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Rev. 1.4
6.6.1.2. 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 (5 (NLIU) [15:0] with this value.
Before going off-hook with the ATD, ATO, or ATA command, the ISOmodem reads the line voltage and compares it
with the stored reference.
To prevent polarity reversals from being detected as a loss of loop current, a debounce timer controlled by U-
registers 50 and 51 is used. However, if the HOI bit is set, a parallel phone intrusion while off-hook will give a LINE
IN USE result code to indicate that the ISOmodem has gone on hook due to a parallel phone intrusion.
6.6.2. Intrusion Explanation
When the ISOmodem is off-hook, the value in U63 [15:8] (LCS) represents loop current. If the ISOmodem is in the
data mode, it is difficult for the host to monitor the LCS value. For this reason, the modem monitors the line for off-
hook intrusions; when an intrusion is detected, the modem can interrupt the host and go back on-hook
automatically, based on parameters set by the host.
To avoid false intrusion detections due to loop transients during the on-hook to off-hook transition, there is a delay
between the ISOmodem going off-hook and the start of the intrusion algorithm. The delay is set by U77 [15:12]
(IST) (Intrusion Settling Time). When the settling time has passed, the off-hook intrusion algorithm monitors the
value of LCS at a sample rate determined by U76 [15:9] (OHSR). The samples are processed one of two ways,
depending on whether UAE [7] (UAEM) is set. If UAEM = 0, the modem declares an intrusion on two consecutive
low-current samples; this is the legacy mode and is compatible with previous versions of the ISOmodem. UAEM =
1 enables an enhanced algorithm that offers increased immunity to false detections. The enhnanced algorithm is
used in applications where glitches of less than 800 ms must be ignored but an intrusion is declared for a one
second drop in loop current.
The enhanced algorithm is enabled by setting UAEM = 1, initializing UAE [5:0] (DCLM) with a non-zero delta
current value and (if it is known in advance) a reference current in UAE [15:8] (ACLM). The modem samples the
loop current (LCS) and compares the sample to the reference value in ACLM. If ACLM = 0, the modem stores the
initial loop current in ACLM and waits for the next sample. The modem stores two samples before it starts looking
for an intrusion. Starting with the third sample, if ACLM ? 0, the modem does the comparison:
LCS < |ACLM| - |DCLM|
and stores the result. ACLM is updated if the above comparison is false for sixteen consecutive values of LCS and
if U76 [8] (FACL) = 1. If this comparison is true for ten consecutive values of LCS, an intrusion is declared. LCS is
sampled at a rate of 8 x OHSR; setting OHSR to a value of 800 ms, an intrusion is detected when the interruption
has continued for at least 1.0 s.
If UAEM = 0 (the default setting), the legacy algorithm is in effect: the modem compares each LVCS sample to the
reference value in U76 [4:0] (ACL). If LVCS is lower than ACL by an amount greater than the value set in U76 [7:5]
(DCL) for two consecutive samples, an intrusion is declared.
The ISOmodem can automatically update ACL with the LVCS value while off-hook (unless an intrusion has
occurred). An ACL value can be written by the host and forced to remain unchanged by setting U76 [8] (FACL) = 1.
If ACL = 0, the ISOmodem does not begin the intrusion algorithm until after two LVCS samples have been
received. .ACL = 0 at the first off-hook event after reset, unless a value is written to it by the host. If the host writes
a non-zero value to ACL prior to the ISOmodem going off-hook, a parallel phone intrusion occurring during the IST
interval and maintained until the end of the IST interval triggers a PPD interrupt.
The legacy intrusion detection algorithm is as follows:
Loop Voltage
Action
0 < LVCS < 6.25 % x U85
Report NO LINE and remain on-hook
6.25 % x U85 < LVCS < 85% x U85 Report LINE IN USE and remain on-hook
85% x U85 < LVCS
Go off-hook and establish connection
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