How can we sure about common mode voltage for RS422/RS485 without ground connection?
In some applications, common-mode voltage bears zero (or nearly zero) significance. For example, and RS422 driver can drive (at the far end) an optoisolator. Since the only load on the twisted pair is the LED in the optoisolator, common-mode voltage need only be held lower than enough to arc across the PC board... one might get 240VAC common-mode induced voltage on the line, yet suffer no ill effects.