Glossary · Requirements and interfaces
Residual interface
Also known as: Interface to existing equipment
German: Restschnittstelle
In automation projects, a residual interface is the interface that remains between a newly engineered, migrated or replaced part of a plant or machine and the unchanged existing part, and that must be defined and tested so both work together.
- Data and interfaces
- Integrated systems
In one sentence
A residual interface is the interface between a new or migrated part of a plant and the unchanged existing part, which must be defined and tested.
Example
When the old control of a press line is replaced station by station, the residual interface lists the handshake signals the new PLC must exchange with the stations still running on the old control.
How it applies
- Engineering: In retrofits and migrations, the residual interface is often poorly documented; it should be captured as Interface requirements before work starts.
- Safety: Safety signals crossing the residual interface (e-stop circuits, enabling signals) need particular care, since the safety function spans old and new parts.
- Documentation: The documentation team should record the residual interface in the as-built documentation, so later changes on either side consider it.
- Commissioning: Test the residual interface signal by signal during migration.
Residual interface vs. interface specification
The Interface specification defines an interface in general. The residual interface is a specific situation: the boundary created by a partial change to an existing system.
Undocumented residual interfaces are a frequent cause of delays and hidden safety gaps during retrofits.