ADM3066E doesn't meet the RS485 standard?

Hello, 

Could you please share your measurement setup for the receiver differential input voltage ? In my experience it can be difficult to get an accurate reading of the receiver threshold of the device without filtering. Ambient noise which couples into the cables can convert to a differential voltage which affects the measurement result. 

The RS-485 specification requires that for a AB voltage of +200mV or greater, the receiver output a logic 1, and for an AB voltage of -200mV or below, the receiver outputs a logic 0. The ADM3066E meets this requirement per the table you have shared above, 

The setup can be improved by connecting to the AB pins with a matched twisted pair cable, to ensure any ambient noise is equally coupled into both the A and B lines, and does not appear as a differential signal. I would typically recommend placing 100nF capacitors on A and B to ground, and a 1uF capacitor between A and B, when measuring this, to filter out any differential noise that manages to make it's way through to the input pins. 

Regards,

Neil 

Hello Neil,

The problem is that it doesn't make sense to me that in this way the protocol doesn't have symmetry around 0V.

The asymmetry is intentionally introduced to provide the failsafe function. There some more detail on this in the RECEIVER FAIL-SAFE section of the ADM3066E datasheet. As the receiver logic low threshold is still no greater than -200mV, this still is designed to the TIA/EIA-485-B specification.

For timing critical applications, the asymmetry can introduce some receiver skew for very slow changing input signals. The ADM3066E has fast output rise/fall time of 6ns, so on a network of purely ADM3066E devices, this will not have an impact. 

Regards,

Neil

OK, I understand.

Thank you for the explanation.

OK, I understand.

Thank you for the explanation.