FX419 is an old part this has been replaced by FX439.

This part is FFSK modem.

FX439     FX439   FX439          FUNCTION
  DW        J     LG/LS

   1         1      1            Xtal/clock
   2         2      2            !Xttal (invert input)
   3         3      3            Tx Syn o/p
             4      4            NC
   4         5      5            Tx signal o/p
                    6            NC
   5         6      7            Tx Data i/p
   6         7      8            Tx enable (invert logic)
   7         8      9            Bandpass o/p (Rx 900-2100 filter)
   8         9     10            Rx enable
   9        10     11            On chip analog bias o/p (Vdd/2)
  10        11     12            GND
  11        12     13            Unclocked data o/p (received asyn data)
  12        13     14            Clocked data o/p
  13        14     15            Carrier detect o/p
  14        15     16            Rx signal i/p
            16     17            NC
  15        17     18            Rx Sync o/p (1200Hz clock in sync with data
  16,17     18     19,20         NC
  18        19     21            Clock rate select input 1-4.032Mhz, 0-1.008Mhz
  19        20     22            Carrier detect time constant
            21     23            NC
  20        22     24            Vdd