Could ALL stand for Alternate, even though misspelt and makes little sense on a DSO.
Triggering off either channel should show both traces in single captures.
Ah yes, that must be it. That TENMA/UDT scope does indeed support ALTernate triggering.
@simon christo: As others have stated, you are probably mixing up the trigger selection with the actual capture and display of the traces. You have to select
one of the channels for triggering (trigger on wither CH1 or CH2), but then the scope will capture
both traces. This applies in single-trigger as well as recurring trigger mode.
Your scope also has the capability of
alternate triggering (which is unusual for a digital scope). In that mode, it will first wait for a trigger event on CH1, then capture CH1, then wait for a trigger event in CH2, then capture CH2. If you select ALTernate triggering in
single-shot mode, the scope will probably capture only one of the traces indeed -- so alternate triggering in single-shot mode is a meaningless combination.
There is
no option to tell the scope "wait for a trigger event on either CH1 or CH2, then capture both traces". I am not aware of a general-purpose scope which can do that. You always have to select
one of the channels to define the trigger event.