9

LTC488/LTC489

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 in three-state. The receiver of the LTC488/

LTC489 has a fail-safe feature which guarantees the out-

put to be in a logic 1 state when the receiver inputs are left

floating (open-circuit). When the input is terminated with

120

Ω and the receiver output must be forced to a known

state, the circuits of Figure 10 can be used.

S

APPLICATI

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 pullup 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,

1.5k). However, some applications need more protection.

The best protection method is to connect a bidirectional

TransZorb

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 instruments, GSI, and come in a variety of break-

down voltages and prices. Be sure to pick a breakdown

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.

TransZorb is a registered trademark of General Instruments, GSI

488/9 F11

120

Ω

DRIVER

Y

Z

488/9 F10

110

Ω

RX

130

Ω

130

Ω

110

Ω

5V

RX

RECEIVER

1.5k

120

Ω

5V

1.5k

RX

120

Ω

5V

C

100k

RECEIVER

RECEIVER

Figure 10. Forcing “0” When All Drivers Are Off