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1318

Single 5V RS232/RS422/AppleTalk® DCE Transceiver

Linear Technology 

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.

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

0.1µF

0.1µF

2

1

8V AT 5mA

3

0.1µF

LTC1318

22

0.1µF

23

24

– 7V AT 5mA

V+

EXTERNAL

DEVICE

V–

LT1318 • F05

Figure 5.

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.

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.

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