The figure below shows a redundancy scheme where neither a separate I/O card, nor a Y-cable patch-panel exists. An external resistor lies between the chip and the transformer for both primary card and Standby card. Because TIP/RING leads for two cards are connected together after the transformer, two resistors are essentially in parallel thus changing the impedance matching. Therefore, the resistor on Standby card needs to be switched out. A solid-state analog switch can be used to switch the resistor in or out, as shown in the figure. The primary card is in normal operation with 120Ω resistor switched in the circuit. Standby card, on the other hand, is in high impedance state, and its resistor is switched out of the circuit by the analog switch. Upon primary card failure, Standby card will return to normal operation (from high-impedance state) and its external resistor is switched in. The opposite will occur in primary card. Refer to application note AN-858 for more details. For other questions not addressed by the Knowledge Base, please submit a technical support request.