From: Rick_Michael_Cortese@cup.portal.com
Newsgroups: comp.sys.atari.8bit
Subject: Re: Connecting an RS232 modem to an Atari?
Date: Tue,  8 Sep 92 11:15:40 PDT
Organization: The Portal System (TM)
  <65456@cup.portal.com>


EXAMPLE #3

This is the one I really recommend building, if you buy a NULL
modem connector you'll be able to use it to hook up with both
computers & a modem.

Explodo view for connection to male
 DB-25

 Solder side of DB-25 (I stick
 everything inside the hood)

   ____
  :    \____
  :         \____
  :              \
  :  1            :
  :             14:
  :               :
  :++2+-------------------------------------+-WHITE
  ::            15:                         :
  ::              :    5.6K                 :
  :: 3+---------------+\/\/\+-+---BLUE      :
  ::  5.6K      16:           :             :
  :+\/\/\/+-+     :           +             :
  :  4+-----+               /---/ 5.1V    /---/
  :         :   17:          /:\  ZENERS   /:\
  :         :     :           :             :
  :  5+-----+     :           :             :
  :             18:           +-------------+
  :               :           :
  :++6            :           :
  ::            19:           :
  ::              :           :
  :: 7+-----------------------+-BLACK &
  :+-----------+20:             ORANGE
  :     *SEE NOTE :
  :  8+-+         :
  :             21:
  :               :
  :  9            :
  :             22:
  :               :
  :  10           :
  :             23:
  :               :
  :  11           :
  :             24:
  :               :
  :  12           :
  :             25:
  :               :
  :  13       ____:
  :      ____/
  :_____/



*NOTE:  Some printers use pin #8 as CARRIER DETECT.  I believe the
NEC Spinwriter is one.  I never ran into any thing that needs it,
but you may want to use pin #8 to #20 if you have a problem.  In
this case DO NOT CONNECT PIN #6!  Maybe use the 4-5-6 connection.
#6 and #8 are both outputs from DTE's so you could have one trying
to bring the other to a +10 or a -10V!  I've seen a reference that
does this, but it doesn't seem like a good idea.  Besides,  the
NEC uses pin #19 & maybe #20 for DTR and is such an odd case, it
may not work anyway.  If you use 5.6K resistors you could connect
just about anything to anything, but why?  If you buy a NULL modem
for connecting your computers, it'll probably take care of the CD
line anyway.  As a final note; if you really want to be safe get a
pin-out for the devices you want to connect.  Make sure you never
have two outputs from either device (or themselves for that
matter) connected to each other.

Here's a list of standard pin assignments for RS-232 to help you
along & double check my connections. *****NOTE***** I do connect
pins 4 & 5 to pin #2 (or #3) through a 5.6K resistor.  This is
necessary because although the joystick port can deliver a 0V
level, it can't drive a +5V by itself.  You could probably also
substitute pin #20 vs #4 for driving pins # 5 & #6 if you never
want to connect to a modem.


PIN #          ASSIGNMENT
BOTH DEVICES
1              PROTECTIVE GROUND
7              SIGNAL GROUND
THE OUTPUTS (DTE DATA TERMINAL EQUIPMENT)
2              TRANSMITTED DATA (TD)
4              REQUEST TO SEND (RTS)
20             DATA TERMINAL READY (DTR)
23             DATA SIGNAL RATE SELECTOR
24             TRANSMITTER SIGNAL ELEMENT TIMING
14             SECONDARY TRANSMITTED DATA
19             SECONDARY REQUEST TO SEND
THE INPUTS (DATA TERMINATING CIRCUIT EQUIPMENT)
3              RECIEVED DATA (RD)
5              CLEAR TO SEND (CTS)
6              DATA SET READY (DSR)
22             RING INDICATOR (RI)
8              CARRIER DETECT (CD)
21             SIGNAL QUALITY DECTOR
23             DATA SIGNAL RATE SELECTOR
15             TRANSMISSION SIGNAL ELEMENT TIMING
17             RECIEVER SIGNAL ELEMENT TIMING
16             SECONDARY RECIEVED DATA
13             SECONDARY CLEAR TO SEND
12             SECONDARY RECIEVED LINE SIGNAL DETECTOR