Glossary · PLC and control programming
Forcing
Also known as: Force, I/O forcing
German: Forcen
In PLC programming and commissioning, forcing is the overriding of an input, output or variable with a fixed value set from the engineering tool, regardless of the actual field signal or program logic, until the force is removed.
- PLC programming
- Functional safety
In one sentence
Forcing overrides a PLC input, output or variable with a fixed value from the engineering tool until the force is removed.
Example
During commissioning, a technician forces a proximity switch input to TRUE because the sensor is not yet mounted, and the controller shows a FORCE indicator.
How it applies
- Commissioning: Forcing lets engineers test outputs and logic before all sensors are wired. Controllers usually show an active force with an LED or a status message.
- Risk: A forced value decouples the program from reality. A forced input can make the logic believe a guard is closed or a tank is empty, and a forced output can move an actuator without the normal interlocks. Forcing must never be used to bypass a Safety function; safety controllers typically restrict or prohibit it.
- Operation and maintenance: Many sites require a permit, a log entry and a check that all forces are removed before handover. The controller's force list is part of the handover checklist.
- Documentation: Describe who may force, how to see active forces and how to remove them. In commissioning reports, record every force that was set and when it was removed.
Forcing vs. writing a variable
Writing (modifying) a variable changes its value once; the program can overwrite it in the next cycle. Forcing keeps the value fixed until it is released, which makes it more powerful and more dangerous.