Modbus slave simulator for Windows

What a Modbus slave simulator is for

A Modbus slave, also called a server, is the device that holds the data: a meter, a drive, a PLC or a sensor. A master asks it for registers and coils. A Modbus slave simulator is a program that plays such a device. It answers requests with values you choose, so you can test a master without the real hardware.

That is useful in many situations: you write a SCADA screen before the equipment arrives, you test a PLC program that reads from a gateway, you check a script on a laptop, or you want to see what your master does when a device answers with an error. Hardware is often busy, far away or too risky to break on purpose. A simulator on your PC is none of these.

What Busscope Slave does

Busscope Slave is a Modbus slave simulator for Windows. It answers over Modbus TCP and over Modbus RTU on a serial port, and it can play several slaves at once, each with its own slave id and its own data.

• Several slave ids: simulate a whole bus of devices on one connection, or several devices behind one IP address.
• Editable values: change coils and registers while the simulator runs and watch the master follow.
• Changing values: let values move over time so a chart or alarm on the master side has something to show.
• Fault injection: make the simulator break answers on purpose, to see how your master, SCADA or PLC program copes.
• Traffic view: every request and response is listed, so you can see what your master really sends.
• Setup files: save a simulated plant and load it again later.

Test a Modbus TCP master on one computer

You do not need a second machine. Start Busscope Slave, open a Modbus TCP connection and let it listen. A master on the same computer connects to 127.0.0.1 and the port you chose. Your SCADA, a script or Busscope Master then sees a device that answers. Because the simulator and the master run on the same PC, a test takes a minute to set up.

Test a Modbus RTU master without a device

For Modbus RTU the simulator needs a serial port. With two real COM ports connected by a null-modem cable, or with a virtual COM port pair, the master uses one end and Busscope Slave the other. Baud rate, parity, data bits and stop bits can be set, and the frame gap can be raised for USB adapters and virtual ports, which tend to deliver bytes in bursts. See the Modbus RTU tester page for the master side of serial testing.

Break things on purpose

Real devices fail in ways that are hard to produce on demand. They stop answering, answer late, send an exception code, or report a value that makes no sense. A master that has only been tested against a healthy device often fails the first time something goes wrong in the field. Fault injection in Busscope Slave lets you produce these cases at will and check three things: does the master show a clear error, does it retry sensibly, and does it recover when the device comes back.

It pairs with the reference page on Modbus exception codes, which lists what each code means and how to fix it, and with the guide to Modbus TCP timeouts.

Other tools for the same job

You have choices, and the right one depends on what you need.

• Modbus Slave from Witte Software simulates up to 100 slave devices in 100 windows according to its product page, over TCP, RTU, ASCII and UDP variants, for $129 per license (one or two licenses) according to its order page.
• diagslave is a free, command-line slave simulator for Windows and Linux. Its page lists Modbus TCP, UDP, RTU, ASCII and RTU over TCP. It is a good fit for scripts and test benches with no user interface.
• Simply Modbus offers a TCP client, an RTU master and a slave simulator.

Busscope Slave has a graphical interface, a view of the traffic and fault injection in one program. It handles TCP and RTU only, so if you need ASCII or UDP on the slave side, one of the tools above fits better. Facts about other tools come from their own pages, read on 6 November 2026, and may have changed. See the comparison page.

Frequently asked questions

Do I need a real device to test against?

No. Busscope Slave plays the device. A Modbus TCP master on the same computer connects to 127.0.0.1. For Modbus RTU use a real COM port or a virtual COM pair.

Can I run a master and a slave on the same PC?

Yes for Modbus TCP. For Modbus RTU you need two connected serial ports, real or virtual.

Can I try it for free?

Yes. Without a license key Busscope Slave runs in evaluation mode. In evaluation mode the simulator keeps working, but fault injection and changing values switch off, and each connection stays up for a limited time. Evaluation use for production or commercial purposes is not permitted. See the download page.

What does it cost?

A license starts at $39 per user during the launch period, which ends on 31 December 2026. It is perpetual. Prices and quantity discounts are on the pricing page.

Does it support Modbus TLS or ASCII?

Not on the slave side. Busscope Slave answers over Modbus TCP and RTU. The master, Busscope Master, supports TLS and ASCII.