groundfungus:
Bad information that is becoming a forum urban legend, probably because many don't understand the TTL operations. The arduino can send tx to a large number of rx, and the arduino can receive rx from a large numbet of tx if each tx has a properly aligned diode in the tx line.
Zoomkat. My bad then. This is the first that I have heard of this and would like to know more about the technique. Would one also need a protocol to identify the sender and/ or receiver?
The key to the TTL serial tx/rx is that both the tx and rx pins are normally held high at 5v when no transmission is occurring. The rx pin is internally "lightly' held high at 5v so that it is not susceptible to line noise. The tx pin is normally at 5v, but goes to ground when it sends a bit. This also pulls the connected rx pin to ground, which tells the rx it is receiving a bit. The tx goes back to 5v at the completion of the bit send. This causes/allows the rx pin to go back to 5v indicating the end of the bit send. So one tx pin can pull a number of connected rx pins low. For a number of tx pins to be connected to a single rx pin, a diode is needed on each tx pin connected to the rx pin. With out the diodes, all the tx pins would be holding the single rx pin high, preventing a single tx pin being able to pull the rx pin low. The diodes (band end connected to the tx pin) prevent the tx pins from feeding 5v to the rx pin keeping it high at 5v. Each tx pin can still pull the rx pin low. With the arduino USB serial connection, a diode can't be put on the internal tx pin, but in my test the USB tx pin did not prevent another external tx pin from pulling the arduino rx pin low. YMMY. You will still need to add identity features to the slave arduino code.