Datasheet

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
120Ω and the receiver output must be forced to a known
state, the circuits of Figure 10 can be used.
U
S
A
O
PP
L
IC
AT
I
WU
U
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
®
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.
TransZorb is a registered trademark of General Instruments, GSI
488/9 F11
120Ω
DRIVER
Y
Z
Figure 11. ESD Protection with TransZorbs
®
488/9 F10
110Ω
RX
130Ω
130Ω
110Ω
5V
RX
RECEIVER
1.5k
120Ω
5V
1.5k
RX
120Ω
5V
C
100k
RECEIVER
RECEIVER
Figure 10. Forcing “0” When All Drivers Are Off