11

LTC1334

S

APPLICATI

I FOR ATIO

Basic Theory of Operation

The LTC1334 has two interface ports. Each port may be

configured as a pair of single-ended RS232 transceivers

or as a differential RS485 transceiver by forcing the

port’s selection input to a low or high, respectively. The

LTC1334 provides two RS232 drivers and two RS232

receivers or one RS485 driver and one RS485 receiver

per port. All the interface drivers feature three-state

outputs. Interface outputs are forced into high imped-

ance when the driver is disabled, in the shutdown mode

or with the power off.

All the interface driver outputs are fault-protected by a

current limiting and thermal shutdown circuit. The ther-

mal shutdown circuit disables both the RS232 and RS485

driver outputs when the die temperature reaches 150

°C.

The thermal shutdown circuit reenables the drivers when

the die temperature cools to 130

°C.

In RS485 mode, shutdown mode or with the power off, the

input resistance of the receiver is 24k. The input resistance

drops to 5k in RS232 mode.

A logic low at the ON/OFF pin shuts down the device and

forces all the outputs into a high impedance state. A logic

high enables the device. An internal 4

µA current source to

Figure 11. RS232/RS485 Interfaces

1

28

5V

LTC1334 • F11

120

Ω

12

21

LTC1334

RX OUT

18

5V

13

10

11

9

8

14

17

16

RS485 I/O

RS232 DR OUT

19

5V

20

15

5V

≥ ±5V INTO

3k

Ω LOAD

5

4

27

26

24

RX OUT

6

RS232 RX IN

23

2

3

RX OUT

7

DR IN

DR ENABLE

DR IN

DR IN

22

RS232 DR OUT

RS232 RX IN

C1

C2

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

In RS485 mode, an internal 4

µA current source pulls the

driver enable pin high if left open. The RS485 receiver has

a 4

µA current source at the noninverting input. If both the

RS485 receiver inputs are open, the output goes to a high

state. Both the current sources are disabled in the RS232

mode. The receiver output B is inactive in RS485 mode

and has a 50k pull-up resistor to provide a known output

state in this mode.

A loopback mode enables internal connections from driver

outputs to receiver inputs for self-test when the LB pin has

a low logic state. The driver outputs are not isolated from

the external loads. This allows transmitter verification

under the loaded condition. An internal 4

µAcurrentsource

pulls the LB pin high if left open and disables the loopback

configuration.

RS232/RS485 Applications

The LTC1334 can support both RS232 and RS485 levels

with a single 5V supply as shown in Figure 11.

Multiprotocol Applications

The LTC1334 is well-suited for software controlled inter-

face mode selection. Each port has a selection pin as

shown in Figure 12. The single-ended transceivers sup-

port both RS232 and EIA562 levels. The differential trans-

ceivers support both RS485 and RS422.