Circuit arrangement for telephone switching systems, in particular small PABX system, for which external connections can be set up to exchanges requiring loop dialling or DTMF dialling

摘要

The invention relates to a circuit arrangement for telephone switching systems, in particular small PABX systems, for which external connections can be set up to exchanges requiring loop dialling or DTMF dialling. A transistor combination (T1, T2) designed in the manner of a bistable multivibrator can be inserted into each external line (L1) leading to PABX systems with loop pulse control. For dialling signal transmission, the corresponding changeover of the multivibrator to the high-impedance or low-impedance condition is performed on the basis of numerical information transmitted in coded form by a subscriber station (A1). If DTMF dialling is required, the primary winding of a transformer (Ün) which is coupled with the line (Ln) concerned is connected in the activated condition to the output of a common oscillator component (OB). The individual transformers are activated alternately by a control unit via a switch (T8, T9) comprising the combination of an npn-transistor and a pnp-transistor. The individual transformers are decoupled with high impedance due to the special design of the switch. <IMAGE>