DS88C120 Datasheet, PDF (7/10 Pages) National Semiconductor (TI) – Dual CMOS Compatible Differential Line Receiver

Application Hints (Continued)
FAIL-SAFE OPERATION
Communication systems require elements of a system to
detect the presence of signals in the transmission lines and
it is desirable to have the system shut-down in a fail-safe
mode if the transmission line is open or shorted To facilitate
the detection of input opens or shorts the DS78C120
DS88C120 incorporates an input threshold voltage offset
This feature will force the line receiver to a specific logic
state if presence of either fault is a condition
Given that the receiver input threshold is g200 mV an input
signal greater than g200 mV insures the receiver will be in
a specific logic state When the offset control input (pins 1
and 15) is connected to V_CC e 5V the input thresholds are
offset from 200 mV to 700 mV referred to the non-inverting
input or b200 mV to b700 mV referred to the inverting
input Therefore if the input is open or shorted the input will
be greater than the input threshold and the receiver will
remain in a specified logic state
The input circuit of the receiver consists of a 5k resistor
terminated to ground through 120X on both inputs This net-
work acts as an attenuator and permits operation with com-
mon-mode input voltages greater than g15V The offset
control input is actually another input to the attenuator but
its resistor value is 56k The offset control input is connect-
ed to the inverting input side of the attenuator and the input
voltage to the amplifier is the sum of the inverting input plus
0 09 times the voltage on the offset control input When the
offset control input is connected to 5V the input amplifier will
see V_{IN(INVERTING)} a 0 45V or V_{IN(INVERTING)} a 0 9V
when the control input is connected to 10V The offset con-
trol input will not significantly affect the differential
performance of the receiver over its common-mode operat-
ing range and will not change the input impedance balance
of the receiver
It is recommended that the receiver be terminated (500X or
less) to insure it will detect an open circuit in the presence
of noise
The offset control can be used to insure fail-safe operation
for unbalanced interface (RS-423) or for balanced interface
(RS-422) operation
For unbalanced operation the receiver would be in an inde-
terminate logic state if the offset control input was open
Connecting the offset to 5V offsets the receiver threshold
0 45V The output is forced to a logic zero state if the input
is open or shorted
For balanced operation with inputs shorted or open receiv-
er C will be in an indeterminate logic state Receivers A and
B will be in a logic zero state allowing the NOR gate to
detect the short or open condition The strobe will disable
receivers A and B and may therefore be used to sample the
fail-safe detector Another method of fail-safe detection
consists of filtering the output of the NOR gate D so it would
not indicate a fault condition when receiver inputs pass
through the threshold region generating an output transient
In a communications system only the control signals are
required to detect input fault condition Advantages of a bal-
anced data transmission system over an unbalanced trans-
mission system are
1 High noise immunity
2 High data ratio
3 Long line lengths
Unbalanced RS-423 and RS-232 Fail-Safe
TL F 5801 – 12
7
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