DS78C120 Datasheet, PDF (7/9 Pages) National Semiconductor (TI) – Dual CMOS Compatible Differential Line Receiver

Application Hints (Continued)
The input circuit of the receiver consists of a 5k resistor ter-
minated to ground through 120Ω on both inputs. This net-
work acts as an attenuator, and permits operation with
common-mode input voltages greater than ±15V. The offset
control input is actually another input to the attenuator, but its
resistor value is 56k. The offset control input is connected to
the inverting input side of the attenuator, and the input volt-
age 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)} + 0.45V or V_{IN(INVERTING)} + 0.9V when the
control input is connected to 10V. The offset control input will
not significantly affect the differential performance of the re-
ceiver over its common-mode operating range, and will not
change the input impedance balance of the receiver.
It is recommended that the receiver be terminated (500Ω 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, receiver
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 fil-
tering 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 re-
quired 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
DS005801-12
7
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