MODEM LOOP CURRENT DETECT SYSTEM TO DETECT AN OFF-HOOK CONDITION IN AN EXTENSION TELEPHONE.

摘要

A telephone line monitoring circuit uses an optoisolator (OPTO 2) to convert the telephone line current to a voltage which is applied to the input of an A/D converter (118). The A/D values are coarse values representing telephone line current. The A/D is sampled at a predetermined interval to identify changes introduced by the telephone company central office, and to take those changes into account when deciding if an extension telephone has gone off-hook. When a customer causes an extension telephone to go off-hook, the line current divides between the extension telephone and the IRD modem (100), and such change is detected.