Specifications

AN93
94 Rev. 0.9
3.3.43. U79 (GEN4)
U79 is a bit-mapped register. Bits 15:6 are reserved.
Bits 5:0 represent the line voltage, loop current, or on-
hook line monitor (see Table 64). While the modem is
on-hook, the value in the LVCS register measures loop
voltage (see Table 65). This value can be used to
determine if a line is connected or if a parallel phone or
other device goes off-hook or on-hook. The accuracy of
the LVCS bits is ±20%. When the modem goes off-hook,
the value in the LVCS register measures loop current.
LVCS can indicate when a parallel phone or other
device goes on-hook or off-hook and detect whether
enough loop current is available for the modem to
operate or if an overload condition exists.
The line voltage monitor full scale may be modified by
changing R5 as follows:
V
MAX
=V
MIN
+ 4.2 (10M + R5 + 1.78k)/(R5 +1.78k)/5
See Table 65. LVCS is backward-compatible with older
ISOmodem
®
revisions. The value is absolute and does
not reflect loop polarity. See U6C (LVS) [15:8] for 1 V/bit
resolution and signed 2s complement format and
U63(LCS) [15:8] for 1.1 mA/bit loop current
measurement. The values for loop voltage and loop
current in U79 are calculated by the modem from the
values in U6C and U63 respectively.
Table 62. U77 Bit Map
Bit Name Function
15:12 IST Intrusion Settling Time (250 ms units) 1 second default.
11 HOI Hang-Up On Intrusion.
0 = ISOmodem does not automatically hang up after an off-hook PPD interrupt.
1 = ISOmodem automatically hangs up after an off-hook PPD interrupt.
10 Reserved Read returns zero.
9AOCAutoOvercurrent.
0 = Disable.
1 = Enable.
8:0 OHT[8:0] Off-Hook Time (1 ms units) 30 ms default.
Table 63. U78 Bit Map
Bit Name Function
15:14 IB Intrusion Blocking.
00 = No intrusion blocking.
01 = Intrusion disabled from start of dial to end of dial.
10 = Intrusion disabled from start of dial to IS register time-out.
11 = Intrusion disabled from start of dial to “CONNECT XXX”, “NO DIALTONE”, or “NO CAR-
RIER”.
13:8 Reserved Read returns zero.
7:0 IS Intrusion Suspend (500 ms units) default = 0 ms.
Table 64. Monitor Mode Values
On-Hook Voltage Monitor Mode Off-Hook Current Monitor Mode
00000 = No line connected.
00001 = Minimum line voltage (VMIN = 2.5 V ±0.5 V).
11111 = Maximum line voltage (87 V ±20%)
00000 = No loop current.
00001 = Minimum loop current.
11110 = Maximum loop current.
11111 = Loop current is excessive (overload). Overload
> 155 mA (60 mA in CTR21 mode).