To Fail-Safe or not Fail-Safe on RS485/422 Transceiver
SHort Answer:  You must add the differential terminators (120//120) for Rx and Tx combined OR use the Fig 6 sol'n for the MAX3095.
 RN7 is redundant.



In the static case with uC outputs defined as inputs, U35 will detect the open circuit with its internal 47k pullup and force a logic high output.  

Meanwhile U4 will always sees this differential Logic High during bootup until the same output "Active High state" is supplied by the uC to U35.

The internal Fail safe circuit uses 47k pullup on P and pulldown on N inputs to detect the open circuit with no differential terminator connected.

I assume there is a DC OK detector to initiate a POR so it could be extended to disable these drivers using G = 0 or perhaps with a R pullup C to 0V to drive G. 

This makes RN7 redundant.

If DC power comes up noisy, I would gate the driver with a DC OK or POR signal or RC delay > Vdc rise time worst case.

I am not certain about all of this, not having tried this condition, but you have my short term understanding.

Here are some historical documents on this subject and other examples.

RS-485: Passive failsafe for an idle bus 
 http://www.ti.com/lit/an/slyt324/slyt324.pdf
SN65HVD3080E http://www.ti.com/lit/ds/symlink/sn65hvd3080e.pdf

But in the end , you may be ok as is.  So when in doubt, test it, before committing to a layout.

ps. I suspect they detect all conditions of shorting and open so an outside normal terminated voltage range and &lt; open input terminated diff voltage range = invalid input using double window detector may be used  to force the output high.  A 1/128 bus load of 47k= 47k/128= an equiv pullup of 367 Ohm pullup and pulldown from 128 receivers. So a driver diff R of 120 Ohms and same for Rx is needed to detect an connection from source diff R to stay below the inside the outside fault thresholds of open circuit. So even for 3" length you need 60 Ohms diff R.

so &lt;+/- 200mV is an invalid level and a threshold 10% below 128/(367+367+128)*5V=742 mVpp or +/- 668mV indicates open source impedance

afk.