DS26F31MQML-SP: RS-422 with Cold spare I/O

HI Sam,

So it depends - its a bit more grey than a yes or no for this question. 

1. Receivers always are going to load the line - the bus facing pins are usually okay when the device is unpowered. RS-422 compliant receivers are rated at +/-7V common mode -so connecting an active device to an unpowered one is generally fine but high impedance is not really correct because they have an impedance and if there is multiple other devices on the line it will effect the overall impedance of the system.

2. A lot of RS-422 specific parts with only drivers will have the VCC = 0V output won't lines - that is specific to most, if not all, RS-422 specific drivers. If they have receivers on the device that statement isn't usually included.

3. RS-485 devices (which are backwards compatible with RS-422 devices and just generally better devices) will have its receivers always loading as well - but generally will have more options to have a higher input impedance (regardless of VCC) - unless you are doing multi-channel (which based on the part you were looking at you may well be) RS-485 devices are usually suggested for RS-422 applications so I wanted to include this on RS-485 devices as well.

So in conclusion: most RS-422 driver only devices will have the same when VCC=0V outputs don't load (which means the differential bus outputs aren't loading the bus) (It should be most if not all but there may be an exception or two) - however if a receiver is on the line there will be inherent loading regardless of VCC but RS-422 complaint receivers can handle -7V to 7V (w.r.t. the unpowered device's ground)  on the line regardless of VCC and if using a RS-485 device (which are usually spec'd with RS-422 specs as well since its backwards compatible) its -7V to 12V. So an active signal can exist on the differential bus as long as its within the common mode ratings of the unpowered device. However console side pins (the single ended data/control inputs) are normally hi-Z and the output of R is just a fet buffer essentially so it should also be high impedance and won't load too much - but most parts are not able to handle signals on the console side pins when VCC = 0V so while they are hi-z most of them have ESD diodes that will break if they become too forward biased (this is described in the abs max tables of datasheets).

Please let me know if you have any other questions!

Best,

Parker Dodson