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參數(shù)資料
| 型號(hào)： | T7256A |
| 英文描述： | Compliance with the New ETSI PSD Requirement |
| 中文描述： | 符合新的ETSI PSD的要求 |
| 文件頁(yè)數(shù)： | 56/116頁(yè) |
| 文件大?。?/td> | 1056K |
| 代理商： | T7256A |
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Data Sheet
January 1998
T7256 Single-Chip NT1 (SCNT1) Transceiver
52
Lucent Technologies Inc.
Application Briefs
T7256 Reference Circuit
A reference circuit illustrating the T7256 in a stand-
alone NT1 application is shown in Figures 20 and 21.
This depicts a complete stand-alone NT1 design with
the exception of power supply circuitry and power sta-
tus monitoring circuitry. A bill of materials for the sche-
matic is shown in Table 29. Note that specific
applications may vary depending on individual require-
ments.
U-Interface
The U-Interface attaches to the board at RJ-45 connec-
tor J1 (see Figure 20). F1 and VR2 provide overcurrent
and overvoltage protection, respectively. These two
devices in combination with transformer T1 provide
protection levels required by FCC Part 68 and UL*
1459. For an in-depth discussion of protection issues,
the following application notes are helpful.
1.
“Overvoltage Protection of Solid-State
Subscriber Loop Circuits,” Lucent Analog Line
Card Components Data Book (CA94-007ALC)
800-372-2447.
2.
Protection of Telecommunications Customer
Premesis Equipment, Raychem
Corporation,
415-361-6900.
C16 is a 1.0
μ
F dc blocking capacitor that is required
per ANSI T1.601, Section 7.5.2.3. The 250 V rating of
C16 is governed by the maximum breakdown voltage
of VR2, since the capacitor must not break down before
VR2. The resistance of R13 (21
) and F1 (12
) pro-
vides a total line-side resistance of 33
, which is
required when using the Lucent 2754H2 transformer
(see the note at the end of Table 29 for information on
R13 values when using other transformers).
On the device side of the U-interface transformer, VR1
provides secondary overvoltage protection of 6.8 V.
Optional capacitors C13 and C14 provide common-
mode noise suppression for applications that are
required to operate in the presence of high common-
mode noise. R6 and R7 provide the necessary external
hybrid resistors.
* UL is a registered trademark of Underwriters Laboratories, Inc.
Raychem is a registered trademark of Raychem Corporation.
S/T-Interface
The S/T-interface attaches to the board at RJ-45 con-
nector J2 (see Figure 21). L1 is a high-frequency com-
mon-mode choke used to minimize EMI. R24 and R25
are 100
terminations required by ITU I.430 Section
8.4. Jumper-selectable resistors R26 and R27 provide
for a 50
termination option instead of the standard
100
termination. This is useful in configurations
where none of the TEs have terminating resistors.
Dual-transformer T2 has a standard footprint that can
accept ISDN transformers from several vendors. On
the device side of the S/T-interface transformer, D2—
D11 and VR3—4 provide overvoltage protection for the
device pins. R20—23 provide current limiting for cases
where one or more of the protection diodes conducts
due to an overvoltage condition. Capacitor C17 pro-
vides suppression of common-mode noise that might
otherwise be introduced onto the receiver input pins,
effectively increasing the receiver's CMRR. Note that
the S/T transformer must have a center tap on the
device side in order to use this scheme. R16 and R17
in combination with R20 and R21, respectively, provide
the 121
of resistance required by the T7256 on each
transmitter output pin. R18 and R19 are the 10 k
,
10% resistors required on the receiver input pins.
MLT Circuit
The metallic loop termination (MLT) circuit (U3 and
related components in Figure 20) provides a dc termi-
nation for the loop per ANSI T1.601, Section 7.5. R14
and R15 are power resistors used to sink current dur-
ing overvoltage fault conditions. The optoisolater (U2)
provides signal isolation and voltage translation of the
signaling pulses used for NT maintenance modes, per
T1.601, Section 6.5. The T7256 interprets these pulses
via an internal ANSI maintenance state machine, and
responds accordingly. For applications outside North
America, the MLT circuit is not required.
Status LED
D1 in Figure 20 is an LED that is controlled by the
STLED pin of the T7256 and indicates the status of the
device (activating, out-of-sync, etc.). Table 28 and Fig-
ure 18 of this data sheet details the possible states of
the STLED pin and the meaning of each state.
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