isolationpower supplyrs422rs485
There are several isolated solutions on the market such as ISO1176DW. These ICs usually pretty big, expensive and consuming.
However I could use non isolated transceiver with isolated power supply for both microcontroller and transceiver.
In some cases this solution can be much cheaper (especially if power supply isolation is given).
Is there any points I didn't aware of?
A switcher that runs from the AC can have its isolated output connected to the neutral. Make sure it is an isolating switcher though.
You can connect the two grounds if they're truly floating wrt each other. Then the CM range is controlled and you're back to the original situation. Several volts of difference is okay (-7 to +12) so any reasonable connection over a short distance should meet this requirement. A high CM range transceiver would still require some kind of control of the common mode voltage.
Isolated DC-DC converters are very common, just do a parametric search at some distributor such as Digikey. I see 176 pages (25 per page) of different isolated DC-DC board-mount modules before filtering for required power and for input and output voltages.
Edit: One of my favorites is the TI (nee Burr-Brown) DCR01 series, which has an LDO (no problems out-of-spec voltages with light loads) and fits in an SOP that is 2.5mm high. It can supply 1W, so 200mA at 5V.
There are also RS-485 hybrid transceiver modules with the DC-DC converter built-in, but they're a bit expensive, and the DC-DC converter is probably a bit inefficient compared to one with a less constrained transformer.
You should connect the isolated ground to the ground on the remote system to control the common mode range.
Best Answer
Isolating RS-485 lines is a good idea since they tend to go long distances and you want to avoid ground loops.
You are right, it doesn't really matter where exactly the isolation occurs. That decision is part of the system-level design, with no one right universal answer.
Sometimes it's easiest to isolate directly at the RS-485 bus. That simplifies the power supplies, but driving the bus can be tricky.
Sometimes it makes sense to have a separate small 5 V power supply just to run the immediate circuitry connected to the RS-485 line. That's what I'm doing in one project now. The main circuitry is locally ground-referenced. A spare PWM output with shutdown input drives a small flyback transformer to make a few 10s of mA at 5 V. When the supply gets above the regulation threshold, it signals thru a opto-isolator, which asserts the PWM shutdown input. This creates the isolated supply with only a few parts and zero firmware cycles after startup.
Sometimes it makes sense to run most of the circuitry at the bus ground, then isolate the few lines going in/out from/to elsewhere.