Glossary · Automation software engineering and architecture
Firm real time
Also known as: Firm real-time system
German: Feste Echtzeit
In real-time systems, firm real time describes timing requirements under which a result delivered after its deadline is useless and is discarded, but occasional deadline misses do not cause a system failure, as long as they stay within defined limits.
- Software engineering
In one sentence
In firm real time, a late result is useless and discarded, but occasional deadline misses within limits do not cause failure.
Example
A vision system inspecting parts on a conveyor must finish each image in time; a late result is discarded and the part is rejected for re-inspection, which is acceptable if rare.
How it applies
- Engineering: Firm real-time requirements need a defined tolerance, for example no more than one missed deadline in a given number of cycles, and a defined handling of late results.
- Operation: Monitoring deadline misses and their frequency shows whether the system still meets its requirements, especially after software changes or higher load.
- Documentation: Specifications should state which requirements are hard, firm or soft, with measurable limits. Where missed deadlines have visible effects, such as rejected parts, operating documentation should explain them.
Firm vs. hard and soft real time
In Hard real time, every deadline must be met; a miss is a failure. In soft real time, late results still have some value, reduced with lateness. Firm real time lies between: late results have no value, but rare misses are tolerated. These classifications are not formally standardized, so define them in the project's requirements.