Fax to Modem interface

** DO NOT CONNECT THIS TO THE PHONE LINE!!!! **
This is only for connecting two telephone equipments TOGETHER!

This circuit is based on circuit posted to sci.electronics.design newsgroup by Paul Grohe ([email protected]).

Building the circuit

Get two phone jacks, a 12V source, and a 680 ohm, 1/2 W resistor. Connect the components to make the circuit below:


Green              680 1/2W    +   12V    -          Red
--------------------/\/\/\------|'|'|'|'-----------------
To Fax                                           To Modem
_________________________________________________________
Red                                                  Green

The circuit can be easily built to a small plastic box with telephone connectors in both ends. The circuit takes about 10-20 mA current, so if the circuit is not in use all the time it is best to use batteries. You can also use small wall transformer to power the circuit. Remeber to take a wall transformer which gives good quality output (regulated model preferred), because if the wall transformer gives out noise, it can be hard on the line (usully you hear 50/60 Hz buzzing).

The 680 ohm resistor (used for limiting the power from 12V source to safe value for telephone electornics) causes some problems because it causes imedance mismatch to the telephone line. If this causes problems, you can add a capacitor to the circuit which makes sure that the circuit apperas to be like piece of wire in voice frequencies you can avoid mismatch caused by that 680 ohm resistor. I added a 22 ohm resistor in series with the capacitor to limit the surge current which appears when the equipments go on-line. For normal FAX to modem usage this modification should not be necessary because equipments should be able to cope with those impedance mismatches the original circuit causes.

                      2.2 uF
                      +||       22 ohm
           +-----------||-------/\/\/\-------+
           |           ||                    |
           |                                 |
Green      |        680 1/2W    + 12V  -     |        Red
-----------+--------/\/\/\------|'|'|'|'-----+-----------
To Fax                                           To Modem
_________________________________________________________
Red                                                  Green

** DO NOT CONNECT THIS CIRCUIT TO YOUR PHONE LINE WHICH GOES TO TELEPHONE COMPANY!!!! **

Using the circuit with two telephones

Just connect the telephones to both ends of the circuit. The circuit gives power to both telephones and provides the audio connection between telephones. The circuit does not provide any ring voltage, so you can't use this circuit to make telephones to ring.

Connecting two modems together with the circuit

You can connect two computer modems using this circuit together. Just connect the first modem as shown in the picture and replace the fax machine with the second modem. You can establish connection between modems by making one modem to make a call by typing ATD command and making other to receive it using ATA command.

Some modems will work (especially those with leased line mode) when they are just wired together without the powering circuit described above. But to be sure that all modems work it is best to use the powering circuit.

Using FAX machine as scanner with you faxmodem

Set the fax machine to send. Load the picture into the fax and dial a digit or two. The fax should start "beeping". Hit the computer fax softwares' "immediate receive" or "off-hook" button. The two should handshake and the fax should go through. Easy.

Be sure to turn off the ID headers, and enable the highest resolution mode and/or greyscale mode on the fax machine.

Another circuit idea

If cable is not too long, try this circuit to connect two modems to each other:

               +---------+---o +9V
               |         |
               \         \
            1k /         / 1k
               \         \
               |   1uF   |
           o---+---| |---+---o 
   modem1                        modem2
           o---+---| |---+---o
               |   1uF   |
               \         \
            1k /         / 1k 
               \         \    
               |         |
               +---------+---o -9V

Those telephone line circuits use two separate sets of 1 kohm resistors to feed the separate working current to both of the modems. The 1 uF capacitors will pass the audio nicely from one line to another.

Possiblities to do the same thing without building the circuit

If you have two telephone lines then it is easy, just use fax to call to your FAXXMODEM. And if you don't have two phone lines, it's still not too hard. If you have your fax machine and pc fax-modem card plugged in to the same phone line with one of those little "T" widgets. You can start to send fax to your own phone number. In that way you get dial tone and line current from your normal telephone line. Then you hit "answer now" on the. When PC answers to the FAX machine, unplug the "T" form telco jack to get rid od overriding busy noise. Now everything should work nicely.

When you use FAX machine as scanner the resultant scanned image might not be worth the trouble. The FAX image in black and white (no gray scale), and the resolution is quite poor.

Tomi Engdahl <[email protected]>