Dear Sir

The waveforms from the oscilloscope are shown as below

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• Hi Jason,

Thanks for the detailed schematic and waveform. Your query is related to the fail-safe feature, and that is actually not a problem.

I've attached the application note AN-960 here, for ADM3485/ADM3485E, both of them have open fail-safe feature which guarantees RO output high when inputs are unconnected. Based on your schematic, we see there are bias resistors network for input bus pins, and at this circumstance, the RO output should be determined by input differential voltage( VIA-VIB), to be more precisely, the triggering threshold should stay somewhere between -200mV and 200mV( Figure 10 of AN-960). Let's calculate the VIA-VIB under different bias resistors(KR21 and KR50):(Page 6 of AN960)
When KR21=KR50=47K, VIA-VIB = 3.3V/(47K*2+0.12K)*0.12K = 4mV
When KR21=KR50=1K, VIA-VIB = 3.3V/(1K*2+0.12K)*0.12K = 187mV
The triggering threshold of ADM3485 should higher than 4mV and lower than 150mV( considering the 50mV input hysteresis). This explains why experiment A outputs low and experiment C outputs high. For experiment D, the triggering threshold of ADM3485E should be below 4mV, different parts might have different threshold.

If you want to keep RO output high when bus is idle, you need calculate the biasing resistor values to keep VIA-VIB>200mV.

Regards,

Neil

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