Glossary of historian and SCADA terms
48 terms used around industrial data, OPC UA, Modbus and battery storage, explained plainly for engineers and the people who work with them.
- Acknowledged value
- A value the historian has made durable, usually by writing it to a log on disk. Until a value is acknowledged it can be lost in a power cut; afterwards it should survive one.See also: Write-ahead log, fsync
- Aggregate
- A value calculated over an interval from raw samples: minimum, maximum, mean, count or a time-weighted average. Trends over long periods draw aggregates because there are more samples than pixels.See also: Interpolation, Envelope
- Asset model
- The plant described as areas, units and equipment, with each signal placed under the thing it measures. It lets people find "Inverter 3, active power" instead of a PLC address.See also: Tag
- Basic256Sha256
- An OPC UA security policy using RSA keys of up to 4,096 bits and SHA-256. Widely supported by controllers such as the Siemens S7-1500; newer AES-based policies are replacing it.See also: Security policy, OPC UA
- BESS
- Battery energy storage system. On the grid, usually containers of battery racks with a power conversion system, connected through a transformer and switchgear, providing trading and grid services. Read what to log on a battery site.See also: PCS, BMS, SoC
- BMS
- Battery management system. Monitors cell voltages and temperatures, state of charge and health, and protects the battery by limiting charge and discharge or opening contactors.See also: BESS, SoC, SoH
- Calculated tag
- A signal the historian computes from other signals, such as total site power or daily energy, and stores like any other.See also: Tag
- Chunk
- A block of compressed samples for one signal over a stretch of time. Historians store data in chunks so that reading a period decodes only the chunks it touches.See also: Compression
- Compression
- Reducing the space data takes. Lossless compression (such as delta encoding) keeps every value exactly; lossy compression (such as deadband or swinging door) discards values judged unimportant. A good historian says which was in force.See also: Deadband, Swinging door
- Data historian
- Software that collects time-stamped values from industrial equipment, stores them efficiently for years, and serves them for trends, reports and analysis. Read what an industrial data historian does.See also: Tag, Sample
- Deadband
- A rule that stores a new value only when it differs from the last stored value by more than a set amount. It saves a lot of space on slow signals and loses detail on fast ones.See also: Compression, Swinging door
- DNP3
- Distributed Network Protocol, a telemetry protocol common in North American utilities and some grid connections, with event buffering and time-stamped data.See also: IEC 60870-5-104
- Envelope
- The band between the minimum and maximum of each interval, drawn around the mean when a long period is shown. It keeps spikes visible that an average would hide.See also: Aggregate
- FAT
- Factory acceptance test. Testing a control system against its specification before it goes to site, often on a bench with simulated field devices.See also: SAT
- fsync
- The operating-system call that forces data written to a file onto the physical disk. How often a historian calls it sets how much it can lose in a power cut.See also: Write-ahead log, Acknowledged value
- G99
- The Great Britain engineering recommendation for connecting generation, including storage, to distribution networks. It sets protection, compliance testing and data requirements.
- Gap marker
- A record in the history that says data is missing from a point in time, and why: a lost connection, a restart, a full disk. Without one, a trend may draw a straight line across the outage.See also: Quality code, Interpolation
- Grafana
- An open-source dashboard tool. Historians often act as a Grafana data source so that dashboards can be built over the stored data.
- HMI
- Human-machine interface. The screens operators use to see and control a process; often running on a panel PC that can also host a historian.See also: SCADA, PLC
- IEC 60870-5-104
- A telecontrol protocol over TCP/IP used between substations, generation sites and network control centres, widely used in Europe for grid connections.See also: IEC 61850, DNP3
- IEC 61850
- A family of standards for substation automation, defining data models and protocols (MMS, GOOSE, sampled values) used by protection relays and bay controllers.See also: IEC 60870-5-104
- Interpolation
- Estimating a value between two stored samples, either as a straight line (linear) or by holding the last value (step). Useful for analysis, and misleading if applied across a gap.See also: Gap marker
- Modbus TCP
- A simple request-and-response protocol over Ethernet for reading and writing registers and coils, used by meters, relays, inverters and gateways. Read about logging Modbus TCP with Vault.See also: Register, Word order
- Monitored item
- In OPC UA, a single node the client has asked the server to watch. Servers limit how many they will handle; an S7-1500's limit depends on the CPU and firmware.See also: Subscription, OPC UA
- NESO
- The National Energy System Operator, which balances Great Britain's electricity system and procures services such as dynamic frequency response from batteries.See also: BESS
- OPC UA
- OPC Unified Architecture, a platform-independent standard for industrial data exchange with a browsable address space, subscriptions and built-in security. Read about logging OPC UA with Vault.See also: Subscription, Security policy, Monitored item
- Parquet
- An open, column-based file format for large tables. Analysis tools such as Python, DuckDB and Power BI read it quickly, which makes it a good export format for history.
- PCS
- Power conversion system: the inverter that converts between the battery's DC and the grid's AC and follows the site controller's power setpoint.See also: BESS
- PLC
- Programmable logic controller. The industrial computer that runs the control logic for a plant and often hosts the OPC UA or Modbus server a historian reads from.See also: HMI, SCADA
- Publishing interval
- In OPC UA, how often the server sends a subscription's changed values to the client. Separate from the sampling interval, which is how often the server checks each value.See also: Sampling interval, Subscription
- Quality code
- A status stored with each value saying whether it can be trusted: good, uncertain or bad, often with a reason such as a communication failure. A bad value should be shown as a gap, not a number.See also: Gap marker
- Register
- A 16-bit memory location in a Modbus device. Values larger than 16 bits span two or four registers, and devices disagree about their order.See also: Word order, Modbus TCP
- Republish
- An OPC UA service that asks the server to resend a notification the client missed, for example after a brief network drop.See also: Subscription
- Sample
- One stored value of one signal, with its timestamp and quality.See also: Tag, Quality code
- Sampling interval
- In OPC UA, how often the server checks a monitored item for a change. A value that changes and changes back between samples is never seen.See also: Publishing interval
- SAT
- Site acceptance test. Testing the installed system on site, with real equipment, before handover.See also: FAT
- SCADA
- Supervisory control and data acquisition. The system that gathers data from a site's controllers and lets operators supervise and control it. Many SCADA packages include a historian; many small sites have only short-term trends.See also: HMI, PLC
- Security mode
- In OPC UA, whether messages are unprotected (None), signed so they cannot be altered (Sign), or signed and encrypted (Sign & Encrypt).See also: Security policy
- Security policy
- The set of algorithms an OPC UA connection uses for signing and encryption, such as Basic256Sha256 or Aes256_Sha256_RsaPss.See also: Security mode, Basic256Sha256
- SoC
- State of charge: how full a battery is, as a percentage of its usable capacity.See also: SoH, BMS
- SoH
- State of health: a battery's remaining capacity compared with when it was new, used to track degradation against a warranty.See also: SoC, BMS
- Sparkplug B
- A specification for carrying industrial data over MQTT with defined topics, payloads and birth and death messages, popular for sending site data to cloud platforms.
- Store-and-forward
- Buffering data locally while a link to a central system is down, then sending it when the link returns, so the central record has no gap.
- Subscription
- In OPC UA, a group of monitored items whose changes the server sends to the client at the publishing interval, instead of the client polling each value.See also: Monitored item, Publishing interval
- Swinging door
- A lossy compression method that drops values lying close to a straight line between stored points. Efficient on smooth signals; it can hide short events.See also: Deadband, Compression
- Tag
- A named signal in a control system or historian, such as an inverter's active power. Many historians are licensed by the number of tags.See also: Sample, Asset model
- Word order
- The order in which a Modbus device places the 16-bit registers of a 32 or 64-bit value. The four common orders are ABCD, BADC, CDAB and DCBA; get it wrong and values are nonsense.See also: Register
- Write-ahead log
- A file a system appends every change to, and flushes to disk, before applying the change elsewhere. After a crash the log is replayed so nothing acknowledged is lost.See also: fsync, Acknowledged value
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