SP504 Datasheet, PDF (17/32 Pages) Sipex Corporation – WAN Multi-Mode Serial Transceiver

Receivers
The SP504 has seven (7) independent re-
ceivers which can be programmed for the
different interface modes. Control for the
mode selection is done via a 4–bit control
word that is independent from the driver
control word. The coding for the drivers and
receivers is identical. Therefore, if the modes
for the drivers and receivers are supposed
to be identical in the application, the control
lines can be tied together.
Like the drivers, the receivers are pre-ar-
ranged for the specific requirements of
the interface. As the operating mode of
the receivers is changed, the electrical
characteristics will change to support the
requirements of clock, data, and control line
receivers. Table 2 shows the mode of each
receiver in the different interface modes that
can be selected.
There are three basic types of receiver cir-
cuits — RS-232, RS-423, and RS-485.
The RS-232 receiver is a single–ended
input with a threshold of 0.8V to 2.4V. The
RS-232 receiver has an operating voltage
range of ±15V and can receive signals up
to 120kbps. The input sensitivity complies
with EIA-RS-232 and V.28 at +3V to -3V.
The input impedance is 3kΩ to 7kΩ. RS-
232 receivers are used in RS-232 mode for
all data, clock and control signals. They
are also used in V.35 mode for control line
signals such as CTS and DSR.
The RS-423 receivers are also single–ended
but have an input threshold as low as
±200mV. The input impedance is guaranteed
to be greater than 4kΩ, with an operating volt-
age range of ±7V. The RS-423 receivers can
operate up to 120kbps. RS-423 receivers are
used in RS-449, EIA-530, EIA-530A and V.36
modes as Category II signals as indicated
by their corresponding specifications.
The third type of receiver is a differential which
supports RS-485. The RS-485 receiver has
an input impedance of 15kΩ and a differential
threshold of ±200mV. Since the characteris-
tics of an RS-422 (V.11) receiver are actually
subsets of RS-485, the receivers for RS-422
requirements are covered by the RS-485
receivers.
RS-422 receivers are used in RS-449,
EIA-530, EIA-530A and V.36 as Category I
signals for receiving clock, data, and some
control line signals. The differential receivers
can receive data up to 10Mbps.
The RS-485 receivers are also used for the
V.35 mode. Unlike the older implementations
of differential or V.35 receivers, the SP504
contains an internal resistor termination net-
work that ensures a V.35 input impedance of
100Ω (±10Ω) and a short-circuit impedance
of 150Ω (±15Ω). The traditional V.35 imple-
mentations required external termination
resistors to acheive the proper V.35 imped-
ances. The internal network is connected
via low on-resistance FET switches when
the decoder is changed to V.35 mode. The
termination network is transparent when all
other modes are selected. The V.35 receiv-
ers can operate up to 10Mbps.
All receivers include a fail-safe feature that
outputs a logic HIGH when the receiver
inputs are open. For single-ended RS-232
receivers, there are internal 5kΩ pull-down
resistors on the inputs which produces a
logic HIGH ("1") at the receiver outputs. The
single-ended RS-423 receivers produce a
logic LOW ("0") on the output when the inputs
are open. This is due to a pull-up device
connected to the input. The differential re-
ceivers have the same internal pull-up device
on the non-inverting input which produces a
logic HIGH ("1") at the receiver output. The
three differential receivers when configured
in V.35 mode (RxD, RxC & SCT) do not have
fail-safe because the internal termination
resistor network is connected.
Decoder
The SP504 has the ability to change the
interface mode of the drivers or receivers via
an 8–bit switch. The decoder for the drivers
and receivers is not latched; it is merely a
combinational logic switch.
The control word can be externally latched
either HIGH or LOW to write the appropriate
code into the SP504. The codes shown in
Tables 1 and 2 are the only specified, valid
modes for the SP504. Undefined codes
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