Z02215
Single Chip Modem with Integrated Controller, Data Pump, and AFE
52
Data Access Arrangement
Figure 10 indicates an example DAA configuration for North America. Isolation
transformer (T1) couples the primary (line) and secondary (modem) sides, while
providing high voltage isolation. This wet transformer (allowing DC current) simpli-
fies the circuit and reduces the cost of the DAA.
On the Secondary side, the transmit (TxA+ and TxA–) and receive (RxA+ and
RxA–) are combined in the 4-wire to 2-wire hybrid circuit.
On the Primary side, the off-hook relay switches the phone line between a local
handset (phone) or the modem. The ring detect circuit consists of DC blocking
capacitor C304, current limiting resistor R305, zener diodes CR303 and CR304,
optocoupler U303, and its reverse protection diode D304. Protection elements
RV301, F301, C301, and C302 (and transformer T1’s isolation) provide higher
voltage capability for approval in some foreign markets. C201 and C202, for
example, may require replacement by Metal Oxide Varistors (MOVs) or Gas Dis-
charge Tubes (GDTs). The shunt relay reduces the DAA impedance during pulse
dialing. This relay is required for certain country approvals. The CID relay pro-
vides a signal path (through C4) to receive Caller ID signals without taking the line
off-hook.
PS001907-0904
Country Configuration
ZILOG [ ZILOG, INC. ]
