parity, stop bits to match the slave device
3. Set Unit ID → click Connect

Read / Write Registers

To read registers:

1. Select function code: FC01 (coils), FC02 (discrete inputs), FC03 (holding), FC04 (input)
2. Enter starting address and quantity
3. Click Read — values appear in the response table

To write registers:

1. Select FC06 (write single) or FC16 (write multiple)
2. Enter starting address and the value(s)
3. Click Write

Modbus TCP TCP/IP

Modbus TCP encapsulates Modbus frames in TCP packets. Default port: 502. No framing (no CRC) — TCP handles error checking. Supports multiple simultaneous master connections.

Modbus RTU Serial

Modbus RTU uses binary framing over serial (RS-232 or RS-485). The frame includes a CRC checksum. All devices on the same bus must use the same baud rate and parity.

Modbus ASCII

ASCII mode encodes each byte as two hex characters. Slower than RTU but more robust on noisy lines. Uses LRC checksum. Select ASCII in the protocol dropdown — same serial settings as RTU.

Modbus UDP

UDP variant of Modbus TCP — no connection state, lower overhead, faster for broadcast. Configure the same as TCP but select UDP as protocol. Useful for multicast polling of multiple devices.

Virtual COM Ports (PC-only RTU testing)

To test RTU without physical hardware, create a virtual COM port pair:

1. Download and install com0com (free, open source)
2. Create a pair: COM10 ↔ COM11
3. In Modbus Simulator Slave: select COM10, baud 9600, no parity
4. In Modbus Simulator Master (or your application): select COM11, same settings
5. Frames sent on one side appear on the other — full RTU loop on a single PC

Multiple Slaves

Modbus Simulator supports up to 8 simultaneous slave instances. Each can have a different Unit ID, protocol, and register set.

1. In Slave mode, click Add Slave (top toolbar)
2. Configure each instance independently
3. Over TCP, all slaves share the same port — Unit ID routes requests to the right instance
4. Over RTU, each slave should be on a separate COM port (or the same bus with different Unit IDs)

Data Formats

Each register cell can display values in multiple formats. Right-click a cell to change:

• Unsigned Int (16-bit) — 0 to 65535
• Signed Int (16-bit) — -32768 to 32767
• Hex — 0x0000 to 0xFFFF
• Binary — 16 bits displayed
• Float32 — two consecutive registers treated as IEEE 754 float
• BCD — binary-coded decimal

Byte order for Float32: configurable in Settings → Big-Endian (AB CD) or Little-Endian (CD AB). Match your device's specification.

Request Log

In Slave mode, the Request Log panel (bottom of the window) shows every poll from the master in real-time:

• Timestamp
• Function code (FC03, FC06, etc.)
• Register address and quantity
• Value written (for write requests)
• Exception returned (if configured)

Click Export Log to save as CSV for debugging or documentation.

Troubleshooting — Connection Issues

Troubleshooting — No Response / Exception Codes

• Exception 01 (Illegal Function) — the device doesn't support that function code
• Exception 02 (Illegal Address) — register address out of range for that device
• Exception 03 (Illegal Data Value) — value outside allowed range
• Timeout / null response — device not reachable, wrong Unit ID, or slow device (increase timeout)

Use the Slave Simulator as a known-good device to verify your master is working correctly, then connect to your real device.

Frequently Asked Questions

How do I activate my license key?

Launch Modbus Simulator → click Enter License Key in the top menu or in the trial expiry prompt → paste your key → click Activate. Requires an internet connection for the first activation only.

Can I use the software on multiple PCs?

Each $99 license covers 1 PC. To use on a second PC, purchase an additional license or contact support for a team discount (3+ licenses at $79 each).

Will it work with my SCADA (Ignition, WinCC, AVEVA, etc.)?

Yes. The simulator exposes a standard Modbus TCP interface on port 502. Any SCADA system with a Modbus TCP driver will connect to it. Point the SCADA's Modbus device to 127.0.0.1:502 when both run on the same PC.

What Windows versions are supported?

Windows 10 (64-bit, version 1903+) and Windows 11. 32-bit Windows is not supported.

Can I simulate a Modbus device without any PLC or hardware?

Yes — that's the primary use case. Run the Slave mode, configure your register values, and your SCADA/HMI talks to it as if it were a real device. No PLC, no RS-485 adapter, no hardware of any kind required for TCP testing.

I need more help — how do I contact support?

Email [email protected] — we typically reply within 24 hours on business days. Include your OS version and a description of what you're testing.