What is MES: functions, signs you have outgrown Excel, custom software or MES on Ignition

A manufacturing execution system (MES) is the plant software at ISA-95 level 3, between ERP and the PLCs: it receives work orders, records what each line actually made, with which materials and which stops, and keeps that record tied to orders, lots and equipment. Plants usually get there by one of three routes: spreadsheets and paper filled in by people, software written for the plant, or a commercial MES product. The example used here for the third route is Ignition with Sepasoft MES modules, the platform covered on TDL's MES page.

What an MES does

Wikipedia describes an MES as a computerised system that tracks and documents how raw materials become finished goods, and notes that this "as-built" record matters in regulated industries such as food, beverage and pharmaceuticals. In ISA-95 terms it sits at level 3, between ERP at level 4 and process control at levels 0 to 2. The ISA-95 article covers the levels, the level 3 activities and the ERP interface, so they are not repeated here.

The function list most often quoted comes from MESA International. Rockwell Automation's Plex eBook on MES gives the eleven functions defined in 1997: operations and detailed sequencing, resource allocation and status, document control, process management, data collection and acquisition, quality management, labour management, dispatching production units, product tracking and genealogy, performance analysis, and maintenance management. The model was updated in 2004 (c-MES) and again in 2008. On 30 June 2026 ISA announced that, effective that day, it has assumed certain assets of MESA International, including its educational programmes and content, as MESA International ceases operation.

On a plant project, the functions usually asked for are these:

OEE and downtime is often the first function a plant uses. Its formula, availability × performance × quality as Vorne's oee.com gives it, and a worked example are in the OEE article. The historian article covers the time-series store that often sits beside an MES.

Signs a plant has outgrown spreadsheets

Most MES functions start life as a spreadsheet or a paper form. In a joint OEE webinar, Inductive Automation and Sepasoft describe early tracking as manual entry on paper, in spreadsheets or on chalkboards, and contrast it with automatic recording, which times downtime events to the second and reduces the time operators spend on data entry. Plex's eBook adds that an ERP link removes the need to type the same data twice. Typical signs that a spreadsheet has reached its limit:

• Figures are typed in at the end of the shift, and short stops never reach the sheet.
• The same numbers are typed again into ERP, and the two disagree at month end.
• Tracing one finished lot back to its raw-material lots takes days.
• Several copies of the workbook circulate, and only one person can repair its formulas.
• Each line uses its own list of stop reasons, so lines cannot be compared.

Three paths

Excel and paper

Spreadsheets need nothing new to start, but the data is typed by people after the event, with no PLC link unless someone builds one. Excel for Microsoft 365 has Show Changes, which lists who changed which cell, when, and the previous value, for the past 365 days. Microsoft also lists its gaps: edits from older or one-time-purchase versions are not tracked, uploading or saving a copy of the file can clear the list, and values changed by VBA may have no previous value. That helps with a shared workbook; it still records only what someone typed.

Custom software

The second route is software written in-house or by a contractor, often a database with web forms and sometimes a direct read of PLC tags. It can follow the plant's own way of working exactly. The risks: the knowledge tends to stay with one or two developers, nobody else can change it without the source code and documentation, and every operating system or database upgrade becomes the owner's project. In the same webinar, Inductive Automation and Sepasoft advise using built-in functions and ready-made components over complex custom solutions where possible; they sell such products, so read that as a vendor's view.

An MES product or MES modules on a platform

Many vendors sell MES software; the two examples here are in no particular order. Siemens describes Opcenter Execution as a family of MES products with variants for discrete, process, electronics, medical device, pharmaceutical and semiconductor production. AVEVA describes its MES as prebuilt, modular capability packages that can be deployed in stages, on premises or in a hybrid setup with the cloud.

The example TDL works with is Ignition with Sepasoft modules:

• Platform. Ignition is licensed by the server, with unlimited clients, tags and projects; modules are bought as needed. Every Sepasoft module requires Ignition, and Sepasoft's Production module is always installed with the others.
• Modules. OEE Downtime, Track & Trace, SPC, Settings & Changeover and Batch Procedure, plus Business Connector, Web Services and the Interface for SAP ERP for links to other systems.
• Licensing. Perpetual licences. OEE Downtime, Track & Trace, SPC and Settings & Changeover are licensed per physical production site, with unlimited lines, machines and users; Batch Procedure and the connector modules are licensed per server. Support plans include upgrades while the plan is active.
• OEE Downtime. Collection can be automatic, manual or mixed; in the most common setup the start and end of each event are recorded automatically and operators split events or add notes. Detection modes decide which machine is blamed when a line stops, for example the first cell to go down.
• Track & Trace, Settings & Changeover. Track & Trace follows product from raw materials to finished goods, including consumables and by-products, and Sepasoft aligns it with ISA-95. Settings & Changeover stores settings and material amounts per product, and its variance log records settings changed at an operator terminal outside the recipe system.
• PLC link. Ignition's Logix driver talks to ControlLogix over EtherNet/IP and is optimised for firmware 21 and later.

Comparison

Vendor facts come from the cited documents; the rest is TDL's assessment.

Configuration has its own trap. Sepasoft's documentation says that renaming an item in the production model creates a new item and disables the old one, and data recorded against the old item is not available to the new one, so it recommends naming every item correctly at the start.

Start small, then add modules

The OEE webinar advises a pilot on the line where the impact is largest, kept simple. TDL's MES page describes the same path: OEE Downtime on one line first, with Track & Trace, Settings & Changeover or SPC added when the plant needs them. The OEE page, with its in-browser calculator, suggests the bottleneck line or one with frequent changeovers.

1. One line, OEE and downtime. Machine states, counts and stop reasons come from the PLC; missing signals are added with small program changes (see connecting existing equipment).
2. More lines, one set of stop reasons. Every line uses the same lists, so lines can be compared; for plants with several sites, Sepasoft lists sharing standardised downtime reasons across sites as a multi-site feature of its Enterprise tier.
3. Further functions when needed. Lot genealogy, quality sampling, settings per product.
4. The ERP link. Orders come down, quantities and material use go up. On Ignition the Sepasoft options are Business Connector, Web Services and the Interface for SAP ERP.

Where a plant uses or chooses Sepasoft modules, TDL connects them to the line PLCs, so machine states, counts, alarms and recipe values come from the existing controllers. TDL also builds OEE dashboards on Ignition, FactoryTalk View SE or AVEVA System Platform with a historian.

Questions to answer before buying

• Which question must the system answer first? For example, why one line loses an hour per shift, or which customers received product from a given raw-material lot.
• What do the PLCs already hold? Machine states, counts, alarm words, and what must be added.
• Who owns the master data? Products, ideal rates, stop reasons and shift calendars each need one owner.
• What crosses to ERP? Each message, its direction and how often.
• Which records must be kept? For how long, who may correct them, and whether an auditor will read them.
• What does the licence cover? Sites, servers, and redundant, development and test systems, which Sepasoft sells as add-on licences.
• Where does the server sit on the network? See the IEC 62443 article.