Pull-down resistor with ADM485
If you cannot make sure that your control circuit (e.g. microcontroller) can always keep DE deasserted unless it's actually transmitting (including startup, unprogrammed and similar conditions) you should by all means add a pulldown to DE to avoid tying up the bus. After all, it's about the worst thing that can happen to a RS-485 bus - no other device can communicate in this state. This is how I do it:



A pullup on /RE ensures that no spurious noise is received by the UART (and also allows putting the driver into a sleep mode if that matters). If your /RE and DE are connected together, pull that line in the "receive" direction (down).

Pullup values can vary if you have special requirements but 10K is standard for a digital signal not driven by an open drain.