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LT1485

Differential Bus Transceiver

Linear Technology 

LTC1485

APPLICATI S I FOR ATIO

The termination resistors are used to generate a DC bias

which forces the receiver output to a known state, in this

case a logic 0. The first method consumes about 208mW

and the second about 8mW. The lowest power solution is

to use an AC termination with a pull-up resistor. Simply

swap the receiver inputs for data protocols ending in

logic 1.

Fault Protection

All of LTC’s RS485 products are protected against ESD

transients up to 2kV using the human body model

(100pF, 1.5kΩ). However, some applications need more

protection. The best protection method is to connect a

bidirectional TransZorb® from each line side pin to ground

(Figure 15).

A TransZorb is a silicon transient voltage suppressor that

has exceptional surge handling capabilities: fast response

DRIVER

A

120Ω

B

1485 F15

Figure 15. ESD Protection with TransZorbs

time and low series resistance. They are available from

General Semiconductor Industries and come in a variety

of breakdown voltages and prices. Be sure to pick a

breakdown voltage higher than the common-mode volt-

age required for your application (typically 12V). Also,

don’t forget to check how much the added parasitic

capacitance will load down the bus.

TransZorb is a registered trademark of General Instruments, GSI

TYPICAL APPLICATI S

RS232 Receiver

RS232

IN

5.6k

RECEIVER

RX

1485 TA02

RS232 to RS485 Level Translator with Hysteresis

220k

RS232

IN

10k

DRIVER

A

120Ω

5.6k

B

1485 TA03

HYSTERESIS = 10k •  VA – VB /R ≈ 19 (kΩ • VOLT)/R

10

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