LTC488/LTC489

9

4889fb

APPLICATIONS INFORMATION

4889 F10

110Ω

RX

130Ω

130Ω

110Ω

5V

RX

RECEIVER

1.5k

120Ω

5V

1.5k

RX

120Ω

5V

C

100k

RECEIVER

RECEIVER

4889 F11

120Ω

DRIVER

Y

Z

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 output to be

in a logic 1 state when the receiver inputs are left floating

(open-circuit). When the input is terminated with 120Ω

and the receiver output must be forced to a known state,

the circuits of Figure 10 can be used.

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 tran-

sients up to 2kV using the human body model (100pF, 1.5k).

However, some applications need more protection. The best

from each line side pin to ground (Figure 11).

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.

Figure 11. ESD Protection with TransZorbs

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