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Connecting two computers using their modems, without a telephone line


This Tutorial Courtesy of John Meshkoff


One method for connecting two computers together to transfer files is by a direct modem to modem connection. Some reasons to do so: the two computers may not have diskette compatibility; one computer may have a modem, but no free serial port; a serial port may be free, but (especially on some modern PC's with Asian made motherboards) the back panel connector may be wired incorrectly; getting two different computers to communicate correctly via serial port connection (i.e., "null-modem") may be difficult; getting files other than text files to transfer via rs-232 may be tricky. A direct modem connection has the advantage that standard terminal programs are designed primarily to work with a modem. Another possibility is that of printing via a fax machine; some PC printers are multi-function: fax/scanner/copier/printer. This is another way an old Mac might access such devices. I have seen this mentioned, but haven't been able to try it yet (I don't yet have a compatible Mac fax program). It might also be possible to use a fax machine as a scanner this way; connect one computer modem to the fax machine.


One necessity for a reliable connection of this type is a "line simulator", a device to provide simulation of a telephone line, supplying the line current that modems are designed to work with. For our purposes, a very simple home-made circuit will do the job (you should be able to find most, if not all, of the parts at a Radio Shack store). This connects between two telephone modular jacks, and the two modems plug into the jacks:



Basic Circuit:


                             +  | | -
                            |   | |     R     |           
                            |  9 to 24V       |            
                            |                 |           
                                C .47uF     






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.


A more elegant solution when using an unregulated power supply is to add a 12 volt regulator such as a 78L12 (or a 7812 if you can't find the low power "L" version) as shown here:


                                  78L12           380 Ohm 1/2 Watt
                                 |         |
     	  +VDC IN >--------+-----|IN     + |----+--/\/\/\/--+--------GREEN
                         +_|_    |___GND___|   _|_         _|_
          -DC  IN >--+    _ _         |        _ _ .1uF    _ _ .47uF
                     |     |  10uF    |         |           |


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).


Once the modems are connected and the terminal programs are up, type AT to be sure you get back "OK" response. Next, type ATX3&C0 (that's C-Zero). C0 may not actually be needed on a PC, but is needed on the Mac; I type the same string at both my PC and Mac without any problems, though. Now, type ATD (the dial command) and enter on one computer; next type ATA and enter on the other. The two computers will connect, and you may now use the terminal to transfer any files. The Mac 68000 faq says Macs can use modem rates up to 56000; I use 19600 on my Plus and PC because it's the fastest available on the Windows 3.1 terminal program I use on my Win95 PC (Hyperterminal would allow faster rates, but is a little more trouble to use; see the link below "issuing Modem AT commands in Windows 95" for how to use it). Use your favorite terminal program on the Mac. I have downloaded Mac programs (in .hqx or .sit format) and transferred to my Plus with this method without any problem.



Powerbook Notes: "In a DOS and Windows World" By Cary Lu, contributing editor, Macworld August 1995 page: 153-154 (available at http://macworld.zdnet.com/ -- Search text: Mac+to+PC+data+transfer)


"Use old phones as an intercom"

"aITs Help Desk FAQs: Communications: issuing Modem AT commands in Windows 95"

Power Systems Group Phone: (650)742-8887 IMATRON, INC., 389 Oyster Point Blvd. Fax: (650)742-1009 So. San Francisco, CA 94080, USA

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