Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 148)

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6.3. Caller ID
The ISOmodem supports all major Caller ID (CID) types. CID is disabled by default (+VCID = 0). Setting +VCID = 1
via the AT+VCID = 1 command enables decoded (formatted) CID, while setting +VCID = 2 causes raw Caller ID
data to be output. The specific CID mode is selected by +VCDT, which is set to the US Telcordia standard by
default. The AT+VCDT = n command is used to define the CID mode according to the decimal values of n defined
in Table 94. U70 [4] (CID) is a sticky bit that is set when a CID preamble is received and cleared with an AT:I
(“Interrupt read”) command.
The following sections describe each CID mode.
6.3.1. Force Caller ID Monitor (Always On)
In this mode, the ISOmodem, when on-hook, continuously monitors the line for the CID mark sequence and FSK
data. This mode can be used in all systems, especially those requiring detection of CID data before the ring burst.
It is also useful for detecting voicemail indicator signals and for supporting Type II Caller ID. In most systems,
“Always On” is the preferred method, since it separates CID detection from ring detection.
6.3.2. Caller ID After Ring Only
The ISOmodem detects the first ring burst, echoes RING to the host, and prepares to detect the CID preamble. On
preamble detection, the modem echoes the CIDM response to the host (indicating the preamble was received and
FSK modulated CID data will follow), and INT is triggered if enabled.
Next the CID algorithm looks for the start bit, assembles the characters, and sends them to the host as they are
received. When the CID burst is finished, there is no more carrier and NO CARRIER is echoed to the host. The
ISOmodem continues to detect subsequent ring bursts, echoes RING to the host, increments the ring counter, S1,
and automatically answers after the number of rings specified in S0.
6.3.3. UK Caller ID with Wetting Pulse
The ISOmodem first detects a line-polarity reversal, echoes FLASH to the host, and triggers the INT pin. The
ISOmodem then searches for the Idle State Tone Alert signal and, when detected, echoes STAS to the host. After
the Idle State Tone Alert Signal is completed, the ISOmodem goes off-hook then on-hook to apply the 15 ms
wetting pulse to the local loop. Next, the ISOmodem prepares to detect the CID preamble. After 50 continuous
mark bits (1s) are detected, the CIDM response is echoed to the host indicating that the mark sequence was
received and that FSK-modulated CID data will follow, and INT is again triggered. Then, the CID algorithm looks for
the start bit, assembles the characters, and sends them to the host as they are received. When the CID burst is
finished, the carrier is lost, and NO CARRIER is echoed to the host. The ISOmodem detects ring bursts, echoes
RING to the host, increments the ring counter, S1, and automatically answers after the number of rings specified in
S0. If the wetting pulse is not required, +VCDT = 0 or 1 can be used in the UK.
6.3.4. Japan Caller ID
The ISOmodem detects a line-polarity reversal and a brief ring burst, then goes off-hook and triggers the INT pin.
CID data are sent using the V.23 specification. After detecting 40 mark bits (1s), the ISOmodem searches for a
start-bit. CIDM is echoed to the host when a start bit is received. The ISOmodem then begins to assemble
characters and sends them to the host. When the CID signal ends, the ISOmodem hangs up and echoes NO
CARRIER to the host. The modem then waits for the normal ring signal. Table 95 shows the AT command strings
that configure the ISOmodem for Japan Caller ID.
Table 94. Caller ID Modes
n +VCDT Settings
0 After ring only (default)
1 Force CID monitor (always on)
2 UK with wetting pulse
3 Japan