Monitor a differential pair for valid & fault conditions

-February 09, 2015

It is advantageous to know if there is a valid signal on a differential signal bus. This Design Idea detects differential data transmission and reports a loss of signal (LOS) to a microcontroller or other monitoring device.

The circuit of Figure 1 detects a continuous differential signal from -7V to +12V with a minimum amplitude difference of 200mV. The circuit detects open inputs, shorted inputs, and inputs stuck at high or low. The circuit’s output is low when any one of these conditions occurs, and high when there is a differential pulse train. Only a single 5V supply is needed.

Figure 1  Circuit detects a continuous differential signal

U1 (MAX3280), an RS-485 receiver, accepts differential signals of 200mV minimum at A and B, where the voltage can be from -7V to +12V. Pin RO of U1 is high in the absence of input signals, because U1 is a true fail-safe device that provides a high output when the inputs are open or shorted. RO can also be high or low when one of the inputs is stuck high and the other is stuck low. R1 and C1 form a lowpass filter to average U1’s output into a DC voltage when the output at RO is a pulse train.

U2 (MAX992), a dual open-drain comparator, is configured as a window comparator, with R2-R4 setting the voltage thresholds to 0.25V and 4.75V. Only when the filtered voltage of a pulse train falls between the two thresholds will the final OUT signal be high.

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