Early access v0.9 · Modbus RTU

A whole rack of Modbus devices.
Zero hardware.

ModbusLab is a Modbus RTU slave simulator & device emulator that turns your PC into a whole rack of virtual Modbus devices. Test masters, commission SCADA systems and debug RS-485 buses with zero hardware beyond one USB / RS-485 adapter.

Start 3-day free trial See pricing
  • 3-day free trial, cancel before it bills
  • Full feature access
  • Multi-slave on one COM port
ModbusLab app icon ModbusLab | RTU Slave Simulator
Bus running
COM1 / 9600 / 8N2
0 received by native engine
15.0 ms before RS-485 TX
85 across four banks
RTU FC 01–06 / 0F / 10 / 11 / 2B
Register bankLive process values
Holding 53Input 32Coils 8
WIRE OFF.NAMEVALUEFORMATACCESS
4000Temperature235Int16R/W
4001Humidity482Int16R/W
4002Modbus address1UInt16R/W
4003Baud rate1UInt16R/W
4004Averaging [s]30UInt16R/W
4005Output 1: Y1400UInt16R/W
// CAPABILITIES

Built for the bench,
not the brochure.

Everything you need to stand in for real hardware while you build, test and commission.

Many slaves, one COM port

Run a whole bus of simulated devices over a single USB/RS-485 adapter. Each device has its own unit ID, register map and behaviour. No daisy-chained hardware.

01 … 247

All four register banks

Holding, input, coils and discrete inputs are fully editable with typed values (UInt16, Int16, floats, bit fields) and live recalculation while the bus is running.

0x · 1x · 3x · 4x

Live RTU frame inspector

Watch every request and response on the physical line, colour-coded and timestamped. Pause, scrub and export the capture to nail down timing bugs.

CRC-16 / MODBUS

Timing under your thumb

Inject response delays per device to reproduce slow sensors, congested buses and timeout edge cases before they bite you in the field.

0 – 10 000 ms

Device profiles & duplication

Model real hardware once, whether it is Lumel, Siemens or your own firmware, then clone it across unit IDs. Save profiles and rebuild whole test rigs in seconds.

PROFILES

Native engine, no Java lag

A Rust core answers frames in microseconds and keeps character-timing clean on real RS-485 lines. Lightweight, offline, and happy on a laptop in a cabinet.

RUST CORE
// THREE STEPS TO A RUNNING BUS

From adapter to answers
in under a minute.

01

Plug in the line

Connect any USB ↔ RS-485 adapter. ModbusLab detects the COM port, then you pick baud, parity and stop bits to match your master.

COM1 · 9600 · 8N1
02

Build your devices

Add slave profiles, assign unit IDs and fill the register banks. Duplicate a device to simulate an entire fleet in one click.

UNIT 01 · UNIT 07 · UNIT 12
03

Start the bus

Hit start and your master sees real devices. Every frame is answered by the native engine and logged to the inspector.

BUS RUNNING
// WHERE THE BUS IS HEADED

RTU today. The whole
Modbus family next.

ModbusLab is actively developed. Lifetime licenses include every future protocol and mode at no extra cost.

SHIPPED

Modbus RTU slave

Multi-slave simulation on one serial port, all four register banks, response-delay control and the live frame inspector.

IN DEVELOPMENT

Modbus TCP

Ethernet & Wi-Fi device emulation. Spin up virtual TCP slaves on your LAN with no adapter required.

PLANNED

Modbus ASCII

Legacy ASCII framing for older serial installations. Emulate them before you replace them.

PLANNED

Master mode

Flip the direction: poll real devices, scan buses and build automated test harnesses from the same interface.

PLANNED

macOS & Linux

Windows ships first. Native builds for macOS and Linux follow once the protocol core is complete.

// PRICING

Cheaper than one
RS-485 converter.

Every plan starts with a 3-day free trial of the full app with no feature locks. Cancel before the trial ends and you pay nothing.

Monthly

For ongoing lab and commissioning work.

$ 5 / month

First 3 days free, cancel before it bills

  • Unlimited simulated slave devices
  • All four register banks + live inspector
  • Response-delay & timing controls
  • Device profiles & duplication
  • All updates while subscribed
  • Cancel anytime
3-day trial, then $5/mo · secured by Stripe
MOST POPULAR

Lifetime

Pay once. Own every version we ever ship.

$ 30 one-time

Try the same app free for 3 days

  • Everything in Monthly
  • All future protocols included: TCP, ASCII & master mode at no extra cost
  • Lifetime updates, no renewals
  • Priority feature voting
  • 14-day money-back guarantee
One payment, forever · secured by Stripe
3-day trialfull features
Cancel anytimebefore it bills
14-day refundon lifetime
Offline applicense-key activated
// QUESTIONS FROM THE FIELD

FAQ

What is a Modbus slave simulator?

A Modbus slave simulator is software that pretends to be one or more real Modbus devices, including sensors, meters and PLCs, on a bus. Instead of wiring physical hardware, your master (SCADA, HMI or PLC) talks to the simulator and it answers requests exactly like the real device would. ModbusLab simulates Modbus RTU slaves over a standard USB/RS-485 adapter, so you can develop and test without any field hardware.

What hardware do I need to run ModbusLab?

A Windows 10/11 64-bit PC and any USB ↔ RS-485 adapter (or a built-in serial port). That's it. One adapter can carry a whole bus of simulated devices. Modbus TCP support, coming next, will need no adapter at all.

How do I test Modbus RTU communication without hardware?

Install a Modbus slave simulator like ModbusLab on a Windows PC, connect a USB ↔ RS-485 adapter and wire it into your two-wire line (or loop back to a second adapter on the same PC). Configure the unit IDs and registers your real devices would have, start the bus, and point your master at the line. It will poll the simulated devices as if they were physical.

Can ModbusLab simulate multiple Modbus devices at once?

Yes. Add as many slave profiles as you need. Each gets its own unit ID (1–247), register banks and response timing. The native engine answers every frame addressed to any of them on the shared COM port, exactly like physical devices wired together.

Which Modbus function codes are supported?

FC 01 (read coils), FC 02 (read discrete inputs), FC 03 (read holding registers), FC 04 (read input registers), FC 05 (write single coil), FC 06 (write single register), FC 0F / 10 (write multiple), FC 11 (report server ID) and FC 2B (read device identification).

Is Modbus TCP or Modbus master mode available yet?

Not yet. ModbusLab currently speaks Modbus RTU (slave). Modbus TCP is in active development, with ASCII and master mode on the roadmap. Lifetime licenses include every future protocol and mode at no extra cost, and monthly subscribers get all updates while subscribed.

Is ModbusLab an alternative to Modbus Poll or ModSim?

Yes, on the slave side. Modbus Poll acts as a master; ModbusLab complements or replaces slave-side tools like Modbus Slave, ModSim32 or Diagslave with a modern interface, many devices on one COM port, per-device response delays and a live frame inspector. A built-in master mode is on the roadmap. Try it free for 3 days.

How does the 3-day free trial work?

Enter your email and card to start a 3-day free trial with every feature unlocked. You won't be charged until the trial ends. Cancel anytime before that and you pay nothing. After the trial, the monthly subscription continues at $5/month, or upgrade to the one-time lifetime license.

Can I get a refund or cancel?

Monthly can be cancelled any time, and you keep access until the period ends. Lifetime comes with a 14-day, no-questions-asked money-back guarantee. Just reply to your receipt email.

Take the bus for a spin.

Get 3 days of full access. Enter your card and pay nothing until the trial ends.
If it doesn't save you a field trip, cancel before it bills.

FREE MODBUS RTU SLAVE SIMULATOR · v0.9.2 · x64 · ~38 MB · WINDOWS 10 / 11
macOS & Linux builds planned. See roadmap