I have a question about table II of ADM1485 Datasheet.
Does RO go to 1 independent of DE level when RE=0 and A,B are open?
This is a half duplex transceiver so ideally you should only be enabling the driver or the receiver, but not both.
So when RE = 0 (receiver enabled), driver should be disabled (DE = 0)
so when RE = 0 and DE = 0, and A and B are open or floating, RO will go 1.
If you're enabling both the driver and the receiver then whatever will be driven into the DI will be looped back on RO after some time delay so it depends on DE and RE in that case if that is your question, but ideally you shouldn't be doing this. So I believe DE should be disabled when you're receiving and only then RO will go high if A and B are open/floating.
On ADM3485E datasheet, the table is given from the receivers perspective as it is labelled as the 'Receiving Truth Table' so we do not really care about what logic signal is applied to the DE input as we are talking about the receiver there. But in reality, when DE and RE are both enabled, it loops back whatever the driver input is so A and B cannot be considered 'open' in this case on the receiver end and the receiver will output whatever's going into the driver's DI pin after some delay. This is the standard behavior of many half-duplex RS485 transceivers. That truth table will hold for full duplex transceivers only where we do not share the 2 lines for receiving and transmitting signals. The receiver and the driver are independent blocks connected by the A and B lines on most RS485 transceivers so the input pins on the driver (e.g DE) should not affect the state of the receiver and vice versa. It will still be receiving signals if the driver is disabled or enabled and the driver would be driving if the receiver is enabled or disabled. The user controls that.