T7256 Agere Systems, T7256 Datasheet - Page 37

• Manufacturer Part Number: T7256
• Description: (T7234 - T7256) Compliance
• Manufacturer: Agere Systems

Lucent Technologies Inc.
February 1998
Questions and Answers
U-Interface
Q14: Where can information be obtained on lightning
A14: Requirements vary among applications and
Q15: ITU-T specification K.21 describes a lightning
A15: The critical component in this test is the trans-
and surge protection requirements for 2B1Q
products?
between countries. ANSI T1.601, Appendix B,
provides a list of applicable specifications to
which you may refer. Also, there are many manu-
facturers of overvoltage protection devices who
are familiar with the specifications and would be
willing to assist in surge protection design. The
ITU-T K series recommendations are also a good
source of information on protection, especially
recommendation K.11, “Principles of Protection
Against Overvoltages and Overcurrents,” which
presents an overview of protection principles.
Also refer to the application notes mentioned in
the U-interface Description section of this data
sheet.
surge test for NT1s (see Figure 1/K.21 and Table
1/K.21, Test #1) in which both Tip and Ring are
connected to the source and a 1.5 kV voltage
surge is applied between this point and the GND
of the NT1. What are the protection consider-
ations for this test? Are the HP and HN pins sus-
ceptible to damage?
former since its breakdown voltage must be
greater than 1.5 kV. Assuming this is the case,
the only voltage that will make it through to the
secondary side of the transformer will be prima-
rily due to the interwinding capacitance of the
transformer coils. This capacitance will look like
an impedance to the common-mode surge and
will therefore limit current on the device side of
the transformer. The device-side voltage will be
clamped by the SM6T6V8CA device. The maxi-
mum breakdown voltage of the SM6T6V8CA is
7.1 V. The 16.9
LOP and LON pins on the T7234 from this volt-
age. However, this voltage will be seen directly on
pins 36 and 31 (HP and HN) on the T7234. The
on-chip protection on these pins consists of out-
put diodes and a pair of polysilicon resistors.
These pins have been thoroughly tested to
(continued)
resistors will help protect the
(continued)
T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
Q16: Can the range of the T7234 on the U-interface be
A16: ANSI Standard T1.601, Section 5.1, states that
ensure that a 7.4 V level will not damage them;
therefore, no third level of protection is needed
between the SM6T6V8CA and the HP and HN
pins.
specified in terms of loss? What is the range over
straight 24 awg wire?
transceivers meeting the U-interface standard are
intended to operate over cables up to the limits of
18 kft (5.5 km) 1300
tance design rules specify that a loop (of single-
or mixed-gauge cable; e.g., 22 awg, 24 awg, and
26 awg) should have a maximum dc resistance of
1300 , a maximum working length of 18 kft, and
a maximum total bridged tap length of 6 kft.
The standard states that, in terms of loss, this is
equivalent to a maximum insertion loss of 42 dB
@ 40 kHz. Lucent Technologies has found that,
for assessing the condition of actual loops in the
field in a 2B1Q system, specifying insertion loss
as 33.4 dB @ 20 kHz more closely models ANSI
circuit operation. This is equivalent to a straight
26 awg cable with 1300
(15.6 kft).
The above goals are for actual loops in the out-
side loop plant. These loops may be subjected to
noise and jitter. In addition, as mentioned above,
there may be bridge taps at various points on the
loop. The T1.601 standard defines 15 loops, plus
the null, or 0-length loop, which are intended to
represent a generic cross section of the actual
loop plant.
A 2B1Q system must perform over all of these
loops in the presence of impairments with an
error rate of <1e–7. Loop #1 (18 kft, where
16.5 kft is 26 awg cable and 1.5 kft is 24 awg
cable) is the longest, so it has the most loss
(37.6 dB @ 20 kHz and 47.5 dB @ 40 kHz). Note
that this is more loss than discussed in the pre-
ceding paragraph. The difference is based on test
requirements vs. field deployment. The test
requirements are somewhat more stringent than
the field goal in order to provide some margin
against severe impairments, complex bridged
taps, etc.
resistance design. Resis-
dc resistance
33