We have found that depending on the message content, it takes varying amount of time for the failsafe function to drive the receiver output high. Adding a biasing network has very little impact. This is a 2 transceiver 485 network with one passive receiver (always enabled). A message is sent from our test 485 transceiver and after the message ends, the transmitter is disabled. This leaves 3 receivers on an inactive bus. We see the LT2881 take on the order of 200usec to go to a failsafe condition (Vdiff <200mV).