I see noting really wrong with what's actually posted but as is usually the case not all the code is posted. Here's an untested example of what I think you're looking for:
Code:
Main:
  HSERIN 5000, Timeout, [char]
  IF (char="A") THEN
    GOSUB somewhere
  ENDIF
Goto Main

Somewhere:
   HSEROUT["I got an A", 13]
RETURN

Timeout:
   HSEROUT["Nothing for the last 5 seconds, I'll just restart and wait 5 more....",13]
Goto Main