MT88E43B
Data Sheet
Bellcore states that it is desirable to be able to turn off CAS detection for an off-hook capable CPE. The disable
switch allows the subscriber who disconnects a service that relies on CAS detection (e.g., CIDCW) but retains the
CPE, to turn off the detector and not be bothered by false detection.
When SW1 in Figure 4 is in the B position the guard time circuit is disabled. The detector will still process
CAS/Alerting tones but the MT88E43 will not signal their presence by ensuring StD is low.
BT specifies that the idle state tone alert signal recognition time should not be less than 20 ms when both tones are
used for detection. That is, both tones must be detected together for at least 20 ms before the signal can be
declared valid. This requirement can be met by setting the t
GP
(refer to Figure 5) to at least 20 ms.
BT also specifies that the TE is required to apply a D.C. wetting pulse and an AC load 15-25 ms after the end of the
alerting signal. If t
ABS
=t
DA
+t
GA
is 15 to 25 ms, the D.C. current wetting pulse and the A.C. load can both be applied
at the falling edge of StD. The maximum
t
DA
is 8ms so t
GA
should be 15-17 ms. Therefore, t
GP
must be greater than
t
GA
. Figure 5(a) shows a possible implementation. The values in Figures 9 and 11 (R2=R3=422K, C=0.1µF) will
meet the BT timing requirements.
MT88E43
Tones detected
From
detector
+
-
V
TGt
Comparator
N
P
V
DD
Q1
St/GT
C
R
Q2
= V
SS
A
ESt
StD
SW1
B
V
SS
Figure 4 - Guard Time Circuit Operation
7
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
