6750|M-200|S100|R255255255
As far as I can see you only have the letters M, S, R and | (vert bar) how do they help addressing multiple Arduinos?
Any idea how many devices can be in the chain?
As many as you like, there is no physical limit but you have to think about propagation delay because at best you will have a one-character delay for every Arduino. So if there are 100 nodes #100 will get the message 99 x byte-times later than #1. At 115200 that's approx 8.6mS later. That may or may not matter in your application.
Rob