In SX Microcontrollers, SX/B Compiler and SX-Key Tool, mrchadwickusa wrote:

Hi Ray,

Whenever you want to get a signal from some place to another place, you have to also provide a return path for the signal current.  That is what the ground does for you.   Assuming you have two separate power supplies like wall warts running the two boards, there is no common connection between them until you deliberately make the connection.

Without the common, your serial signal is just lifting the whole receiving SX circuit up and down, so the receiving SX never saw the signal change.  Been there, done that!

The SX also has input protection diodes which will couple the signal power from an input to the power pin on the SX when the receiving SX is off.  That is why your power light lights on the receiver even with power removed.  Generally there is not enough supplied by the transmitting SX to power the receiver, just enough to light the LED.

MRC

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