Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 161)

AN93
Rev. 1.3 161
6.6. Intrusion/Parallel Phone Detection
The modem may share a telephone line with a variety of other devices, especially 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.
6.6.1. 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 line (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 allows the system to adapt to different or changing local loop
conditions. An on-chip adaptive monitoring algorithm may be enabled with the %V2 command.
6.6.1.1. 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 ISOmodem reads the line voltage and compares it to U83
(NOLN) [15:0] and U84 (LIUS) [15:0].
A debouncing 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 ISOmodem has gone on-hook due
to a parallel phone intrusion.
Note: This method may not be as desireable as method 2, particularly for low-voltage lines.
Pros:
 Easy to understand and predict
 Allows reference level control
Cons:
 Chosen levels must work for all lines—not adaptive
Loop Voltage Action
0 < LVCS < U83 Report NO LINE and
remain on-hook
U83 < LVCS < U84
(U register) Report LINE IN USE and
remain on-hook
U84 < LVCS Go off-hook and establish
connection