Browse Prior Art Database

Subscriber Line Interface Circuit

IP.com Disclosure Number: IPCOM000042914D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Ferry, M: AUTHOR [+2]

Abstract

A low cost subscriber line interface circuit (SLIC) is described that is attachable to time division switching systems or to space division switching ones. Basically, the SLIC is a current feed circuit powered by a battery (-48 volts). It provides for single- to double-ended conversion between the network and the telephone station. Its design prevents transmission of line impairment signals into the network circuit and into other station lines. Referring to the figure, SLIC, by means of A and B wires, connects telephone set 10 to the network (NETW). Two mains blocks, A and B, perform, different functions as described hereunder. Block A performs a function in DC and another one in AC. It comprises an operational amplifier OA1 driving transistor T1.

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Subscriber Line Interface Circuit

A low cost subscriber line interface circuit (SLIC) is described that is attachable to time division switching systems or to space division switching ones. Basically, the SLIC is a current feed circuit powered by a battery (-48 volts). It provides for single- to double-ended conversion between the network and the telephone station. Its design prevents transmission of line impairment signals into the network circuit and into other station lines. Referring to the figure, SLIC, by means of A and B wires, connects telephone set 10 to the network (NETW). Two mains blocks, A and B, perform, different functions as described hereunder. Block A performs a function in DC and another one in AC. It comprises an operational amplifier OA1 driving transistor T1. The (+) input of OA1 is biased to the ground level by resistors R2, R3 (200 kilo-ohms each), while the (-) input is connected to node M through resistor Ra and to the battery through resistor Rb (both 200 kilo-ohms). In DC, block A operates as an emitter follower amplifier and stabilizes the potential of node M at ground level. This level is defined by the voltage drop in resistor R7 (40 ohms) created by current I circulating in transistor T2, resistor R7, resistor R5 (40 ohms), line impedance, resistor R4 (40 ohms) and transistor T1. Block A also performs a function in AC; i.e., it performs an inverting function. When NETW produces a signal at node M, i.e., on (-) input of OA1, this signal is transformed into a signal of the same amplitude, but in phase opposition, that applied at...