Minh, at 00:20 on Fri, Feb 06 1998, you wrote to All ...
MV> is there a difference between a null modem cable and a serial 
MV> (female-female) cable ? i'm trying to link two PCs together for basic 
file 
MV> xfers.
yes, nullmodems have TX RX and handshaking lines crossed over. A straight 
through lead wouldn't. The cables are easy to make and can also be bought 
from most computer dealers. A parallel file transfer cable is also available. 
You need to ask for Laplink(tm) or Fastlynx cables.
This is from the Fastlynx docs
Eight or nine conductor cable works well, and allows for addition of
connections if they should be needed in the future. To use the crimp type
connections, you need the tool to crimp the pins onto the wires.  If you plan
on making several cable in your lifetime, then the tool is well worth it. 
Otherwise, you're probably better off paying someone else to make it.  Scout
your local electronics store.  I've had good results with the connectors from
made by AMP, sold by your finer electronics stores.  Also, Belden cable is
some of the best stuff to buy, if your retailer sells it.
If the computers you are connecting have 9 pin serial connectors, you need
the female (with holes) connectors on both ends of your cable. (IBM's and
compatibles have male connectors for their 9 pin serial ports). If yours are
female, make sure the connector is not a video connector instead of a serial
port connector.  25 pin COM ports on IBM's and clones are typically male,
also.  Whether you make a cable with 9 pin or 25 pin connectors (or one at
each end), pre-made adapters can be purchased to convert from 9 pin to 25
pin, and vice versa.  I made all mine with 9 pin ends, and use 9 pin to 25
pin adapters with good results.
TIP: Buy enough cable! As long as you're making it, make a length of about 8
or 10 feet. I've made cables, when using shield, about 35' in length 
with no problems. Better a little too long than too short. Avoid excess 
length,
however. 
TIP: If you end up with Radio Shack parts, wrap electrical tape around the
cable at each end to fit underneath the connector.  The connectors and cable
at Radio Shack don't fit real snug together, allowing the cable to slip out
and put stress on the electrical connections.  You'll see what I mean when
you are closing the connector together.
TIP: If you are using Radio Shack cable, try to use the shield (the braided
wire around the outside of the cable as the ground connection.  This task is
made easier if you use Belden cable - it comes with a wire that runs along
the shield, which is foil instead of that crummy braided wire.  By using the
shielded part of the cable as the ground connection, it helps protect from
electrical noise.  This is more important with longer cables.
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              The following is part of the docs for FastLynx:               
-------------------------- Cable Specifications ----------------------------
Following is a description of the pin connections for a FastLynx 7-wire
serial cable.  The cable is a 4-headed cable with a 9-pin and 25-pin female
connector on both ends.  The cable is wired as follows:
    9 pin   25 pin          25 pin  9 pin
    -----   ------          ------  -----
    pin 5   pin 7     pin 7   pin 5   (Ground - Ground)
    pin 3   pin 2     pin 3   pin 2   (Transmit - Receive)
    pin 7   pin 4     pin 5   pin 8   (RTS - CTS)
    pin 6   pin 6     pin 20  pin 4   (DSR - DTR)
    pin 2   pin 3     pin 2   pin 3   (Receive - Transmit)
    pin 8   pin 5     pin 4   pin 7   (CTS - RTS)
    pin 4   pin 20    pin 6   pin 6   (DTR - DSR)
The ground wire is connected to the same pin on both ends.  The last three
wires are a reverse of the prior three.
Following is a description of the pin connections for a FastLynx parallel
cable.  The cable has a male DB25 connector at both ends.
            25 pin          25 pin
            ------          ------
            pin 2     pin 15
            pin 3     pin 13
            pin 4     pin 12
            pin 5     pin 10
            pin 6     pin 11
            pin 15    pin 2
            pin 13    pin 3
            pin 12    pin 4
            pin 10    pin 5
            pin 11    pin 6
            pin 25    pin 25
The second set of 5 wires is the reverse of the first set.
     ******************************************************************
     The following cable will allow transfers using LapLink III.  However, it
doesn't support the feature of installing the software from the remote.  The
FastLynx cable above does work with all the features of FastLynx.  The
following cable merely transmits and receives data.  It cheats by jumping
connections at each end to trick the computer into thinking it's connected to
another computer.  The FastLynx cable above allows the 2 PC's to actually
communicate.  However, I haven't gotten LapLink III to install software from
remote with FastLynx's cable, either.  FastLynx does it just fine when using
a FastLynx type cable. Update 11/13/90: Uncle Heavy uploaded diagrams to make
a true Laplink III cable. The instructions are identical to the FastLynx 
cable.
                            IMPORTANT! See *
                                    ³
                                    ³
Connector 1                         ³                         Connector 2 
-----------                         V                         -----------
Transmit Data  2  2  Transmit Data 
                                        \/ 
Receive Data   3  3  Receive Data  
             
RTS            4  4  RTS           
                       ³                           ³
CTS            5  5  CTS           
                       ³                           ³
DSR            6  6  DSR           
                       ³    º                 º    ³
Ground         7  7  Ground        
                       ³    º                 º    ³
CD             8  8  CD            
                            º See         See º
DTR            20  20  DTR

Explanation:
------------
*Connect pin #2 of one connector to pin #3 at the other end. This is known as
a "pin 2 to 3 crossover". That way one computer receives what the other is
transmitting.
At each end, connect pins #4, #5, & #8 together. 
Also at each end, connect pins #6 & #20.   
** If you are using a nine pin connector, this connection is not needed as
there is no pin #20. A connection to pin #6 is not needed.           
TIP: Before you get too far, cut off about 3/4 inch of cable off one end of
your length of cable. Then, strip the insulation and foil from this piece or
use tweezers or forceps to remove the 9 wires from inside. Strip the
insulation off both ends of 4 of these wires, 6 if making a 25 pin connector
cable. These short pieces of wire will be needed to make the jumpers at each
connector. Twist one end of each of 2 wires together, and solder them both
pin #5. Then one wire can go to pin #4, and the other to pin #8 as in the
diagram.
In case you want to know:
-------------------------
TD  =  Transmit Data
RD  =  Receive Data
RTS =  Request To Send
CTS =  Clear To Send
DSR =  Data Set Ready
CD  =  Carrier Detect
DTR =  Data Terminal Ready
I received instructions from a BBS one day on how to construct a null modem
cable, and decided to add some comments of my own.  Then, I stumbled upon the
FastLynx documentation.  I've made their cable, and it works quite well (at
least the serial cable does.  I think I made the parallel cable a little
long.  Back to the crimping tool!).  So, this text is a culmination of all
three - the original file, my comments and ideas, and part of the FastLynx
documentation.               -"Joe's Cat"
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Regards, Peter
peter.lc@uq.net.au  ----  http://www.uq.net.au/~zzplanec
--- Msgedsq 3.10
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