Glossary · System coordination, integration and orchestration
Request signal
Also known as: Request bit
German: Anforderungssignal
In industrial automation, a request signal is a signal by which a machine, controller or operator asks another party to perform an action or grant access, such as requesting a part, a transfer or entry into a shared zone; the request is answered by an acknowledgment, permission or rejection.
- System integration
In one sentence
A request signal asks another machine or controller to perform an action or grant access, and expects acknowledgment or permission in return.
Example
The robot sets its request signal for the conveyor zone; the conveyor stops the segment, sets the permission signal and the robot enters.
How it applies
- Engineering: Define how long a request stays active, whether it is edge- or level-triggered, and what happens if it is withdrawn before an answer. Conflicting simultaneous requests need Resource arbitration.
- Commissioning: Test requests that are never answered (timeout), answered late or withdrawn, and simultaneous requests from several parties.
- Operation: Pending requests that never complete are a typical cause of line stops; show them in diagnostics.
- Documentation: Include request signals with their timing and answers in the interface specification or Interface control document (ICD), ideally as a sequence diagram.
Request signal vs. ready signal
A request signal expresses a wish to act and expects a reply. A Ready signal states that the sender can act, without asking for anything. Together with a Permission signal, they form the basic building blocks of a Machine handshake.