Question:
What is the maximum number of of nodes which can be supported using solely the MAX14852/4/6/8 transceivers with true fail-safe receiver inputs? Specifically, will the internal fail safe bias techniques impede the maximum number of nodes for a design? Can we implement a total of 128 nodes of RS485 if using these Maxim transceivers?
Answer:
Maxim RS485 transceivers with true fail-safe receiver inputs do not affect the maximum number of nodes, and here is why:
The receiver is 1/4-unit load so MAX14852, MAX14854, MAX14856, MAX14858 all have the capability of supporting 128 nodes. The integrated fail-safe circuitry for the above mentioned transceivers are not implemented via fail-safe biasing using resistors, which would have caused common-mode loading and reduce the number of transceivers. Instead, the mechanism by which fail-safe is guaranteed is that if the differential receiver voltage (VA-VB) is greater than or equal to -10mV, RXD is logic-high. The receiver threshold is fixed between -10mV and -200mV. When the inputs are short, VA-VB = 0 >= -10mV and therefore RXD is logic-high. In the case when terminated bus with all transmitters are disabled, the receiver's differential input voltage is pulled to zero by termination resistors. Due to the receiver thresholds of the devices, this results in a logic-high at RXD. Please refer to True Fail-Safe section on page 16 of the datasheet for similar information.