SP522 Datasheet, PDF (10/23 Pages) Sipex Corporation – Low-Cost Programmable Multi-Protocol Transceivers

Unused driver inputs can be left floating; pull–
up resistors to +5V is internally connected on
the inputs so that the output is at a logic low
("0"). For differential drivers, the non-inverting
output will be at a logic high ("1").
Receivers
The SP522 has two (2) independent receivers
which can be programmed for the different
interface modes. Control for the mode selection
is done by DP0 and DP1.
Like the drivers, the receivers are pre-arranged
for the specific requirements of the interface. As
the operating mode of the receivers is changed,
the electrical characteristics will change to
support the required interface. Figures 10 to 13
show the mode of each receiver in the different
interface modes that can be selected.
There are three basic types of receiver circuits
— RS-232, RS-423, and RS-485.
The RS-232 receiver is a single–ended input
with a threshold of 0.8V to 3.0V. 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.
The RS-423 receivers are also single–ended but
have an input threshold as low as +300mV. The
input impedance is guaranteed to be greater than
4KΩ, with an operating voltage range of +7V.
The RS-423 receivers can operate up to 120Kbps.
RS423 receivers can be used in RS-449, EIA530,
EIA-530A and V.36 applications 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 +300mV. Since the characteristics
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 applications for
RS-449, EIA530, 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.
All receivers include a fail-safe feature that
output a known logic state when the receiver
inputs are unconnected. 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 receivers have the
same internal pull-up device on the non-inverting
input which produces a logic high ("1") at the
receiver output.
The receivers also have enable pins which allow
for convenient half-duplex configurations. The
receivers are tri-stated when the ENR1 and
ENR2 pins are at a logic high ("1"). For the
SP524, ENR3 and ENR4 are used for the
additional two receivers.
In addition to the separate enable lines on each
transceiver, there is a latch enable pin,
LATCH_EN, which is used for enabling and
disabling the decoder control inputs (DP0, DP1)
and transceiver enable pins. This pin will
default to a logic high ("1") if not being used.
Loopback
The SP522 and SP524 contain a loopback fea-
ture that allows the driver outputs to "loopback"
to the receiver inputs for diagnostic testing in
the application. The loopback function is
activated when the LOOPBCK pin is low. When
in loopback mode, the driver outputs are
tri-stated and the receiver inputs are deactivated.
The receiver input impedance while in loopback
will be a minimum of 12kΩ. The loopback
function can be initiated during any mode of
operation, RS-232, RS-423 or RS-422. The
travel path of the transceivers in loopback is
shown on Figure 9. The loopback function
overrides the separate enable pins for the drivers
or receivers. When LOOPBCK is at a logic low
("0"), the device will be configured in loopback
regardless whether the transceiver is enabled or
disabled. If the loopback function is not
required, the LOOPBCK pin will default to a
logic high ("1") state.
Rev. 7/10/03
SP522 Multi–Mode Serial Transceivers
10
© Copyright 2003 Sipex Corporation