# What is a PLC gateway?

> A PLC gateway reads data from PLCs and other controllers in their own protocols and makes it available to other systems in a common one, such as OPC UA, MQTT or Modbus TCP. This guide explains how it works, where it is used and what to look for.

## How a PLC gateway works

PLCs speak different protocols: Siemens S7, Modbus, Mitsubishi MC, Omron FINS, EtherNet/IP and many more. A gateway polls or subscribes to each controller in its native protocol, turns raw addresses into named tags with units and quality, and publishes them on an open interface. MES, ERP, SCADA, historians and analytics then read one source instead of talking to every machine.

## Hardware or software gateway?

Hardware gateways are small boxes, usually one per machine or per protocol pair, and suit simple conversions such as Modbus RTU to Modbus TCP. Software gateways run on an industrial PC, a server or an ARM device and handle many machines and protocols at once, with tag browsing, security, history and central configuration.

## Why OPC UA?

OPC UA is the common way for MES, ERP and SCADA to read industrial data: a browsable address space, data types, encryption with certificates and user accounts. A gateway that exposes every machine through one OPC UA server gives these systems a single, secure endpoint.

## Where PLC gateways are used

- **Old and new machines together:** Machines of different brands and generations deliver their data in one structure, without replacing their controllers. ([Learn more](https://plcgateway.com/protocols.md))
- **MES and ERP integration:** Production counts, machine states and process values reach MES and ERP through one OPC UA endpoint. ([Learn more](https://plcgateway.com/connect.md))
- **MQTT, IIoT and cloud:** Values are published to an MQTT broker or a REST API for IIoT platforms, dashboards and data lakes. ([Learn more](https://plcgateway.com/integrations.md))
- **AI on plant data:** With an MCP server, AI assistants read live values, history and alarms and answer questions about the plant. ([Learn more](https://plcgateway.com/ai-mcp.md))

## What to look for in a PLC gateway

- **Protocol coverage:** Native drivers for the controllers you have today, including older models, plus OPC UA and OPC DA for everything else.
- **Security:** Encrypted OPC UA with certificates, user roles, and write access only where it is really needed.
- **Engineering effort:** Browse tags instead of typing addresses, import tag lists from CSV, and test a connection in seconds.
- **Outputs:** OPC UA plus MQTT, Modbus TCP, REST or webhooks, so new systems can be added later without new interfaces.

## Questions about PLC gateways

### What is the difference between a PLC gateway and an OPC UA server?

An OPC UA server is one of the outputs of a gateway. The gateway is the part that talks to the PLCs in their own protocols; many gateways, including PLCGateway Connect, then publish the data through a built-in OPC UA server.

### Do I have to change the PLC program?

Usually not. The gateway reads existing data blocks, registers or tags. Some controllers need access to be enabled first, for example PUT/GET on the Siemens S7-1200 and S7-1500.

### Can a PLC gateway write values to the machine?

Yes, when writing is allowed for that tag and that user. Writes should be limited to values that are meant to be changed, such as setpoints or recipe numbers.

### Does the data have to go to the cloud?

No. A software gateway can run entirely on your premises; MQTT and cloud outputs are optional.

## See the gateway with your own machines

Tell us which controllers you run. We will show you PLCGateway Connect with your protocols and your use case. → https://plcgateway.com/contact

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Source: https://plcgateway.com/plc-gateway