**ShowPlay Reference Manual, Chapter 3 (ControlVariables): atrig_chan**
===== Atrig_chan -- External trigger channel =====
The ATRIG_CHAN variable is used to specify which external signal line to use for triggering the start of a paradigm. 

ATRIG_CHAN is used in conjunction with TRIG_FLAG and [[jvs:cigal:showplay:manual:chapter3:trigtype|TRIGTYPE]], which indicate when and how to look for external trigger signals. Other parameters ([[jvs:cigal:showplay:manual:chapter3:atrig_val|ATRIG_VAL]], [[jvs:cigal:showplay:manual:chapter3:dstartmask|DSTARTMASK]]) are also used to specify exactly what aspect of the external signal qualifies as a trigger.

**See Also:**\\
[[jvs:cigal:showplay:manual:chapter3:trig_flag|Trig_flag]],
[[jvs:cigal:showplay:manual:chapter3:trigtype|Trigtype]],
[[jvs:cigal:showplay:manual:chapter3:atrig_val|Atrig_val]],
[[jvs:cigal:showplay:manual:chapter3:dstartmask|Dstartmask]])\\
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