Dom,
It's a minor layout error:

SETB  ENMOV A,DATA  ;Read first part of the return value (high nibble)
 CLR   ENANL A,#0F0h ;Only high nibble is usable

should read

SETB  EN
MOV A,DATA  ;Read first part of the return value (high nibble)
CLR   EN
ANL A,#0F0h ;Only high nibble is usable

regards
Patrick