| 摘要 |
An secured fax adapter apparatus uses a relay circuit to selectively couple a G3 fax to a secured line via a secured telephone unit (STU). The G3 fax can alternatively be coupled to a public switched telephone network (PSTN) via a PSTN port. The secured fax adapter has an enhanced DAA having a controller, a fax communication circuit, a current limited off hook detection circuit, a relay circuit for selectively coupling the external device ports, and a ring generation circuit for producing a square wave for ring generation. In one embodiment, the current-limited off-hook detection circuit uses a symmetrical network of transistors coupled to diodes to limit the current drawn by a load. In another embodiment, the relay circuit has four relays for selectively coupling the G3 fax to the fax communication circuit, the STU to the PSTN, the G3 fax to the PSTN, or the STU to the fax communication circuit. The relay circuit has an additional relay for coupling the off-hook detection circuit to the fax communication circuit.
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