Hello,
To do what you are describing is possible as long as signal A is a digital trigger. The trigger that arms the board's acquisition engine is called the "start trigger". See this location for the attribute name and possible source values: http://zone.ni.com/reference/en-XX/help/370592G-01/scopefunc/niscope_attr_acq_arm_source/
The trigger that signals the start of each record you want to record is the "reference trigger". This trigger can come from the same digital sources as the start trigger but can also be generated from an analog comparator monitoring channel 0, 1 or the trigger input BNC on your 5122. See this location for the name of the reference trigger attribute and possible source values: http://zone.ni.com/reference/en-XX/help/370592E-01/scopefunc/niscope_attr_trigger_source/
So if you wanted signal B to be an analog signal that you do a simple edge trigger on, but signal B is not the signal you want to acquire you would feed signal B into the external trigger BNC and set NISCOPE_ATTR_TRIGGER_SOURCE to VAL_EXTERNAL. You would also need to set other attributes to define the type of analog trigger you want. You can find these attributes here: http://zone.ni.com/reference/en-XX/help/370592E-01/scopefunc/attribute_defaults_512x/ All the attributes related to the reference trigger have the prefix NISCOPE_ATTR_TRIGGER.
If you want signal B to also be one of the signals you acquire then you would feed it into channel 0 or 1 and set NISCOPE_ATTR_TRIGGER_SOURCE to 0 or 1 respectively.
If you have labview available here is a link to a demo that is similar to what you are looking for: http://zone.ni.com/devzone/cda/epd/p/id/2998
If you require signal A to be an analog trigger as well then you would need to use a second board and have it export a reference trigger to a PXI trigger line or to a PFI line. You could use many different boards to do this depending on the levels and frequency of signal A.
Hope this helps!
-Matt
