When pre samples are selected, the trigger system is not activated yet until a specified amount of samples are measured after starting the measurement. This amount of samples is called the trigger hold-off. When the input signal(s) meet the trigger requirements during the trigger hold-off period, this will not generate a trigger and the system will remain sampling pre samples. After the trigger hold-off has passed, at the first occasion that the trigger conditions are met, the system will start measuring post samples. This will ensure that at least that specified amount of pre samples will be measured.
The trigger hold-off can is set as a number of samples. Two special settings are also available: Presamples valid or Off.
In the setting Presamples valid, the trigger hold-off value will always be equal to the number of pre samples that are selected. This will ensure that the complete pre sample buffer is always filled with measured samples.
When trigger hold-off is switched off, the trigger system is activated immediately after starting the measurement. The system will not first fill the pre sample buffer before the trigger system is activated. Depending on the measured signal and the moment the measurement was started, this may result in a situation where not all pre samples are measured, but the first pre samples remain 0 V.
Trigger hold-off = off is useful when measuring once-only events, because in this way never a trigger will be missed. If the system would first fill the pre samples buffer before activating the trigger system, it might happen that the event that should cause a trigger would take place while still filling the pre samples buffer and therefore not generate a trigger. No measurement would take place.
Not all TiePie engineering instruments support trigger hold-off. When trigger hold-off is not supported, the trigger system functions as if trigger hold-off was switched off.