What is a process historian? Buffering, deadband and FactoryTalk Historian vs AVEVA PI vs Ignition

A process historian is a time-series database for plant data: it records each tag's value with a timestamp and a quality or status, can keep it for years and returns trends, averages and interpolated values on request. FactoryTalk Historian, the AVEVA PI System and the Ignition historian all do this job, with different storage engines, collection paths, asset models and licence models. Buffering, deadband and compression come first below, then a comparison drawn from each vendor's own documentation.

What a historian stores

Wikipedia describes an operational historian as a time-series database for process data, deployed close to the control system, that captures instrumentation and control data as tags (or points). AVEVA's Data Archive documentation defines an event as a timestamp, a value and a status; Ignition's documentation says each historical record holds the value, its quality and a millisecond timestamp.

Where a historian sits among the ISA-95 levels is covered in the ISA-95 article. Its data can feed an OEE calculation and for the production records of an MES.

Why not just log tags into an SQL table?

You can, and Ignition's SQL Historian does store history in MySQL, SQL Server or PostgreSQL. What a historian adds is the logic around the storage:

• Volume. A tag scanned every second gives 86,400 values a day. A historian decides per tag which values to keep and manages the tables itself: Ignition's Historian Core module creates, indexes, partitions and retires them automatically.
• Retrieval. Inductive Automation advises querying history through Ignition's own tools, which handle time alignment, interpolation, deduplication and aggregation.
• Time weighting. When values are stored only on change, a plain average of rows weights a value that held for an hour the same as one that lasted a second. Ignition offers a time-weighted average, and PI Web API summaries can be time-weighted or event-weighted.
• Interpolation. AVEVA describes the original signal being reproduced by interpolating between stored events, so a value can be worked out for times between them.

How data gets in, and the limits of buffering

In each product the path has three stages: a collector reads the controller and filters values, a buffer holds data while the archive cannot be reached, and the archive stores it.

Collectors are the FactoryTalk Live Data Interface in Historian SE (Rockwell controllers, plus OPC UA and OPC DA servers), PI interfaces, PI connectors and AVEVA adapters in the PI System, and Ignition's own drivers, including an Allen-Bradley Logix driver for ControlLogix and CompactLogix from firmware v20.19.

PI interfaces buffer to disk through the PI Buffer Subsystem while Data Archive is unavailable and send the data when the connection returns; AVEVA advises a buffer drive without the operating system or page file, and Rockwell's Historian SE redundancy guide says the same (a local drive other than C). Ignition's Store and Forward passes history through a memory buffer and a disk cache, removes data only after a successful database write, keeps the original order and quarantines records that fail repeatedly.

Buffers are finite:

• The AVEVA Adapter for OPC UA defaults to 1024 MB of buffer per endpoint. At the limit it deletes the oldest buffer file, and if it cannot buffer at all it logs errors indicating data loss.
• Ignition's Store and Forward limits by time, count or file size. The default value 0 sets no limit of its own; when a configured limit is reached, the default action refuses new data and the alternative evicts the oldest.
• Ignition 8.3's Core Historian writes straight to disk through a write-ahead log and does not use Store and Forward; history sent to another gateway through the Remote Historian provider does.
• FactoryTalk Historian ME stores firmware and data on an 8 GB mSATA disk and reports the days left until it is full.

A collector buffer covers only an outage between collector and archive: values never read, because the collector PC or its PLC link was down, are in no buffer. Test any "no data loss" promise with a real outage.

Deadband and compression: how much detail to keep

Historians filter in two places, set per tag.

Exception (deadband) at the collector. A PI interface ignores values inside a deadband set by ExcDev (engineering units) or ExcDevPercent (of span), with ExcMin and ExcMax as minimum and maximum report intervals. When a value leaves the deadband, the previous value is sent too. AVEVA's UniInt guidance is an ExcDev slightly smaller than the instrument's precision.

Compression at the archive. The PI Snapshot Subsystem runs a swinging-door test with CompDev, CompMin and CompMax and keeps the events needed to redraw the signal within CompDev; stored values are the actual values received. AVEVA's guidance: compression on for noisy signals, off for manual entries, totals and lab results, and one CompDev does not fit all measurements.

Ignition combines a sample mode, minimum and maximum time between samples, and a deadband. The Discrete style stores a value when it differs from the last stored value by at least the deadband; the Analog style uses the deadband as a compression threshold in a modified sliding-window algorithm. For the Core Historian, Inductive Automation recommends the Discrete style or periodic sampling.

A wider deadband stores fewer points and loses detail. In the example below, 32 temperature samples taken once per second rise from 50 °C to 55 °C, then make a two-second excursion to 56.6 °C. A 0.5 °C deadband stores 11 points and keeps the excursion. A 2.0 °C deadband stores 3, and the excursion, within 2.0 °C of the last stored 54.8 °C, never reaches the archive.

The three platforms side by side

FactoryTalk Historian (Rockwell Automation) comes in two editions. Site Edition (SE) is the plant and enterprise historian. Rockwell's 2020 security advisory PN889 states that Historian SE includes the OSIsoft PI Server with its PI Data Archive component, and AVEVA's partner page describes FactoryTalk Historian as powered by the AVEVA PI System. The SE product profile describes auto discovery of Logix controllers and tags, and server collectives for redundancy. Machine Edition (ME) is a ControlLogix module (1756-HIST2G) that collects from up to five local and five remote controllers at scan rates down to 10 ms and can transfer up to 2500 points to Historian SE or a PI server.

AVEVA PI System. PI Server consists of Data Archive, PI Asset Framework (PI AF) and PI Notifications Service; the documentation cited here is for PI Server 2024 R2. PI AF models equipment as elements, built from templates, whose attributes reference PI points, formulas or relational data.

Ignition historian (Inductive Automation). In 8.1 the Tag Historian module stores history in an SQL database. In 8.3 the Historian Core module works with several providers: the embedded Core Historian based on QuestDB, the SQL Historian (separate module), the SQLite Internal Historian (Legacy) for edge or small systems, a Remote Historian on another gateway and a Historian Splitter that writes to two providers.

The SE licensing details come from Rockwell's ordering guide dated April 2022; check the current guide before quoting them.

How to choose

• Plant built on Logix and FactoryTalk. Historian SE discovers Logix controllers and tags; non-Rockwell devices are read through third-party OPC servers, which the ordering guide lists as a separate purchase. Historian ME covers capture in the chassis without a PC.
• PI already runs at corporate level. Feed the existing PI Server through interfaces or adapters. Historian ME can transfer to a PI server, and FactoryTalk View SE can trend from a PI Server.
• Ignition SCADA, or IT wants data in SQL. Inductive Automation positions the SQL Historian for reporting, external tools and long-term storage; the Core Historian keeps history inside the gateway.
• One site or several. One site can run any of the three on a single server. Across sites, Historian SE has an SE to SE interface that replicates a site server to a target server, PI AF can build one asset model over one or more Data Archive servers, and Ignition's Remote Historian sends history to another gateway.

Other historians exist, for example Canary, AVEVA Historian and Proficy Historian; they are not evaluated here.

Applying it on a project

• Tag list and naming. One name per signal across PLC, historian and MES; templates (PI AF, Ignition UDTs) so that a hundred pumps are configured once.
• Deadband per signal type. Start from instrument precision; record every change for states and counters; no compression for manual entries, totals and lab values. Compare raw and stored trends on real data before go-live.
• Retention. Decide how long each class of data is kept, then size disks and pruning.
• Time. AVEVA requires host clocks synchronised to Data Archive and recommends NTP for the Data Archive clock; events older than the current snapshot bypass compression as out of order.
• Buffering. Size the buffer for the longest expected outage, put it on a non-system drive, and disconnect the archive during FAT to check that the gap fills in order.
• Backups. Data Archive backups include archive files and configuration; an SQL historian is backed up with its database.
• Readers. List them (trends, Excel, OEE, MES) before licensing, because each product counts readers its own way: the 2022 Historian SE ordering guide requires a client access licence (CAL) for third-party applications that read through OLE DB or OPC, Ignition does not count clients, and AVEVA's PI Web API documentation refers licensing questions to the account manager.

TDL's SCADA and historian page lists the historians it works with: FactoryTalk Historian, AVEVA Historian and the AVEVA PI System. The Ignition page describes the Tag Historian on standard SQL databases, and the AVEVA page covers the AVEVA PI System.