The MAX3280E, when unpowered, will not affect communications on the RS-485 bus to which it is connected. A concern with any RS-485 receiver is how it will present itself to an active RS-485 line when the receiver is not powered. Theoretically, if there is a path from the receiver input to VCC, the bypass cap at VCC could charge up and power the receiver. This is not the case for the MAX3280E. If you apply a voltage to the receiver inputs that is within their absolute maximum and minimums, the part will remain unpowered and the input cap will not charge.