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 180

Ω termination resistor is provided in the DS78LS120

line receiver. To use the termination resistor, connect pins 2

and 3 together and pins 13 and 14 together. The 180

Ω

resistor provides a good compromise between line reflec-

tions, power dissipation in the driver, and IR drop in the

transmission line. If power dissipation and IR drop are still a

concern, a capacitor 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), and the termination resistor value is 180

Ω, the

capacitor value should be 1852 pF. For additional application

details, refer to application notes AN-22 and AN-108.

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 DS78LS120 in-

corporates 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.

specific logic state. When the offset control input (pins 1 and

15) is connected to V

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.

The input circuit of the receiver consists of a 5k resistor

terminated to ground through 120

Ω on both inputs. This

network acts as an attenuator, and permits operation with

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

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

control input is connected to 10V. The offset control input will

not significantly affect the differential performance of the

receiver 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 (500

Ω 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 fail-safe offset pin to 5V, offsets the receiver

threshold to 0.45V. The output is forced to a logic zero state

if the input is open or shorted.

Unbalanced RS-423 and RS-232 Fail-Safe

00749911

00749910

FIGURE 2.

www.national.com

5