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Detection of RS-485 signal loss

Posted: 03 Jun 2009 ?? ?Print Version ?Bookmark and Share

Keywords:signal loss detection? RS-485? differential window comparator?

Fault isolation and safety shutdown protocols are critical in many industrial, telecommunication, automotive and data processing systems. While built-in test routines may provide fault isolation when the system is offline, real-time fault detection requires continuous monitoring of signals. These systems often use RS-485 to share data between sensors, actuators, single-board computers or communication processors.

RS-485 signals are differential, using two signal wires to transmit data, and detection of valid signal levels requires a differential window comparator. Designing this circuit function is complicated by the wide common-mode range of RS-485 signals and, in many cases, the availability of only positive supply rails.

This article shows how a differential window comparator can be constructed with the passive-failsafe feature of two SN65HVD3088E RS-485 transceivers and an AND gate. It also provides theory of operation, the basic circuit schematic, test results and other design considerations.

View the PDF document for more information.

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