Here you go:

Pin              R1:         R2: & R3           R4:                       
         P:
-----              -----         -------------           -----            
                    ---
   1              DTR          DTR                DTR always on (+10v)    
Strobe (inverted)
   2              CD             nc                   nc                  
             Data Bit 0
   3              TX              TX                  Send Data           
         Data Bit 1
   4              RX              RX                 Receive Data         
       Data Bit 2      
   5              GND           GND              GND                      
      Data Bit 3
   6              DSR           DSR                 nc                    
         Data Bit 4
   7              RTS             nc                 RTS always on (+10v) 
  Data Bit 5
   8              CTS             nc                   nc                 
            Data Bit 6
   9               nc               nc                 -8v                
                    nc
  10                                                                      
                       nc
  11                                                                      
                  GND
  12                                                                      
                  Fault (inverted)        13                              
                                                          Busy
  14                                                                      
                    nc
  15                                                                      
                  Data Bit 7

NOTES:

 On printer, pin-out assumes Atari 825 printer and cable may have to be
customized for other printers.  Also, pin 12 must be +5v for port to
operate.  If printer doesn't need this line, connect pin 9 to pin 12.

For most printers, the cable connections are as follows (from the P: R:
Connection Manual):

Atari 850 side (DB15P)                   Printer side (36-pin Centronics
male)
----------------------------------                 
-----------------------------------------------------
            1                                                             
   1
            2                                                             
   2
            3                                                             
   3
            4                                                             
   4
            5                                                             
   5
            6                                                             
   6
            7                                                             
   7
            8                                                             
   8
           11                                                             
 16
           12                                                             
 32
           13                                                             
 11
           15                                                             
  9            

Also from the P: R: Connection Manual, the connections for R1: and R2:
(and presumably for R3: on the 850):

Atari 850 side (DB9P)                            Modem Side (DB25P)
--------------------------------                           
-------------------------------
           1                                                            20
           2                                                             
8
           3                                                             
2
           4                                                             
3
           5                                                             
7
           6                                                             
6
           7                                                             
4
           8                                                             
5
No connection to shield                               Frame to the shield
wire

The 850 manual also gives this hookup for R4: with a 20 mA loop device:

      Pin                               Function
      -----                               ------------
        1                                Send data + \_______ 20 mA
        3                                Send data -  /
        7                                Receive Data + \______ 20 mA
        9                                Receive Data -  /