ISA-88 — a line of units, a unit of modules
The equipment model from IEC 61512: cell, unit, module, channel. The twin is built on it — from the line down to a single axis.
A common language for equipment and procedures
ISA-88 is a standard for batch process control, IEC 61512 in its international edition. Its main idea is to separate equipment from the procedure performed on it. Then the same procedure can run on different equipment, and equipment is described once.
The standard was born in batch production, but its physical model became a common language of automation: engineers across industries understand “cell”, “unit”, “module” the same way.
In SyncTwin the standard defines how the twin is built: from the line down to a single axis, each level is an object with its own properties, owner and links, not a row in a common table.
- Physical modelProcess cell → unit → equipment module → control module.
- Procedural modelThe cell procedure commands the phases of units.
- WhyUnits from different vendors are described the same way and assembled into one line.
What consists of what and what does what
| Physical model | What it is | Procedural model |
|---|---|---|
| Process cell | The area where the whole process runs | Procedure |
| Unit | A piece of equipment with its own cycle | Unit procedure |
| Equipment module | A group of devices performing an action | Operation |
| Control module | A single device: axis, sensor, valve | Phase — the smallest action with parameters |
Above the cell the standard places the enterprise, site and area; at line level they are not needed and are not created in the twin.
Twin rules per ISA-88
| ISA-88 level | With us |
|---|---|
| Process Cell | Line, project — one line zone |
| Unit | Unit: machine, robot, conveyor, positioner |
| Equipment Module | Mechanism with an executor: a cutting program or SFC logic |
| Control Module | Channel: an axis or I/O with a role and profile |
- A unit has exactly one leading moduleIf there is a cutting program, it leads and calls the others; if not, the unit’s SFC program leads. Two leaders in one unit would fight over the axes.
- An axis belongs to exactly one moduleChanging the axis owner — only at rest, in two program steps.
- The line sees only what is declaredWhat is inside a unit does not matter to the line — only its variables.
- Tooling is a module with its own logicA turret or gripper is a unit module called by its owner; a mill with a turret is one unit.
One unit — one box machine
All modules of a unit live in one real-time loop; the whole unit moves between machines as project data.
A palletizer is a unit with one module
Simple equipment is not split artificially: a unit may consist of one module.
A channel is a role, not an address
A control module names the meaning of a channel — “part present”, “grip confirmed”; the terminal is assigned by commissioning.
The equipment kind decides what is allowed
Equipment consists of three parts: the mechanism — a reference body model, the instance — your machine with axis binding and tooling, the executors — what runs it.
| Kind | Allowed executors |
|---|---|
| Mill, lathe, laser | Cutting program |
| Robot | Motion program and unit logic |
| Positioner, track, conveyor, feeder | Unit logic |
A positioner that turns the part under the torch during the seam is an external robot axis and part of the cell’s seam program.
A phase is a unit skill with parameters
Input parameters
Points, layer, program — read and changed from outside, without editing the unit program.
Reported values
What the unit reports at the end of a phase; live values — during execution.
Time estimate before start-up
A machine — by the program toolpath, a robot — by the path and joint limits, a conveyor — by path and speed. The cell procedure calculates the line cycle from them.
Phases with time estimates and the line cycle calculation are the platform canon and a subject of the implementation; the unit model, modules and channels work now.
The procedure commands phases, units execute them
The cell procedure is written in SFC, like the unit logic. A procedure step is a unit, its phase command and a completion condition by tag. The procedure does not know how the unit performs the phase: it knows only the interface.
That is why a unit can be replaced by another one — of the same kind or a virtual one — without rewriting the procedure: the executor changes, not the command.
| Procedure step | Unit | Phase command | Completion |
|---|---|---|---|
| Feed the blank | Robot | Transfer into the vice | “Transfer done” tag |
| Machine | Machine | Part program | Complete state |
| Remove the part | Robot | Transfer to a pallet | “Transfer done” tag |
The example is illustrative — it shows how a procedure is built, not the program of a specific cell.
Why the twin is built by the standard
Describe once
A unit kind is described once — mechanism, channels, interface; every instance gets them from the kind.
Different manufacturers — one language
Units from different suppliers join the line the same way: through their interface, not through someone else’s internal logic.
Exchange without translation
ISA-88 roles go into the AutomationML line description — another engineering system sees the same levels.
Questions about the model
Do unit programs need rewriting for the line?
No. The unit program declares its ports and formal places; actual points and links to neighbours are set by the project. A unit does not know its neighbours.
How to describe a device without a program — for example, a camera?
A device unit without a program only writes tags and registers in the box’s local network. In a project it takes part on a par with the other units.
What if a robot needs both a seam and a transfer?
A unit has one leading module at any moment: the seam is led by the motion program, transferring by the automation block. Between them — rest; the axis owner is not changed on the move.
Discuss an implementation
Describe the line in units — we will build its ISA-88 model and propose an R&D stage with price and timeline.
Request accessDiscuss an implementation
Describe the unit or line — we will tell you what we will verify and in what time. Target indicators are fixed in the implementation specification.
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