Thank YOU Tom,
Both are mega board sitting by solderless cards, both (mega and solderless) mounted on a plastic support platform from sparkfun
Yes the two board's grounds are connected.
Again, issue is while master is undergoing code download. In this situation, there is noise AND outputs (master Tx) are (i think) threestated.
So if you are not downloading code, you have no noise?
That is, when everything is loaded, and running, you have no noise?
While downloading code, turn the other controller OFF.
Trying to run any controller while downloading code to it will usually end in problems?
Sorry for the umteenth time, can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
This is a very important part of troubleshooting.