About ADM485

Question

Hi there, 
 I have a question about ADM485, RS-485 transceiver. 
 http://www.analog.com/media/en/technical-documentation/data-sheets/ADM485.pdf 
 The datasheet says this device has fail-safe feature. 
 Could you be explain what is the fail safe feature of ADM485? 
 I also found application note, it mentions true-fail-safe feature. 
 http://www.bdtic.com.cn/download/adi/AN-960.pdf 
 ADM485's fail safe has this true fail safe feature? 
 If it's not, what is the differences? 
 Best, 
 Tarzan

ConalW · Accepted Answer

ADM485 only has open-circuit failsafe. That means, in the specific case where there is nothing connected between A and B pins on the ADM485 (no termination resistor, no other nodes), then the receiver output will be high (failsafe state) instead of indeterminate. 
 ADM485 does not include short circuit or terminated input failsafe (also called "true" fail safe). Other transceivers such as the ADM4850-ADM4857 family have this implemented, by having an offset threshold on the receiver, i.e. as long as the differential voltage is >= 30 mV, the receiver output will be high. When the bus is idle but terminated, or A shorted to B, then there differential voltage is approx. 0V. In this case, ADM485 has an indeterminate receiver output, but ADM4850-ADM4857 have a failsafe output high. 
 ADM4850-ADM4857 do include the open-circuit failsafe as well - this is implemented on all ADI RS-485 transceivers.