Quote Originally Posted by skimask View Post
I'd use a small battery pack rather than a cap, trickle charge the pack off the serial line when you can, diode block it so the pack can't feed back into the data line. Cap's are too unreliable in cases like this.
You might be right about the caps being unreliable, but the current is so low on the lines it might take some time to charge the battery pack.

Question - I thought RS485 voltage was negative without a signal on the line?

My original idea came from thinking about RFID chips.