Silicon Laboratories SI2493/57/34/15/04, SI2494/39 Typical Loop Voltage LVS Transfer Function

AN93

Bit 15 represents the polarity of the Tip-Ring voltage, and a reversal of this bit represents a Tip-Ring polarity reversal. LVS = 0x0000 if the Tip-Ring voltage is less than 3.0 V and, in the on-hook state, can be taken as “no line connected.”

The ISOmodem reports the on-hook line voltage with the LVS bits in two’s complement. LVS has a full scale of 87 V with an LSB of 1 V. The first code step (going from 0 to 1) is offset so that a 0 indicates a line voltage of less than 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 15.

When the ISOmodem 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. Line-current sensing is detailed in Figure 16 and Table 32.

	128
	112
	96
	80
LVS Bits	64
	48
	32
	16
	0
	0	16	32	48	64	80	96	112	128

Tip/Ring Voltage (Volts)

Figure 15. Typical Loop Voltage LVS Transfer Function

40 Rev. 1.3