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[Tutorial] DIY crossover modem cable - Printable Version

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+---- Thread: [Tutorial] DIY crossover modem cable (/showthread.php?tid=1839)

[Tutorial] DIY crossover modem cable - SpookyZalost - August 16th, 2017

what I'm about to present to you guys is a really old trick but handy if you've got some old computers that you either want to get on the internet or want to link without Ethernet, effectively allowing you to dial into them in the same way you used to dial into an ISP.

now there's a couple things you need to know, first of all you're going to need to generate a line voltage on the phone cable, this is normally handled by the normal setup with the phone company but since you aren't dialing out of your house it's something to work around.

some modems actually can generate this for you so a bit of research is needed but most PCI/ISA/PCI-E etc modems are software based and only have the hardware required to send data.

Your Mileage May Vary.

second thing to know is that your data limit will be based on the slowest modem on the link, 33K or more is fairly standard though.

now that the basics are out of the way here's a couple circuits which you will need to splice into your modem to modem RJ11 (phone cable), cable.

the supplies to construct them are as listed

  • Telephone cord ( straight... about 10 or 15 feet ( 3 1/2 meters ) )
  • Capacitor ( .47 �FD - 1 �FD Electrolytic (Very Important))
  • Resistor ( 330 ohms - 390 ohms )
  • 9V battery snap connector(standard or heavy duty)
    I say go with the heavy duty snap connector because the connectors pull out too easily on the cheaper version
  • 9V battery
  • Utility knife ( 'Exacto' knife)
  • Electrical tape or solder w/ soldering iron.
you'll need to construct one of these two the first being the least complicated.

Code:
BASIC CIRCUIT

               +  | | -
          GREEN----------+---||||---/\/\/--+-------------RED
                           |   | |     R     |          
                           |  9 to 24V       |            
                           |                 |          
                           +---||------------+          
                               C .47uF    

            RED-----------------------------------------------GREEN

Note that the resistor value depends on the actual voltage used. For 24V about 1K Ohms max will give at most 24mA (12V @ 500 Ohms, 9V @ 380 Ohms); the resistance of the modem circuit will reduce this slightly (you may need to reduce the resistor value, but if it works with the values mentioned, leave it at that; I'm using 380 Ohms with a voltage input of 14V). The telephone company guarantees about 20mA minimum in an actual phone line, and we want to be about the same minimum. Note also that the battery shown can be replaced by a "wall wart" power supply; most of these are un-regulated (my "9V @ 130mA" plug in DC supply gives about 14V on this circuit when connected to a telepone for testing) and consequently will need an electrolytic capacitor of about 2200 uF across the power supply + and - terminals to reduce the "ripple" voltage (i.e., AC "noise"); be sure to match the polarity of the Electrolytic to the polarity of the power supply.

this second circuit has some extra stuff in case you end up using an unregulated power supply.
note that it includes a 12v regulator

Code:
Better circuit, but more complicated                                  78L12           380 Ohm 1/2 Watt
                                 _________
                                |         |
     +VDC IN >--------+-----|IN     + |----+--/\/\/\/--+--------GREEN
                        +_|_    |___GND___|   _|_         _|_
         -DC  IN >--+    _ _         |        _ _ .1uF    _ _ .47uF
                    |     |  10uF    |         |           |
       RED----------+-----+----------+---------+-----------+
       

       GREEN----------------------------------------------------------RED

In the circuits above, the 10uF is a Tantalum, but you can substitute electrolytic. The 0.1uF is ceramic, and the 0.47uF should be an audio grade bypass; I used a polypropelene capacitor here. Make sure the capacitors have voltage rating at least equal to the power supply you use; a voltage regulator needs at least two volts more at its input than its output, so the above needs at least 14 volts in. Most regulators will take up to about 30 volts max on their input, but the greater the differential, the greater the power loss and subsequent heat dissipation in the regulator. The 78L12 is designed for 200mA max, whereas the 7812 is about 1 to 1.5 A max (when used with an adequate heat sink (an aluminum "radiator" designed for this purpose).

here's an image of what your example should look like, this is the basic one listed above.
[Image: voltinducc.jpg]

finally once you've built your crossover modem cable you'll need to setup your network enabled PC as a router for your incoming modem connections.

effectively what you want to do is set it up so that any information requests coming in via the modem output onto the network and then the internet.

the setup I use utilizes PPP protocol and IP tables under linux.

here's an example of the command you enter to set it up.


Code:
pppd 115200 netmask $LOCAL_NET_MASK crtscts connect 'chat -v -f /etc/admin/network/winclient.chat' lock local ms-dns $LOCAL_IP ms-wins $LOCAL_IP persist proxyarp silent $PPP_IFACE_ADDRESS $TTY noauth

you will need to know your network router's information but I can confirm this works to use WIFI among other things, works with an old laptop to connect my 286 to the internet for example.

you will need to do some research on PPPD and setting up network passthrough, but once you're setup you only need to do it once.

an alternative to the above method would be to use the same software side code but use RS 232 serial ports with a crossover cable, though these are harder to find these days.

one last thing, if you want the easiest way to transfer files onto your older computer I highly recommend telnet, provided you set it up right and with some research you can setup a BBS based file server and telnet files from one computer to another, the server can even run off a flash drive, just make sure it knows where the downloadable files are.