Triggering

The 'TRIGGERS' button in the Display Setup window (or the A-D menu) opens a window for setting options for automatic triggering of recording.  This allows the program to wait until some condition is met before it starts sampling data.  The 'trigger' is a value for one of the channels; data in that channel must meet the user-set trigger condition -- either exceed or fall below the trigger value -- before LabHelper begins sampling.

The trigger window allows you to:

• turn triggering on or off
• select the channel to be scanned for the trigger value.  This can be one of the regular data channels or -- if the number of regular channels is less than 16 -- it can be an unassigned A-D hardware input. 
• set the triggering mode (whether data must exceed or fall below the trigger value)
• select the trigger value itself.  Note that for regular chart channels the trigger value is converted into the normal units for that channel -- it is NOT the raw input voltage.  For oscilloscope channels, or if you are triggering directly from an A-D hardware input, the trigger value is read as a raw input voltage only. 

  If you are using Multichannel Oscilloscope mode, or discontinuous sampling (high sample rates) in Oscilloscope mode, another selection box appears:

These choices specify how a discontinuous oscilloscope display responds to a trigger condition.

• The default mode is to sample and display one screenfull of data and then wait for the next trigger condition to occur. 
• The second option is to deactivate triggering and start sampling continuously (the trigger will 'reappear' if the program is halted with the 'esc' key and then data collection is resumed. 
• A third option is to sample and save one or more screenfulls of data after the trigger event, and then wait for the next trigger condition to occur. 
• The last option is to sample and save one or more screenfulls of data after the trigger event, and then start sampling continuously. 

When all of these options are correctly set, click the 'Triggering OK' button.

When sampling is started (or restarted), the program draws the normal channel display on the screen.  It displays the triggering conditions at the top of the screen, along with a continuously updated readout of the current value of the trigger channel (in Chart mode; in Oscilloscope mode there is no trigger channel readout).  As soon as the trigger condition is met, normal sampling begins.

Note that you can interrupt trigger scanning and manually initiate sampling from the keyboard.  Hitting the spacebar, the 'return' key, or clicking the mouse (or the button) will immediately initiate normal data collection.  Hitting the 'esc' key will stop scanning and open the OPTIONS window.

• If the autorepeat option is selected in Chart mode, triggering is operative for the FIRST data file only.  For subsequent files, triggering is switched off. 
• The same applies in continuous Oscilloscope mode -- once the scope channel is triggered, it runs continuously
• In discontinuous Oscilloscope mode (i.e., at very high sampling rates) or in Multichannel Oscilloscope mode, triggering is operative for each screenfull of data.  Clicking the mouse or hitting the space bar (or receiving a trigger value) will start the sampling and display another screenfull of data.  Alternately, hitting the 'return' key will temporarily switch off triggering. 

   When using triggering in Multichannel Oscilloscope mode, the response of the computer to keyboard commands is greatly slowed.  This is because the computer only checks for keyboard input between screenfulls of data, not when actually recording.

If the trigger condition is constantly met, the computer will continually restart sampling, leaving little time to check the keyboard.  To insure a response to the 'tab' or 'escape' key, hold the keys down until the computer does what you want.

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