SN65176B: how to understand the input resistance spec for receiver?

The A/B input schematic is shown in figure 8-3. If you tried to measure the resistance between an input and GND, you would get at least 12 kΩ.

This specification is not very helpful. What is important to know is that the SN65176B's inputs corresponds to one RS-485 Unit Load, so you could have up to 32 such receivers on a bus.

Hi Aki,

The input resistance is, as Clemens mentioned, measuring impedance between input and GND of differential receiver. 

This specification is important for the RS-485 system with respect to bus loading and bias resistor sizing. 

The standard allows up to 32 Unit loads on an RS-485 bus - 12K ohms is equal to 1 Unit Load - so you can have 32 1Unit load devices on an RS-485 bus - this is equivalent to loading the bus with a 375 Ohm resistor.  In most applications you don't need to use the actual resistance value - just the UL value - which for this device would be 1 - more modern devices will have fractional UL values (1/2, 1/4, and 1/8 UL - which allows 64, 128, and 256 receivers on the bus respectively) 

If your customer is applying an idle fail-safe bias to the bus  (which is adding a pull-up resistor on A and a pull-down resistor on B to prevent undefined inputs when the bus is idling - more information here: https://www.ti.com/lit/an/slyt324/slyt324.pdf) the fail-safe bias resistors also must be included in the total bus loading calculations - meaning that adding these resistors will decrease the amount of differential receivers allowed on the bus. 

Please let me know if you have any other questions!

Best,

Parker Dodson