when you don't include modedefs.bas SEROUT serpin,4,... is the same as SEROUT serpin,N2400...


what is your taget device to receive $a0 ???
$a0= 160 (decimal) = BLANK character in ASCII table

maybe this is why.

try PORTB.2,n2400,["hello ",#10,10,13]

it suppose to display hello 10

let me know