DS78C120 Datasheet, PDF (6/9 Pages) National Semiconductor (TI) – Dual CMOS Compatible Differential Line Receiver

Application Hints (Continued)
LINE DRIVERS
Line drivers which will interface with the DS78C120 are
listed below.
Balanced Drivers
DS26LS31Quad RS-422 Line Driver
DS7830, DS8830Dual TTL
DS7831, DS8831Dual TRI-STATE ^® TTL
DS7832, DS8832Dual TRI-STATE TTL
DS1691A, DS3691- Dual RS-422
DS1692, DS3692 Dual TRI-STATE
RS-422
DS3587, DS3487Quad TRI-STATE RS-422
Unbalanced Drivers
DS1488Quad RS-232
DS14C88Quad RS-232
DS75150Dual RS-232
RESPONSE CONTROL AND HYSTERESIS
In unbalanced (RS-232/RS-423) applications it is recom-
mended that the rise time and fall time of the line driver be
controlled to reduce cross-talk. Elimination of switching
noise is accomplished in the DS78C120 by the 50 mV of
hysteresis incorporated in the output gate. This eliminates
the oscillations which may appear in a line receiver due to
the input signal slowly varying about the threshold level for
extended periods of time.
High frequency noise which is superimposed on the input
signal which may exceed 50 mV can be reduced in ampli-
tude by filtering the device input. On the DS78C120, a high
impedance response control pin in the input amplifier is
available to filter the input signal without affecting the termi-
nation impedance of the transmission line. Noise pulse width
rejection vs the value of the response control capacitor is
shown in Figure 1 and Figure 2. This combination of filters
followed by hysteresis will optimize performance in a worse
case noise environment.
TRANSMISSION LINE TERMINATION
On a transmission line which is electrically long, it is advis-
able to terminate the line in its characteristic impedance to
prevent signal reflection and its associated noise/cross-talk.
A 180Ω termination resistor is provided in the DS78C120 line
receiver. To use the termination resistor, connect pins 2 and
3 together and pins 13 and 14 together. The 180Ω resistor
provides a good compromise between line reflections, power
dissipation in the driver, and IR drop in the transmission line.
If power dissipation and IR drop are still a concern, a capaci-
tor may be connected in series with the resistor to minimize
power loss.
The value of the capacitor is recommended to be the line
length (time) divided by 3 times the resistor value. Example:
if the transmission line is 1,000 feet long, (approximately
1000 ns) the capacitor value should be 1852 pF. For addi-
tional application details, refer to application notes AN-22
and AN-108.
DS005801-9
FIGURE 1. Noise Pulse Width vs
Response Control Capacitor
DS005801-10
FIGURE 2.
DS005801-11
FAIL-SAFE OPERATION
Communication systems require elements of a system to de-
tect 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 incor-
porates 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 ±200 mV, an input
signal greater than ±200 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 = 5V, the input thresholds are offset
from 200 mV to 700 mV, referred to the non-inverting input,
or −200 mV to −700 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.
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