LTC1318
APPLICATION INFORMATION
Interface Standards
The LTC1318 provides compatibility with both RS232 and
RS422/AppleTalk/LocalTalk standards in a single chip,
enabling a system to communicate using either protocol
as necessary. The LTC1318 provides two RS232 single-
ended drivers, one RS422 differential driver, and two
receivers. One of the receivers is a dedicated RS232
single-ended receiver, while the other can be configured
for RS232 (single-ended) or RS422 (differential) opera-
tion by controlling the logic state of the select pin. All
single-ended drivers and receivers meet the RS232C
specification for output swing, load driving capacity and
input range, and can additionally transmit and receive
signals as high as 100kbaud. The differential driver and
receiver can interface to both RS422 and AppleTalk net-
works, and can transmit and receive signals at rates
exceeding 2Mbaud.
Fault Protection
The LTC1318 incorporates many protection features to
make it as “bustproof” as possible. All driver outputs and
receiver inputs are protected against ESD strikes to
±6kV,
eliminating the need for external protection devices in
most applications. All driver outputs can be taken outside
the power supply rails without damage and will not allow
current to be forced back into the supplies, preventing the
output fault from affecting other logic circuits using the
same power supply. Additionally, the driver outputs enter
a high impedance state when the power is removed,
preventing the system from loading the data lines when it
is shut off. All driver and receiver outputs are protected
against short circuits to ground or to the supply rails.
Charge Pump Power Supply
The LTC1318 includes an on-board charge pump to gen-
erate the voltages necessary for true RS232 compatible
output swing. This charge pump requires just four exter-
nal 0.1µF capacitors to operate; two flying caps connected
Configurable RS422/RS232 Receiver
There are two line receivers in the LTC1318. One is a
dedicated RS232 receiver; the other can receive both
single-ended RS232 signals and differential RS422 sig-
nals. This second receiver has two inputs: RXD
+
(pin 8)
and RXD
–
(pin 9) to accept differential signals. The RXD
+
input is disabled in single-ended mode. The receiver mode
is set by the RXMODE (pin 15). A low level on RXMODE
configures the receiver in differential mode; it accepts
input at RXD
+
and RXD
–
and outputs the data at RXDO (pin
17). A high level at RXMODE forces the receiver into
single-ended mode; RXD
+
is disabled, pin 9 switches
identity from RXD
–
to RXI2, and pin 17 switches from
RXDO to RXO2, the single-ended data output. In this mode
the receiver accepts RS232 signals at RXI2 and outputs
the data through RXO2. The receiver becomes inverting in
single-ended mode. This receiver can switch between its
two modes within 100ns, allowing the system to sense the
input signal and configure itself accordingly.
to the C1
+
/C1
–
and C2
+
/C2
–
pins, and two hold caps, one
from V
+
to ground and one from V
–
to ground. The charge
pump has enough extra capacity to drive light external
loads and still meet RS232 specifications; it will support a
10mA load from V
+
to ground or a 5mA from V
+
to V
–
(Figure 5).
5V
2
0.1µF
3
LTC1318
22
0.1µF
23
24
– 7V AT 5mA
LT1318 • F05
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
W
U U
1
8V AT 5mA
V
+
0.1µF
EXTERNAL
DEVICE
0.1µF
V
–
Figure 5.
7
LINER [ LINEAR TECHNOLOGY ]
