T7256 Agere Systems, T7256 Datasheet - Page 44

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

T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
Questions and Answers
S/T-Interface
A34: (continued)
40
T7903/T7250C Transmit to T7234 Receive
a) Transmitter Load:
b) Receiver Levels:
No External TE
Connected
Unterminated
External
TE Connected
Terminated (100 )
External TE
Connected
If we use the same S/T-transmitter line inter-
face circuit as in the normal (stand-alone TE)
case, the transmitter will see the load that it
expects to drive and is thus optimized in terms
of the load. The 100
user selected per the following table:
The T7234 S/T line interface transformer has
a turns ratio of 2.5. The T7234 receiver thus
expects to see nominal pulse levels of 750 mV
x 2.5 = 1.875 V at the device side of the trans-
former.
T7903: The T7903 transmitter (or an external
TE on a 0-length loop) will drive 750 mV
pulses on the S/T line, and that voltage
reflected back to the device side of the trans-
former is 750 mV x 2.0 = 1.5 V. If the T7234
receiver is connected to the device side of the
transformer as shown in Figure 22, it will see
1.5 V instead of 1.875 V when a 750 mV pulse
is present on the line. Thus, there is an inher-
ent pulse attenuation in this scheme of 1.9 dB
at the T7234 receiver.
We need to be sure that the receiver will have
adequate sensitivity to detect pulses from an
external TE that is some distance away. Refer-
ring to ITU I.430, this circuit can only be used
Configuration
(No External NT1 Connected)
Integrated NT1 Used as NT1
(continued)
Installed
Installed
Installed
terminations must be
JMP1
(continued)
Installed
Installed
Installed
JMP2
Not
c) Receiver Bias:
T7234 Transmit to T7903/T7250C Receive
a) Transmitter Load:
in a short passive bus (SPB) mode when
using the onboard NT1, because there is a
local TE (the T7903), so any external TE that
is also used will result in a passive bus config-
uration. ITU-T I.430 states that the maximum
attenuation in SPB configuration is 3.5 dB.
Combining this with the inherent 1.9 dB atten-
uation results in a total possible signal attenu-
ation of 5.4 dB. The receiver must have a
sensitivity of at least 7.5 dB per ITU-T I.430
Section 8.6.2.3, so 5.4 dB attenuation will
present no problem in this case.
T7250C: The T7250C transmitter (or an exter-
nal TE on a 0-length loop) will drive 750 mV
pulses on the S/T line, and that voltage
reflected back to the device side of the trans-
former is 750 mV x 2.5 = 1.875 V. If theT7234
receiver is connected to the device side of the
transformer as shown in Figure 23, it will see
the 1.875 V pulse level it expects when a
750 mV pulse is present on the line.
In the T7903 to T7234 Direct-Connect section
we showed that the receiver is biased by
about 0.67 dB from nominal due to the direct
connect of the T7903 to the T7234. Assuming
the receiver sensitivity decreases by this
much and combining this with the maximum
5.4 dB attenuation found in the previous sec-
tion results in a total of 6.07 dB of required
sensitivity, which is still within the 7.5 dB
requirement on the receiver.
The T7234 S/T line interface transformer nor-
mally has a turns ratio of 2.5, and the trans-
mitter drives a line-side load of 50 .
Reflecting this impedance to the device side
of the transformer results in 312.5
N
242
resistors, results in a total of 554.5
transmitter typically drives. So, to optimize the
transmitter part of the circuit based on the
load the transmitter expects to drive, the
transmitter should see a total resistance of
approximately 554.5 .
2
). This resistance, combined with the
total resistance of the device-side
Lucent Technologies Inc.
February 1998
(50
that the
x