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4 THEORY OF OPERATION
4.1 Introduction
The F271XA is a subscriber side telephone modem with an SFP port for fiber-optic
connections. These Modems are designed to work with F270XA exchange side telephone modems
or another F271XA in pairs and interface with audio grade signals. These units are housed in
VERSITRON FOM II chassis: HF-1, HF-2SS and HF-20A or in a VMX20 Versimux II chassis.
Chassis descriptions are found in section 1.2.3 .
4.2 Power Supply and Alarm Circuit
4.2.1
Power Supply Circuit
When 12VDC is applied to the circuit, the unit should draw between 200mA and 1000mA
depending on the modem state. The unit has a series of switching power supply circuits, which
converts 12V to 3.3VDC, 1.8VDC, and 1.2VDC.
4.2.2
Alarm Circuit
This unit does not have an alarm circuit. Therefore, the Alarm signal on the P1 connector
will not be driven by this unit.
4.3 Functional Description
The unit consists of the following circuits:
4.3.1 – Ring Detection
4.3.2 – Ring-Down
4.3.3 – Analog-to-Digital Conversion
4.3.4 – FPGA
4.3.5 – Fiber-Optic Transceiver Circuit
4.3.6 – Backplane Transceiver Circuit
4.3.1
Ring Detection
The F271XA detects, through its FPGA, a ring signal coming through the fiber interface for
the telephone port and then enables the ringing signal.
4.3.2
Ring-Down
In a ring-down configuration the F271XA detects an off-hook condition and asserts the ring
signal for transmission over the fiber to the down-stream F271XA. On the other end the F271XA
handles the ringing signal as described above ( 4.3.1 ).
4.3.3
Analog-to-Digital Conversion
Analog to Digital conversion is handled by the phone CODEC chipset. A 16-bit sample is
gathered at an 8kHz sampling rate. On the return side Digital to Analog conversion is handled in
a similar way.
