Free OPC UA Client: 6 Tools Compared and Tested

You have an OPC UA server on a PLC, and you need to confirm it exposes what the vendor promised. So you need a client.

The good OPC UA clients are free. That is not the hard part. The hard part is that your first connection almost always fails — and not for a reason any tool comparison mentions.

This guide covers the six clients worth installing, and the certificate problem that stops most people before they read a single value.

An OPC UA client connects to an OPC UA server and interacts with its data: browsing the address space, reading and writing node values, calling methods, and subscribing to value changes. The protocol is defined in IEC 62541.

What Actually Blocks You: The Certificate Handshake

OPC UA is not Modbus. Entering an IP address and a port is not enough. The protocol requires mutual X.509 certificate authentication as soon as you leave the no-security mode.

Here is what happens on a first attempt:

1. The client requests the endpoint list from the server
2. The client receives the server certificate and refuses to trust it
3. You accept the server certificate on the client side
4. The client sends its own certificate — and the server rejects it
5. The client certificate lands in a rejected folder on the server
6. You have to move it manually into the trusted folder

Step 6 is the one nobody documents. Until you do it, you loop.

The standard file-based certificate store uses four folders: own for your own certificate and private key, trusted for certificates you accept, issuers for intermediate CAs, and rejected for everything that failed. Both ends have their own store. A connection needs each side to trust the other.

Error Codes and What They Really Mean

The Method That Works

Always start with SecurityPolicy: None and MessageSecurityMode: None. If that connects, the network path and the server are fine. You have eliminated the firewall, the address, and the port in one step.

Only then move up to Basic256Sha256 or Aes256_Sha256_RsaPss and deal with certificates. Many servers disable no-security mode in production, but during commissioning it is almost always available — and it is your best diagnostic tool.

One trap worth knowing: Basic128Rsa15 and Basic256 are deprecated in the specification. Some older servers offer only those, and some newer clients refuse to connect to them. If you see no compatible endpoint at all, that is usually why.

Comparison Table

The GUI Clients

UaExpert

UaExpert is the reference tool. If you install only one client, install this one.

It browses the address space, reads and writes node values, calls methods, manages subscriptions, and covers the Data Access, Alarms & Conditions, and Historical Access service sets. Plugins add XML NodeSet export, performance testing, and data logging.

The interface is built on Qt and split into panes: project, address space, attributes, references, and a central area for specialized views such as DataAccess, Alarms, and Historical Trend.

Two practical notes. Registration is required to download, but it is free with no functional limits. And the certificate store sits in the application configuration folder — that is where you go to find the client certificate the server needs to trust.

Vendor documentation from Siemens and ABB routinely names UaExpert as the tool to validate an OPC UA server, which makes it the de facto interoperability reference.

Prosys OPC UA Client

Prosys OPC UA Client discovers servers, lists endpoints, lets you pick the security policy and message mode, then browses the address space. It reads and writes values on demand or cyclically, configures subscriptions, calls server methods, and reads historical data when the server provides it.

What sets it apart is its logging and diagnostic views. When a connection fails, it tells you why in usable terms, where other clients return a raw status code. For troubleshooting between an OT device and a SCADA system or historian, that is where it saves the most time.

Prosys also ships a free Simulation Server, covered further down. Together they make a complete test bench with no hardware at all.

Softing OPC UA Demo Client

Softing OPC UA Demo Client is a free demonstration client for validating a connection and browsing an address space.

It is lighter than UaExpert and less complete. Keep it for a quick sanity check, or as a second opinion when one client refuses a connection another accepts — implementations do not react identically to a badly configured server, and comparing two of them narrows the problem fast.

opcua-client-gui

opcua-client-gui is a PyQt graphical client from the FreeOpcUa project, built on the Python library. It handles browsing, subscriptions, and variable writes.

Its value is not the interface, which is rougher than UaExpert. Its value is that you are already inside the Python ecosystem: when manual testing stops being enough, you move to a script using the same library, with nothing new to learn.

License is GPL-3.0 for the GUI tools and LGPL-3.0 for the core libraries. That distinction matters if you ship code inside a product delivered to a customer.

opcua-commander

opcua-commander is a terminal-based client built on node-opcua, under the MIT license.

The use case is narrow and badly underrated: you are on an SSH session into a Linux gateway inside a cabinet, with no graphical desktop, and you need to confirm an OPC UA server is answering. No GUI client helps you there. This one runs in the terminal.

On a remote site, that can save a trip.

Matrikon OPC UA Explorer

Matrikon OPC UA Explorer is a free test client for OPC UA servers, from Honeywell.

Do not confuse it with Matrikon OPC Explorer, which is an OPC Classic tool — DA, HDA, and A&E over COM/DCOM — and does not speak OPC UA at all. The two products carry nearly identical names and come from the same vendor. The mix-up is extremely common, including in tool roundups.

If your plant mixes OPC Classic and OPC UA, having both tools from one vendor keeps things simple.

Free SDKs for Developers

When manual testing stops being enough, you write code. Four free, serious stacks:

One thing to watch: python-opcua is deprecated. Plenty of tutorials still reference it. Use opcua-asyncio for anything new.

MPL 2.0 and MIT are the most permissive of the four for commercial products. LGPL and EPL need more attention before you build a product on them.

Testing Without Hardware

You do not always have a PLC in front of you. Three free OPC UA servers for working dry:

Prosys OPC UA Simulation Server — free, exposes a full address space with values that move. The quickest to get running.

open62541 demo server — included in the project examples. Good for testing an embedded stack.

opcua-asyncio — a few lines of Python expose a server with your own nodes. Useful when you want to reproduce the exact structure of a real device.

This is also the right way to practice with certificates. You break and repair the security configuration as many times as you need, without touching a live installation.

What to Check Before You Choose

The first three are non-negotiable. The rest depends on your work.

What Free Clients Do Not Do

No industrial data collection. A test client is not a data server. Feeding a historian or a SCADA system continuously needs a gateway or a platform.

No advanced scripting in the GUI clients. Replaying a test sequence means dropping to an SDK.

No support. Nobody to call when a connection refuses to establish the night before a commissioning.

No certificate fleet management. On an installation with twenty OPC UA servers, managing trust stores by hand becomes unworkable. That is the problem commercial certificate management infrastructure solves, and it is a real one rather than a sales argument.

For testing, troubleshooting, learning, and development, free is enough. Engineers who work in OPC UA daily generally keep UaExpert and one SDK, and nothing else.

Frequently Asked Questions