SP3508 Datasheet, PDF (26/36 Pages) Sipex Corporation – Rugged 3.3V, 20Mbps, 8 Channel Multiprotocol Transceiver with Programmable DCE/DTE and Termination Resistors

These drivers are current sources that
drive loop current through a differential pair
resulting in a 550mV differential voltage at
the receiver. These drivers also incorporate
fixed termination networks for each driver in
order to set the V_OH and V_OL depending on
load conditions. This termination network is
basically a “Y” configuration consisting of
two 51Ω resistors connected in series and
a 124Ω resistor connected between the two
50Ω resistors to GND. Filtering can be done
on these pins to reduce common mode noise
transmitted over the transmission line by
connecting a capacitor to ground.
The drivers also have separate enable pins
which simplifies half-duplex configurations
for some applications, especially program-
mable DTE/DCE. The enable pins will either
enable or disable the output of the drivers
according to the appropriate active logic
illustrated on Figure 44. The enable pins
have internal pull-up and pull-down devices,
depending on the active polarity of the re-
ceiver, that enable the driver upon power-on
if the enable lines are left floating. During
disabled conditions, the driver outputs will
be at a high impedance 3-state.
The driver inputs are both TTL or CMOS
compatible. All driver inputs have an internal
pull-up resistor so that the output will be at
a defined state at logic LOW (“0”). Unused
driver inputs can be left floating. The inter-
nal pull-up resistor value is approximately
500kΩ.
Receivers
The SP3508 has eight enhanced inde-
pendent receivers. Control for the mode
selection is done via a three-bit con-
trol word that is the same as the driver
control word. Therefore, the modes for
the drivers and receivers are identical in the
application. Like the drivers, the receivers
are prearranged for the specific requirements
of the synchronous serial interface. As the
operating mode of the receivers is changed,
the electrical characteristics will change
to support the required serial interface
protocols of the receivers. Table 1 shows
the mode of each receiver in the different
FEATURES
interface modes that can be selected.
There are two basic types of receiver cir-
cuits—ITU-T-V .28 (RS-232) and ITU-T-V.11,
(RS-422).
The RS-232 (V.28) receiver is single-ended
and accepts RS-232 signals from the RS-
232 driver. The RS-232 receiver has an
operating input voltage range of +15V and
can receive signals downs to +3V. The input
sensitivity complies with RS-232 and V.28
at +3V. The input impedance is 3kΩ to 7kΩ
in accordance to RS-232 and V.28. The re-
ceiver output produces a TTL/CMOS signal
with a +2.4V minimum for a logic “1” and a
+0.4V maximum for a logic “0”. The RS-232
(V.28) protocol uses these receivers for all
data, clock and control signals. They are also
used in V.35 mode for control line signals:
CTS, DSR, LL, and RL. The RS-232 receiv-
ers can operate over 120kbps.
The second type of receiver is a differential
type that can be configured internally to
support ITU-T-V.10 and CCITT-V.35
depending on its input conditions. This
receiver has a typical input impedance
of 10kΩ and a differential threshold of
less than +200mV, which complies with
the ITU-T-V.11 (RS-422) specifications.
V.11 receivers are used in RS-449/V.36,
EIA-530, EIA-530A and X.21 as Category I
signals for receiving clock, data, and some
control line signals not covered by Category
II V.10 circuits. The differential V.11 trans-
ceiver has improved architecture that allows
over 20Mbps transmission rates.
Receivers dedicated for data and clock (RxD,
RxC, TxC) incorporate internal termination
for V.11. The termination resistor is typi-
cally 120Ω connected between the A and
B inputs. The termination is essential for
minimizing crosstalk and signal reflection
over the transmission line . The minimum
value is guaranteed to exceed 100Ω, thus
complying with the V.11 and RS-422 speci-
fications. This resistor is invoked when the
receiver is operating as a V.11 receiver, in
modes EIA-530, EIA-530A, RS-449/V.36,
and X.21.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com

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