Every value a SCADA system shows you - a pressure on a screen, a level in a trend, an alarm in a log - is a tag. The tag database is the master list that defines every one of them: what it is called, where it comes from, what units it is in, and how it behaves. It is the single most important configuration artifact in a SCADA system. This guide explains what tags and the tag database are, and why they matter.
What Is a Tag Database? in one line: A tag database is the central configuration store in a SCADA system that defines every data point (tag) it monitors - each tag's name, source device and address, data type, engineering units, scaling, alarm limits, and other metadata. It maps raw device addresses to meaningful, named process points.
A tag is a named representation of a single piece of process data - one instrument reading, one status, one setpoint. Rather than referring to 'Modbus holding register 40001 on device at 10.2.3.4,' a SCADA system defines a tag like WELL_3_TUBING_PRESS that points to that address, so operators and displays work with meaningful names instead of raw protocol addresses. The tag database is where that mapping and everything about the tag lives.
Tags come in types. Analog tags hold continuous values (pressures, temperatures, flows) with engineering units and scaling. Discrete or digital tags hold on/off states (a pump running, a valve open). Some systems add string tags, and derived or calculated tags whose value is computed from other tags rather than read from a device. Each tag typically carries a current value, a timestamp, and a data-quality flag indicating whether the reading is good, stale, or in error.
Beyond the address mapping, each tag entry carries the metadata that makes the value usable. Engineering units and scaling convert the raw device number (say a 0-65535 count or a 4-20 mA value) into a real pressure in psi. Alarm limits (high, high-high, low, low-low) define when the value is abnormal. Descriptions, area or asset assignments, poll rates or deadbands, and access permissions round out the record.
Because so much depends on it, the tag database is the backbone of a SCADA configuration - displays, trends, alarms, and reports all reference tags by name. Large systems have tens or hundreds of thousands of tags, and licensing is often counted in tags. This is why tag templating and structures (defining a reusable 'well' object with a standard set of tags, then instantiating it per well) are so valuable: they let you configure a hundred identical wellsites once instead of a hundred times.
A clean, well-structured tag database with consistent naming conventions is the difference between a SCADA system that scales and one that becomes unmaintainable. Consistent tag names (a naming standard like AREA_ASSET_MEASUREMENT) let anyone find a point, let templates apply cleanly, and let history and alarms be organized logically. A chaotic tag database, by contrast, makes every change risky and every new hire slow to onboard.
For a distributed operator adding wells continuously, templated tags are essential to onboarding new sites quickly. Merobix models field data as named tags with units, scaling, and alarm limits, reading them from Modbus, DNP3, OPC UA, EtherNet/IP, S7, and MQTT devices - so the same tag structure can front many identical sites and new pads are added by instantiation rather than one-off wiring.
A tag is a named data point representing a single process value - one instrument reading, status, or setpoint. Instead of referring to a raw protocol address, the SCADA system gives it a meaningful name and attaches its units, scaling, alarm limits, and current value with a timestamp and quality flag.
It contains the definition of every tag: the tag name, the source device and address, data type, engineering units and scaling, alarm limits, descriptions, area or asset assignment, poll rate or deadband, and access permissions. Displays, trends, alarms, and reports all reference these tags by name.
A consistent naming convention (like AREA_ASSET_MEASUREMENT) makes the system searchable, lets templates apply cleanly across identical sites, and keeps history and alarms organized. It is what allows a SCADA system to scale to tens of thousands of tags and many similar sites without becoming unmaintainable.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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