CPC5604
Figure 3 Caller ID Protocol
DC characteristics
The LiteLinkTM is designed to meet various country DC
characteristics including the CTR-21 standard. The pins
that control the VI characteristics and current limiting are
designated ZDC and DCS. Meeting DC requirements
are achieved by selecting the appropriate resistors RZDC
(R16) and RDCS (R20) respectively. Resistor values can
also be switched in and out with the CPC5601device or
optocouplers which enables international compliance
under software control. Suggested resistor values for
various countries are listed in table 1. The VI profile on
Tip and Ring is described by the following equation:
2 s
500ms
475ms
2 s
3 s
SINE WAVE
CALLER ID MESSAGE
2ND RING
1ST RING
RING
CID
RDCS+12MΩ
VLINE = VBRIDGE
+
0.5V+ (ILINE - 8mA)RZDC
(RDCS
)
Example: ILINE = 20mA, VBRIDGE = 1.2V, RDCS
1.69MW, RZDC = 8W, VLINE = 6.0V.
=
This CID signal is then processed by the host and, after
processing, the host will deactivate the CID signal. At
this point the host can answer the call if desired by
asserting the OH pin on the LiteLinkTM. It’s important to
note that when the LiteLinkTM goes off-hook, it auto-
matically disconnects the snoop path from both the RX
and Ring outputs. Signals appearing on the telephone
line are now coupled through the optical isolation barri-
er in the LiteLinkTM and not via the capacitors in the
snoop path.
CID gain from Tip and Ring to Rx+ and Rx- is deter-
mined by:
10 RRXF
GAIN =
(RSNOOP)2 +
1
(2πf CS)2
Where f = CID signal frequency
For example, with RRXF = 75KW, RSNOOP = 1.4MW,
CS = 220pF, and f = 600Hz calculated GAIN = 0.707 or
a loss of -3dB at Rx+ and Rx-. This implies that the
snoop frequency response is 600Hz. Gain is expressed
in decibels by:
XXX
18
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