DS2291
022798 2/10
Once the Long Loop Stik has recovered data from the
T1 line, it can decode B8ZS code words and check for
bipolar violations and carrier loss. If the B8ZS pin is tied
high, the DS2291 will automatically replace incoming
B8ZS code words with eight zeros. If the B8ZS pin is tied
low or left open, no replacement occurs. Bipolar viola-
tions are reported via the BPV pin. The BPV pin will tran-
sition high for a full T1 bit period (648 ns) each time a vio-
lation is detected. Bipolar violations inherent in B8ZS
code words are not reported if the B8ZS pin is tied high.
The DS2291 also checks for carrier loss. The RCL pin
will transition high when the DS2291 detects 192 con-
secutive zeros at RX+ and RX-.
The recovered clock and data are passed to the dejitter
circuitry. If the DJA is tied low or left open, the DS2291
will attenuate the jitter present at RX+ and RX- accord-
ing to the curves outlined in Figure 3. These curves
meet the latest T1 specifications. If the DJA pin is tied
high, the DS2291 will not attenuate jitter. Hence, all the
jitter inherent in the signal at RX+ and RX- will be passed
to RCLK, RPOS, and RNEG. If the recovered clock at
RCLK is used to transmit data onto T1 lines, it is recom-
mended that the dejitter circuitry be enabled (DJA = 0).
The dejitter circuitry contains a 128-bit buffer. This buffer
can be recentered on command via the RST
pin. In nor-
mal applications, the RST is left open or tied high. The
Buffer Limit (BL) output will transition high when the
DS2291 is receiving more than 120 unit intervals
peak-to-peak (Ulpp) of jitter at RX+ and RX-. As long as
the incoming jitter is less than 120Ulpp, the BL pin will
remain low.
The DS2291 contains a data mux that allows data to be
routed from either the T1 recovery circuitry or from a lo-
cal source. The mux is helpful locating faults in a sys-
tem. For example, it could be used to implement a “local”
loopback.
Two typical applications with the DS2291 are shown in
Figure 4 and Figure 5. In both applications, the DS2291
is used to recover data from T1 lines up to 6,000 feet in
length. The application in Figure 4 is with an unpro-
tected interface; it might be used in T1 test equipment.
The application in Figure 5 is with the DS2290 T1 Isola-
tion Stik, which provides all the necessary protection as
required by FCC Part 68. This could be used in a Chan-
nel Service Unit (CSU) or in similar types of equipment
in which full surge and isolation protection is required.
SINGLE IN-LINE CONNECTOR
The DS2291 is designed to connect directly into a
30-pin single in-line connector. These connectors are
available from a number of vendors.
