SN65HVD74: SN65HVD74DR internal impedance

The RS-485 specification does not characterize the I/Os with a single resistance value. See figure 1, and the |V_OD| and I_I specifications.

The output short-circuit current is guaranteed to be less than 150 mA; it's typically about 100 mA (see figure 1). Whether the device can handle this depends on the ambient temperature. There is no time limit; the T_J limit must no be exceeded even for a short time.

Hi Arpitha,

The differential receiver has a bus loading of 1/8th unit load - which can thought of as a minimum of 96k ohms of input impedance - it will be higher than that through most of the operation but for RS-485 systems it's generally easier to convert the unit loading into 1 impedance and 1/8 Unit load ~ 96k ohm minimum. 

The differential driver will have a typical output impedance of ~8.8 ohms on each output of the driver - under higher temperatures this could be different - but we don't generally spec the direct output impedance - this is derived from the VOD specification. 

The single ended receiver output "R" has a typical output impedance of 37.5 ohms from VCC to R (when outputting logic high) and a max of ~112.5 ohms (derived from VOH and IOH values in datasheet)

When the "R" pin outputs a low the output impedance (really input impedance since it will be sinking current on a low output) is typically 25 ohms and has a max of 50 ohms. 

Both of the receiver "R" pin specs assume |+/-8mA| current - the values could be different under different measured currents, but for approximation it generally is okay. 

Clemens is correct on the total current limit and TJ analysis, so I have nothing to add on from what they said. 

Best,

Parker Dodson