SP3508 Datasheet, PDF (23/33 Pages) Sipex Corporation – Rugged 3.3V, 20Mbps, 8 Channel Multiprotocol Transceiver with Programmable DCE/DTE and Termination Resistors

The fourth type of drivers are V.35 differential
drivers. There are only three available on the
SP3508 for data and clock (TxD, TxCE, and TxC
in DCE mode). 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 programmable
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 receiver, 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 internal pull-up
resistor value is approximately 500kΩ.
Receivers
The SP3508 has eight enhanced independent
receivers. Control for the mode selection is done
via a three-bit control 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
FEATURES
will change to support the required serial interface
protocols of the receivers. Table 1 shows
the mode of each receiver in the different
interface modes that can be selected. There are
two basic types of receiver circuits—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 receiver 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 receivers
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 transceiver 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 typically 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
specifications. 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.
Date: 06/14/04
SP3508 Enhanced WAN Multi–Mode Serial Transceiver
23
© Copyright 2004 Sipex Corporation