Analysis of valid and fault conditions of differential signals

It is useful to know if there is a valid signal on a differential signal bus. This example is to detect differential data transmission and report loss of signal (LOS) information to a microcontroller or other monitoring device.

The circuit shown in Figure 1 is used to detect continuous differential signals with a minimum amplitude difference of 200mV and an absolute value ranging from -7V to +12V. The circuit can detect input open circuit, input short circuit, and input held high or low. When any of these conditions occur, the output of the circuit is low, and when there is a differential pulse train, it is high. The circuit requires only a single 5V power supply.

    Figure 1 Continuous differential signal detection circuit

    OPEN

    SHORTED

    ONE STUCK HIGH ONE STUCK LOW One end remains high, the other end remains low

    DIFFERENTIAL SIGNAL TRAIN Differential signal train

    OUT Output

    LOW Low level

    HIGH High level

    U1=FAIL-SAFE RS-485 RECEIVER RS-485 receiver with automatic fail-safe function

    U2 = OPEN-DRAIN, DUAL COMPARATOR Open-drain dual comparator

U1 (MAX3280) is an RS-485 receiver that accepts differential signals with an amplitude difference of at least 200mV between the A and B terminals and an absolute voltage range of -7V to +12V. When there is no input signal, U1's pin RO will be high because U1 is a true fail-safe device that will output a high level when the input is open or shorted. R0 can also output a high or low level when one of the input pins is held high and the other is held low. R1 and C1 form a low-pass filter that can smooth the output of U1 to a DC voltage when the output of RO is a pulse train.

U2 (MAX992) is a dual open-drain comparator configured as a window comparator with the voltage thresholds set to 0.25V and 4.75V by the R2-R4 circuit. The final OUT signal goes high only when the filtered pulse train voltage falls between these two thresholds.