Пример SetText, GetText строкового поля

procedure TForm1.Table1IDGetText(Sender: TField;
  var Text: OpenString; DisplayText: Boolean);
var
  s: string;
begin
  s := (Sender as TStringField).Value;
  case s[1] of
    'A': Text := '1';
    'B': Text := '2';
    'C': Text := '3';
  else
    Text := '9'
  end;
end;

procedure TForm1.Table1IDSetText(Sender: TField; const Text: string);
var
  s: string;
begin
  case Text[1] of
    '1': s := 'A';
    '2': s := 'B';
    '3': s := 'C';
  else
    s := 'Z'
  end;
  (Sender as TStringField).Value := s;
end;