LTC491

10

Fault Protection

All of LTC’s RS485 products are protected against ESD

transients up to 2kV using the human body model (100pF,

1.5k

Ω). However, some applications need more

protection. The best protection method is to connect a

bidirectional TransZorb from each line side pin to ground

(Figure 16).

LTC491 • TA17

120

Ω

DRIVER

Z

Y

Figure 16. ESD Protection with TransZorbs

A TransZorb is a silicon transient voltage suppressor that

has exceptional surge handling capabilities, fast response

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 break-

down voltage higher than the common mode voltage

required for your application (typically 12V). Also, don’t

forget to check how much the added parasitic capacitance

will load down the bus.

Receiver Open-Circuit Fail-Safe

Some data encoding schemes require that the output of

the receiver maintains a known state (usually a logic 1)

when the data is finished transmitting and all drivers on the

line are forced into three-state. The receiver of the LTC491

has a fail-safe feature which guarantees the output to be in

a logic 1 state when the receiver inputs are left floating

(open-circuit). However, when the cable is terminated

with 120

Ω, the differential inputs to the receiver are

shorted together, not left floating. Because the receiver

has about 70mV of hysteresis, the receiver output will

maintain the last data bit received.

140

Ω

RECEIVER

RX

+5V

1.5k

Ω

RECEIVER

RX

+5V

110

Ω

130

Ω

110

Ω

130

Ω

LTC491 • TA16

120

Ω

RECEIVER

RX

C

+5V

100k

Ω

1.5k

Ω

Figure 15. Forcing “O” When All Drivers are Off

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.

S

APPLICATI

I FOR ATIO