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A MONOLITHIC subscriber line interface circuit (SLIC) for use in PABX or CO applications, recently fabricated with 80V dielectric isolation technology, will be described. The SLIC provides battery feed with power denial control and loop current limiting, overvoltage protection (with some external devices), ringing control, line supervision with offhook, ring trip and ground key detection and 214 wire conversion. In addition, an uncommitted opamp is included for external connection of a balance network.the SLIC. The Tip Feed (TF) and Ring Feed (RF) outputs are low impedance and require two series 300a resistors that limit current to the secondary protection during over-voltage surges and present a 600Q impedance to the loop. Over-voltages of up to 1kV into the Tip and Ring terminals are met by a high impedance thin-film resistor network which provides 2/4 wire conversion by sensing voltages, which are representative of Tip and Ring currents, across R B F~ and R B F~. Battery feed is accomplished by maintaining the dc component of TF and RF at -4V and -44V respectively.