Specifications

AN93
12 Rev. 0.9
2.2.8. Ringer Network
R7 and R8 comprise the ringer network. These
components determine the modem’s on-hook
impedance at TIP and RING. These components are
selected to present a high impedance to the line, and
care must be taken to ensure the circuit board area
around these components is clean and free of
contaminants, such as solder flux and solder flakes.
Leakage on RNG1 (Si3018/10 pin 8) and RNG2
(Si3018/10 pin 9) can impair modem performance. R7
and R8 are also used by the modem to monitor the line
voltage.
2.2.9. Line Voltage/Loop Current Sensing
There are two methods for line voltage and loop current
sensing. The first method is the legacy mode using
U79(LVCS)[4:0]. The legacy mode is intended for
backward compatibility in applications originally
designed for the previous generation ISOmodem. This
mode is used in the intrusion detection algorithm
implemented on the device.
The second method of measuring line voltage and loop
current takes advantage of the improved resolution
available on the Si3018 and Si3010 DAA chips.
U63(LCS)[15:8] represents the value of off-hook loop
current as a non-polar binary number with 1.1 mA/bit
resolution. Accuracy is not guaranteed if the loop
current is less than the minimum required for normal
DAA operation. U6C(LVS)[15:8] represents the value of
on-hook and off-hook loop voltage as a signed, 2s
complement number with a resolution of 1 V/bit. Bit 15
represents the polarity of the TIP\RING voltage, and a
reversal of this bit represents a TIP\RING polarity
reversal. LVS = 0000h if the TIP\RING voltage is less
than 3.0 V and, in the on-hook state, can be taken as
“no line connected.”
2.2.10. Legacy Mode
The Si2493/57/34/15/04 has the ability to measure both
line voltage and loop current. The 8-bit LVCS register,
U79(LVCS) [7:0], reports line voltage measurements
when on-hook and loop current measurements when
off-hook.
Using the LVCS bits, the user can determine the
following:
 When on-hook, detect if a line is connected.
 When on-hook, detect if a parallel phone is off-hook.
 When off-hook, detect if a parallel phone goes on or
off-hook.
 Detect if enough loop current is available to operate.
2.2.10.1. Line Voltage Measurement
The Si2493/57/34/15/04 reports the on-hook line
voltage with the LVS bits in 2s complement. LVS has a
full scale of 87 V with an LSB of 1 V. The first code
(0 → 1) is skewed such that a 0 indicates the line
voltage is < 3 V. The accuracy of the LVS bits is ±10%.
The user can read these bits directly through the LVS
register. A typical transfer function is shown in Figure 3.
2.2.10.2. Loop Current Measurement
When the Si2493/57/34/15/04 is off-hook, the LCS bits
measure loop current in 1.1 mA/bit resolution. These
bits enable the user to detect another phone going off-
hook by monitoring the dc loop current. The line voltage
sense transfer function is shown in Figure 3, and the
line current sense is detailed in Figure 4 and Table 4.