THVD1552: Do we need Pull up and Pull down resistors for A and B pins?

The fail-safe is guaranteed to work when the differential input voltage is zero.

External fail-safe resistors are optional. You can add them if you want to increase the noise margin.

With 10 kΩ resistors, the voltage drop over the termination resistors when idle will be about 60 mV, i.e., you have increased the noise margin from 20 mV to 80 mV.

Subhasis Pramanik said:

For receiver fail safe operation, do we need to add pull up or pull down resistor in A/B lines.

As Clemens has stated, external fail safe resistors are optional. I do think adding them to the receivers as DNP is a good idea since it's better to have them available if you do think you need them. 

Subhasis Pramanik said:

I understand that the fail safe operation can be done by internal biasing of the tranceiver. 

Correct. You don't need to do anything extra to turn on the fail safe operation. 

Subhasis Pramanik said:

If we consider the VHYS is 50mV(TYP) & VTH+ is -20mV(MAX), does that mean the output will be in indeterminate condition if differential input is -20mV~-70mv range?

Technically the Routput should still be high if you are between those ranges but its not guaranteed since Vhys is provided as a typical value. 

Subhasis Pramanik said:

For better noise immunity, is it better to add pull up and pull down resistor? If so, what will be the range of the resistor. Currently I am considering to add 10k resistor.

Note: the calculation I used below assumes termination on both end points for RS485 half duplex systems. In a full duplex system you normally have only one termination resistor. So the 15mV I calculated would be 30mV if you only used 1x 120ohm termination.

You can calculate the extra noise immunity value by basically using the equation V=IxR. You first need to calculate the total current flow there is when the bus is idle. This can be done by adding the pull up resistor, pull down resistor, and the termination resistor together then dividing by Vcc (V of the pull up resistor). For 10k up, 10k down, and a 60 ohm termination resistor(120//120=60 ohm) Rtotal is 20,060 ohms. I_total = 5V/20kohm = 250uA. So If you have a 60 ohm eq resistance(in RS485 we generally use 2 termination resistors of 120 ohm which equates to 60 ohms), then 25uA x 60 ohm = 15mV. So when the bus is idle, the voltage across the termination resistor will be about 15mV. 

Most people opt for around 200mV of noise immunity during idle but it just depends on how much noise you expect from the environment. If you sized the resistors to be 1k both up and down then you would get around 150mV with the hysteresis included that would get you around 220mV. 

The document below walks through how to calculate this using equations if you want to get a more indepth look. 

https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1730491366211&ref_url=https%253A%252F%252Fwww.google.com%252F

My assumption is 1k pull up and pull down resistors will probably be good enough. (10k is probably too weak)

-Bobby