[plt-scheme] Reading voltages on a serial port...

Maybe this is way off topic but here goes... What I
would like to do is to read the voltage of a pin on
the serial port so that if the pin reads high it
returns a boolean value (positive or negative logic is
fine).

Can someone point me in the right direction?  The
following program yields nothing when run...

(define serial (open-input-file "/dev/ttyS0"))

(define (read-loop in-port)
  (display (read-line in-port))
  (newline)
  (read-loop in-port))

(read-loop serial)


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