Specifications

AN93
88 Rev. 0.8
U79 (GEN4)
U79 is a bit-mapped register. Bits 15:5 are reserved.
Bits 4:0 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 2’s complement output.
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).
Table 65. U79 Bit Map
Bit Name Function
15:5 Reserved Read returns zero.
4:0 LVCS Line Voltage Current Sense.
On-Hook = Voltage Monitor (2.75 V/bit).
Off-Hook = Loop Current Monitor (3 mA/bit).