MDT

What is MDT

• MDT stands for Modified Data Tag. It is bit 7 of the attribute byte.
• The terminal sets it to 1 when the user types data into the field (or modifies it).
• On RECEIVE MAP, only fields with MDT on are sent back to the program - untouched fields are not transmitted.
• This keeps the input data stream small and tells the program exactly which fields the user touched.

The FSET option

• Coding FSET in ATTRB turns the MDT on before the map is sent to the terminal.
• The field's contents are then transmitted back even if the user did not change them.
• Use FSET on display-only fields whose values the program needs back - for example a customer number shown for reference.
• Do not code FSET on every field, or RECEIVE MAP will return all fields and you lose the 'what changed' information.
• A field holding hexadecimal zeros (low-value) is never transmitted, even with MDT on.

The DUMMY field trick

• The last DFHMDF in a map should define a 1-byte field named DUMMY with ATTRB=(DRK,PROT,FSET) and INITIAL=' '.
• DRK and PROT make it invisible and unchangeable; FSET guarantees its MDT is on.
• This ensures at least one byte is always sent back, so RECEIVE MAP never raises the MAPFAIL condition.
• In the symbolic map it appears as DUMMYL / DUMMYF / DUMMYA / DUMMYI / DUMMYO like any named field.
• Example:-
  
  DUMMY DFHMDF POS=(24,79), X LENGTH=1, X ATTRB=(DRK,PROT,FSET), X INITIAL=' '