Specifications

AN93
146 Rev. 0.8
Intrusion/Parallel Phone Detection
Example
The modem may share a telephone line with a variety of
other devices, particularly telephones. In most cases,
the modem has a lower priority for access to the phone
line. Someone dialing 911 in an emergency, for
example, has a higher priority than a set-top box
updating billing information. If someone is using a
telephone, the modem should not go off-hook. If
someone picks up a phone while the modem is
connected or dialing, the modem should drop the
connection and allow the phone call to proceed. The
modem must monitor the phone line for intrusion in both
the on-hook and off-hook conditions.
Intrusion Detection—On-Hook Condition
When the ISOmodem is on-hook, the U79[4:0] (LVCS)
value represents TIP-RING voltage, the ISOmodem is
in the command mode, and the host can easily monitor
LVCS with the AT:R79 command. A typical local loop
has a TIP-to-RING voltage greater than 40 V if all
devices sharing the loop (telephones, FAX machines,
modems, etc.) are on-hook. The typical local loop has a
large dc impedance that causes the TIP-RING voltage
to drop below 25 V when a device goes off-hook. The
host can monitor LVCS to determine whether the TIP-
RING voltage is approximately 40 V or something less
than 25 V to determine if a parallel device is off-hook.
This type of monitoring may also be performed with the
%V1 command. Alternatively, the host could be
programmed to periodically monitor LVCS and store the
maximum value as the “all devices on-hook” line voltage
and establish the on-hook intrusion threshold as a
fraction (possibly 50%) of that value. This would allow
the system to adapt to different or changing local loop
conditions. An on-chip adaptive monitoring algorithm
may be enabled with the %V2 command.
Line not present/in use indication (method 1 - fixed)
If enabled with %V1, this feature checks the line status
before going off-hook and again before dialing. Before
going off-hook with the ATD, ATO, or ATA command, the
Si2493/57/34/15/04 reads the line voltage and
compares it to U83 (NOLN)[15:0] and U84 (LIUS)[15:0].
A debounce timer controlled by U-registers 50 and 51
prevents polarity reversals from being detected as a
loss of loop current. The intrusion detection algorithm
continues to operate if U77(HOI)[11] is set. In this case,
a parallel phone intrusion while off-hook gives a “LINE
IN USE” result code to indicate the Si2493/57/34/15/04
has gone on-hook due to a parallel phone intrusion.
Note: This method may not be as good as method 2, particu-
larly for low-voltage lines.
Pros:
 Easy to understand and predict
 Allows reference level control
Cons:
 Chosen levels must work for all lines—Not adaptive
Line not present/in use indication
(method 2—Adaptive)
This method is enabled through %V2. This feature
checks the line status before going off-hook and again
before dialing. While on-hook, the part monitors line
voltage and updates U85(NLIU)[15:0] with this value.
Before going off-hook with the ATD, ATO, or ATA
command, the Si2493/57/34/15/04 reads the line
voltage and compares it with the stored reference.
Loop voltage Action
0 <
LVCS < U83 Report “NO LINE” remain on-hook
U83 < LVCS <
U84
(U-register)
Report “LINE IN USE” remain on-
hook
U84 < LVCS Go off-hook and establish connec-
tion
Loop Voltage Action
0 <
LVCS < 6.25% x U85 Report “NO LINE”
remain on-hook
6.25% x ref. < LVCS <
85% x
U85
Report “LINE IN
USE” remain on-hook
85% x ref. < LVCS Go off-hook and
establish connection