Fig. 1 - FXS Implementation

Fig. 2 - FXO Implementation

Fig. 3 - Possible Alternative Sound Card Interface
(Saves 3 Capacitors)
For Both FXO & FXS

Fig. 4 - Minimum FXS Implementation
One Way - Receive Only
(Rotary Dial Phones Only)


1. In the FXO implementation, if your CO line applies ringing current, your sound card may be TOASTED if you don't remove it from the circuit during the ringing cycle. Alternatively, you may want to disconnect the network "F" terminal.

2. If your soundcard has DC blocking caps in the audio in and out lines, you should be able to eliminate the capacitors (see Fig. 3). Some sound cards seem to have them and some don't. In most cases you can tell by a close examination.

3. You may be able to improve the hybrid null by clipping the strap between P-Q on the network, and/or adding additional resistance and/or capacitance in the circuit, essentially making your own balance network.

4. Purists may want to add resistors to simulate the transmitter and receiver. Likewise, you may want to supply line loop current to the network C and RR terminals in the FXS circuit (Fig. 1).

5. Note that you can have up to two FXO and/or FXS ports, using a stereo soundcard (that is, your choice of two FXS ports, two FXO ports, or one FXO and one FXS).

6. Audio levels aren't at all critical as far as the hardware's concerned; you use the Linux mixer program to set the proper levels in software...

Questions? Email ikj1234i at yahoo dot com