I borrowed Q1 form Sparkfun:https://www.sparkfun.com/products/11959
But before I did I was confused by why it worked and was a good idea.
I probably need to start with some background; RS485 defines the range between -200mV and +200mV as "no-mans land," e.g. if the differential pair is inside this range some devices see it as high others see it as low. Some years ago almost everyone decided this was a bad idea and started using fail-safe biasing.http://electronicdesign.com/communications/implement-idle-bus-failsafe-multipoint-networks
Fail-safe will force the bus into a known state when not driven.
Later chips like the one I am using (INTERSIL, ISL3172EIBZ) reduced the undefined range (-200mV thru -50mV), which also removes the need to bias for Fail safe.
So now that the bus has fail-safe transceivers I can power it off in the default high state (job of Q1). It is a huge deal, because driving the RS485 bus at 3V takes a lot of current with 100 Ohms at each end. So now my project could be battery powered. I only have to drive the low state when data is sent. I don't see much signal loss over the 600 feet of CAT5 I have. How far will it go? RS485 is rated for 4000 feet, but that is with 20 AWG twisted pair I think, and CAT5 is 23AWG.