6-pin modified modular jack (MMJ) used for serial ports on the VAXstation.

DEC carries four DB-to-MMJ adaptors.  They are internally wired as follows

                   Rdy Out  TX+  TX-  RX-  RX+  Rdy In            
 Adaptor   Gender     1      2    3    4    5     6       Use with:
--------------------------------------------------------------------------
 H8575-A     F      20      2    7    7    3    6&8     VTxxx terminal
 H8571-C     M       6      3    7    7    2     20     DEC printer
 H8571-D     M       6      3    7    7    2     20     Modem
 H8571-E     M      20      2    7    7    3    6&8     Female terminal
                                                        or LaserWriter
--------------------------------------------------------------------------

RS-232 using DB-25 connectors:
                                                DTE           DCE
                                             Terminal        Modem
                                            or computer
Pin Number Signal Name
    2          TD        Transmit Data                   -->
    3          RD        Receive Data                    <--
    7          GND       Ground                          ---
    6          DSR       Data Set Ready                  <--
    8          DCD       Data Carrier Detect             <--
   20          DTR       Data Terminal Ready             -->