T7256 Agere Systems, T7256 Datasheet - Page 45

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

Lucent Technologies Inc.
February 1998
Questions and Answers
S/T-Interface
A34: (continued)
No External TE
Connected
Unterminated
External TE Connected
Terminated (100 )
External TE Connected
T7903: In this case, the T7234 transmitter is
driving into a transformer with a turns ratio of
2.0. The pulse amplitude that the transmitter
must generate on the device side of the trans-
former is 1.5 V (resulting in a 750 mV pulse on
the line in accordance with the standards).
The T7234 transmitter circuit is a current
source of 6.0 mA. To generate a voltage of
1.5 V with 6.0 mA requires a resistance of
1.5/0.006 = 250 , which is 62.5
reflected to the device side of the transformer.
This impedance should consist of jumper-
selectable 100
trated in Figure 22. The table below lists the
jumper settings for each possible configura-
tion.
The total impedance the T7234 must drive
(from the first paragraph of this section) is
554.5 , and the impedance across the trans-
former leads is 250
graph). The remaining 554.5 – 250 = 304.5
is divided equally between the positive and
negative transmitter outputs, requiring 152
in each leg. We can accomplish this with a
143
bridge and a 10
the bridge. The resistance is split in this way to
provide 10
diode bridge when the bridge is conducting
(similar to the T7903 transmitter circuit).
Configuration
(No External NT1 Connected)
Integrated NT1 Used as NT1
resistor on the device side of the diode
(continued)
of current limiting through the
and 167
resistor on the line side of
(from the second para-
Installed
Installed
Installed
(continued)
JMP3
resistors as illus-
T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
when
Installed
Installed
Installed
JMP4
Not
T7250C: Referring to Figure 23, if we use the
same T7234 S/T transmitter line interface circuit
as in the normal (stand-alone NT) case, the
T7234 transmitter will see the 554.5
normally expects to drive and is thus optimized in
terms of the load. The 100
are user selected per the preceding table.
b) Receiver Levels:
c) Receiver Bias:
The T7903 will see the correct pulse levels
by design. In the preceding section, the T7234
transmit circuit was designed to produce
750 mV pulses on the line. The T7903
receiver is attached directly to the device side
of the transformer, so it will see the 1.5 V
pulse levels that it expects to see.
T7903: The T7903 S/T line interface trans-
former has a turns ratio of 2.0. The T7903
receiver thus expects to see nominal pulse
levels of 750 mV x 2.0 = 1.5 V at the device
side of the transformer. The T7234 transmitter
section was designed to produce 750 mV
pulses on the S/T line (as would an external
TE on a 0-length loop). That voltage reflected
back to the device side of the T7901 trans-
former is 750 mV x 2.0 = 1.5 V, so the T7901
sees the pulse level it expects when a 750 mV
pulse is present on the line.
T7250C: The T7250C S/T line interface trans-
former has a turns ratio of 2.5. The T7250C
receiver thus expects to see nominal pulse
levels of 750 mV x 2.5 = 1.875 V at the device
side of the transformer. The T7234 transmitter
section was designed to produce 750 mV
pulses on the S/T line (as would an external
TE on a 0-length loop). That voltage reflected
back to the device side of the T7250C trans-
former is 750 mV x 2.5 = 1.875 V, so the
T7250C sees the pulse level it expects when a
750 mV pulse is present on the line.
The receiver bias is sufficiently small that it is
not an issue (see preceding sections).
terminations shown
load that it
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