I created a schematic for the MAX3485 with the ESP32, but I have a question about the selection of biasing resistor(R39,R40) values. Some people use 4.7k, 10k, and 20k ohm resistors, but I'm unsure what the exact value should be and how much current is required. Does anyone have insights or suggestions on this? Thank you!
There are no exact values that must be used.
The bias resistor values are calculated based on what you want as the idle bus differential voltage for improved noise margin, and that depends on in which kind of noisy environment you are in and how many devices you must support on the bus.
Some chips have failsafe features that do not even need any bias resistors.
The current needed can be calculated simply with ohm's law, they are just three resistors between supply and ground.
So it completely depends on if your device is the host that should terminate and have bias resistors or not, or the device under some host control that should not have bias resistors at all and no termination - unless your device cannot work when bus is shorted or disconnected.
And all bias resistors reduce the amount of devices you can connect to bus.
If you have a specific bus which requires the use of bias resistors by definition, the resistor values are also defined in the bus specification.
The pull-up on TE/#RE pins will mean the MCU jams the bus at boot until your firmware initializes the IO pin.
