Datain[1] is second byte in string.
If you wait number 7, then that byte isn't in datain array.
If you sent 7,6,12, then in datain first byte is 6, second is 12, so you need something like this
Code:
HSERIN [WAIT($07), str DATAIN\8]
if datain[0] = $06 then high portc.3
And now if you send 7,then 6, and 6 more byte portc will go high.