DS78LS120 Datasheet, PDF (6/9 Pages) National Semiconductor (TI) – Dual Differential Line Receiver (Noise Filtering and Fail-Safe)

Application Hints (Continued)
Balanced RS-422 Fail-Safe
00749912
00749913
For balanced operation with inputs open or shorted, 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 open or short 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 NOR gate D so it would not indicate a
fault condition when receiver inputs pass through the thresh-
old region, generating an output transient.
00749914
In a communications system, only the control signals are
required to detect input fault conditions. Advantages of a
balanced data transmission system over an unbalanced
transmission system are:
1. High noise immunity
2. High data ratio
3. Long line lengths
www.national.com
6