T7256 Agere Systems, T7256 Datasheet - Page 43

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

Lucent Technologies Inc.
February 1998
Questions and Answers
S/T-Interface
A34: (continued)
T7234 Transmit to T7903/T7250C Receive
b) Receiver Levels:
c) Resistor Selection:
a) Transmitter Load:
The T7234 S/T line interface transformer has
a turns ratio of 2.5, and the transmitter drives
a line-side load of 50 . Reflecting this imped-
ance to the device side of the transformer
results in 312.5
tance, combined with the 242
tance of the device-side resistors, results in a
total of 554.5
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 .
T7903: The S/T line interface transformer has
a turns ratio of 2.0. The receiver expects to
see nominal pulse levels of 750 mV x 2.0 =
1.5 V. 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 .
T7250C: The S/T line interface transformer
has a turns ratio of 2.5. The receiver expects
to see nominal pulse levels of 750 mV x 2.5 =
1.875 V. The T7234 transmitter circuit is a cur-
rent source of 6.0 mA. To generate a voltage
of 1.875 V with 6.0 mA requires a resistance
of 1.875/0.006 = 312.5 .
In this section, the term receiver implies not
only the receive section on the chip, but also
the external 10 k resistors connected to the
receiver. These resistors remain unchanged
from the standard line interface circuit in order
to maintain the same total receiver imped-
ance.
T7903: Ideally, the T7234 transmitter should
be driving into 554.5 , and the T7903
receiver wants to see the levels that would
result if the transmitter drove 6 mA through
250 . So, the total transmit path resistance
should be divided into three resistors. The first
is the resistor across which the receiver is
(continued)
that the transmitter typically
(50
x N
(continued)
2
). This resis-
total resis-
T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
d) Receiver Bias:
T7903/T7250C to T7234 Direct Connect with
External S/T-Interface Provided
First, we need to address the issue of the trans-
former turns ratio.
T7903: The T7903 uses a 2.0:1 transformer, and
the T7234 uses a 2.5:1 transformer. It is desirable
to be able to use a dual transformer, so we want
the transmit- and receive-side transformers to
have the same turns ratio. Also, it may be desir-
able to use a product with this arrangement as
just a TE (with an external NT1, i.e., no U-inter-
face connected to the integrated NT1). Therefore,
we will select a 2.0:1 turns ratio transformer to
ensure T7903 pulses of sufficient amplitude on
the line side of the transformer and ensure that
an external transmitter won’t overdrive the T7903
receiver inputs.
T7250C: The T7250C and T7234 both use a
2.5:1 transformer, which simplifies the analysis
for this case.
connected and should be approximately
250
levels. A standard 249
this case. The remainder of the 554.5
should be divided equally between two other
series resistors in the transmit path, and
(554.5
chosen for the two other series resistors as
illustrated in Figure 20.
T7250C: Ideally, the T7234 transmitter should
be driving into 554.5 , and the T7250C
receiver wants to see the levels which would
result if the transmitter drove 6 mA through
312.5 . So, the total transmit path resistance
should be divided into three resistors. The first
is the resistor across which the receiver is
connected and should be approximately
312.5
levels. A standard 309
this case. The remainder of the 554.5
should be divided equally between two other
series resistors in the transmit path, and
(554.5
chosen for the two other series resistors as
illustrated in Figure 21.
The receiver bias is not an issue for the same
reasons discussed in the T7903/T7250C
Transmit to T7234 Receive section.
so that the receiver sees the correct
so that the receiver sees the correct
– 249 )/2 is 152.7 , so 150
– 309 )/2 is 122.6 , so 121
value is adequate for
value is adequate for
is
is
39