THVD1552: Safe state with open input

Hi Zach,

The resistor selection is a trade-off between the amplitude of the differential offset introduced (larger tends to improve noise immunity) and the effective impedance of the termination (lower tends to decrease the number of nodes that can be supported). You can find more information in the following blogs:

e2e.ti.com/.../rs-485-basics-two-ways-to-fail-safe-bias-your-network

e2e.ti.com/.../how-much-loading-can-an-auto-polarity-rs-485-bus-support

With the THVD1552 in particular, a large additional offset is generally not needed. In fact, even without biasing resistors it will still interpret an open input as a logic "high" value since its receiver input switching threshold is set below 0 V. Of course, additional resistors may still be useful in improving noise immunity in the idle state or in adding failsafe biasing to other nodes on the network that may not have it integrated into the transceiver.

Please let us know if you have any questions on this.

Regards,
Max