Specifications

Si2457/34/15/04

Rev. 1.3 19

4.11. Caller ID Operation

The Si2457/34/15/04 supports full type I and type II

caller ID detection and decode. Caller ID is supported

for the US Bellcore, European ETSI, UK, and Japanese

protocols and is enabled via the +VCID, +VCDT, and

+PCW commands.

4.12. Parallel Phone Detection

The ISOmodem

is able to detect when another

telephone, modem, or other device is using the phone

line. This allows the host to avoid interrupting another

phone call when the phone line is already in use and to

intelligently handle an interruption when the ISOmodem

is using the phone line.

4.12.1. On-Hook Line-in-use Detection

When the ISOmodem is sharing the telephone line with

other devices, it is important that it not interrupt a call in

progress. To detect whether another device is using the

shared telephone line, the host can use the ISOmodem

to monitor the TIP-RING dc voltage with the line voltage

sense (LVS) register (U6C, bits 15:8).

The LVS bits have a resolution of 1 V per bit with an

accuracy of approximately ±10%. Bits 0 through 6 of

this 8-bit signed twos complement number indicate the

value of the line voltage, and the sign bit (bit 7) indicates

the polarity of TIP and RING. The ISOmodem can also

monitor the TIP-RING dc voltage using the LVCS

also the %Vn commands for automatic line-in-use

detection.

4.12.2. Off-Hook Intrusion Detection

When the ISOmodem is off-hook, an algorithm is

implemented in the ISOmodem to automatically monitor

the TIP-RING loop current via the LVCS register. During

the off-hook state, the LVCS register switches from

representing the TIP-RING voltage to representing the

TIP-RING current. See Figure 10 on page 20. Upon

detecting an intrusion, the ISOmodem alerts the host of

the condition via the INT pin.

Figure 9. Loop Voltage

0 3 6 9 12151821242830 33363942454751 54576063666972 100

75 78 81 84 87

LVCS

BITS

Loop Voltage (V)