How to Use Modbus Poll: Step-by-Step Setup Tutorial

Modbus Poll by Witte Software is one of the most widely used Modbus master applications for polling and testing Modbus TCP, RTU, and ASCII devices. If you've just downloaded it and aren't sure where to start, this walkthrough covers the full setup — from connecting to a device to reading your first register — along with what to do once your testing needs grow beyond simple polling.

What You Need Before You Start

• Modbus Poll installed (Windows, 30-day free tier available from the vendor)
• A Modbus device to connect to — a real PLC, energy meter, or gateway, or a slave simulator if you don't have hardware on hand yet
• The device's connection details: IP address and port for Modbus TCP, or COM port, baud rate, and parity for Modbus RTU/ASCII
• The register map you want to read: slave/unit ID, function code, starting address, and quantity

If you don't have a physical device available, you can point Modbus Poll at a slave simulator running on the same PC or LAN — see how to simulate a Modbus slave without a PLC for a walkthrough of setting one up in a few minutes.

Step 1: Open Connection Setup

Launch Modbus Poll and open Connection → Connect (or the connection icon on the toolbar). This opens the Connection Setup dialog where you choose the transport and target device.

Step 2: Choose the Connection Type

Modbus Poll supports three connection modes:

• Modbus TCP/IP — enter the slave device's IP address and port (502 by default)
• Modbus RTU — select the COM port and configure baud rate, data bits, parity, and stop bits to match the device (9600 8E1 is the most common default)
• Modbus ASCII — same serial parameters as RTU, but with ASCII framing instead of binary

These settings must match exactly on both ends of the connection. A mismatched baud rate or parity setting is the most common reason a serial connection fails silently — no error, just no data.

Step 3: Create a Poll Definition

Once connected, open Setup → Read/Write Definition (or use the New Window icon) to create your first poll. You'll configure:

• Slave ID — the target device's Modbus unit/slave address (1–247)
• Function — the function code to use: 01 (Read Coils), 02 (Read Discrete Inputs), 03 (Read Holding Registers), 04 (Read Input Registers), 05/06/15/16 for writes
• Address — the starting register address (0-based in the dialog, even though documentation often shows 1-based labels like 40001)
• Quantity — how many consecutive registers or coils to read, up to 125 for a single FC03/FC04 request

Click OK and Modbus Poll opens a window that polls this definition continuously (the default interval is 1000ms, adjustable under Connection → Auto Poll Interval).

Step 4: Read the Values

Each row in the poll window corresponds to one register or coil. By default, values display as signed 16-bit decimal. Right-click any cell to change the display format — unsigned, hex, binary, float (for 32-bit values spanning two registers), or long integer. If your readings look wrong (garbage values, wildly out-of-range numbers), the cause is almost always a byte-order or data-type mismatch rather than a communication problem — see our guide on Modbus byte order for how to diagnose it.

Step 5: Write to a Register (Optional)

To write a value, double-click a cell in a window configured with a write-capable function code (05, 06, 15, or 16), or use Setup → Write Multiple Registers/Coils to send a batch write in a single frame. Writes take effect immediately — there's no confirmation step, so double-check the slave ID and address before sending anything to a live production device.

Step 6: Add More Poll Windows

You can open multiple poll windows simultaneously, each targeting a different slave ID, function code, or address range — useful when a single device exposes both holding registers and input registers, or when you're polling several slaves on the same serial bus. Modbus Poll cycles through all open windows on the shared connection.

Common Setup Issues

Symptom	Likely Cause
Connection times out immediately	Wrong IP/port, or nothing listening on the target port — see Modbus connection refused
Serial connection shows no error but no data updates	Baud rate, parity, or stop bits mismatched between master and slave
Illegal Data Address exception	Starting address or quantity out of range for that register type on the device
Values update but look nonsensical	Wrong display format selected, or byte-order mismatch on multi-register values
Works once, then stops responding	Slave device dropped the connection or hit an internal limit on request rate

What Modbus Poll Doesn't Do

Modbus Poll is a master-only tool — it polls and writes to existing devices, but it cannot simulate a slave device itself. If you need to test your own master application, SCADA driver, or HMI screen before real hardware is available, you'll need a separate slave simulator. Witte Software sells one as a companion product (Modbus Slave, licensed separately); some engineers instead use an all-in-one tool that includes both master polling and slave simulation in a single license. Our Modbus Poll alternative comparison covers the tradeoffs, and our breakdown of Modbus Poll's pricing covers what the master+slave combination actually costs.

Frequently Asked Questions

Is Modbus Poll free to use?

Modbus Poll offers a 30-day free tier with full functionality. After the trial, a license is required to continue using it. It is master-only — simulating a slave device requires the separate Modbus Slave tool from the same vendor. See our review of what the free download actually includes.

How do I connect Modbus Poll to a device?

Open Connection Setup from the toolbar, choose Modbus TCP/IP or a serial mode (RTU/ASCII), enter the target IP address and port (TCP) or COM port and baud rate (serial), then click OK to connect.

What is a poll definition in Modbus Poll?

A poll definition is a configured read/write request: slave ID, function code, starting address, and quantity of registers. Each open window in Modbus Poll represents one poll definition that runs continuously at a set interval.

Can Modbus Poll simulate a slave device?

No. Modbus Poll is a master (client) application only. To simulate a slave device for testing your own master software or SCADA system, you need a separate slave simulator — either the vendor's own Modbus Slave tool or an all-in-one tool like ModbusSimulator that includes both roles.

Related Guides

• Best Modbus Poll Alternative — side-by-side feature and pricing comparison
• Modbus Poll Pricing — what master + slave actually costs
• Modbus Poll Free Download Review — trial limits explained
• How to Simulate a Modbus Slave Without a PLC — set up a target device to poll against