When I send data to an Arduino Uno via the serial monitor, the serial data goes down the same USB cable used to program the Arduino. So there must be a device on the Arduino board like a FTDI or USB-capable AVR that drives the RXD pin of the Arduino. So my question is: When I want two Arduino boards to communicate with each other over serial, many tutorials say just connect RX and TX on one Arduino to TX to RX on the other. So how come the device on the board that normally drives the RXD pin doesn't "fight" the other Arduino that is also now driving the RXD line?
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2 Answers
The Atmega16U2 that does the USB communications is connected to Rx and Tx via 1k resistors, so when communicating with another Arduino it "overpowers" the USB interface. See image:
By the way, note that the Tx and Rx LEDs are connected to the Atmega16U2, not the Atmega328P, so they do not light when doing Arduino-to-Arduino communications.
Measuring with an empty sketch on my Uno, the current between Tx and Gnd was 0.14 mA, not 5 mA. So that suggests that the Atmega16U2 sets the pins to high-impedance when not actually receiving data from the USB interface.
It's logical it would do that, because the 5 mA you mention (in a comment) could be a nuisance if you wanted to use that pin for something else.
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Awesome, thanks! You were both very quick. So does this mean that it is safe to power the Arduinos via the USB even when they are wired for serial communications with another Arduino? And that the ATmega16U2 and the other Arduino will fight, but only with ~5mA? yesterday
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1Yes, I think that's done all the time. However, and this is important, because the Arduino-to-Arduino connection will overpower the USB interface, you will have trouble programming it (the one you are trying to program) while they are connected.– Nick Gammon ♦yesterday
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1It's also possible, although I'm not certain, that the USB chip would set the pins to high-impedance when not actually in use. In fact, I just measured on my Uno, the current between Tx and Gnd was 0.14 mA, not 5 mA. It's logical it would do that, because the 5 mA you mention could be a nuisance if you wanted to use that pin for something else.– Nick Gammon ♦yesterday
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@NeilStockbridge So those 1k resistors are probably there as a precaution against you trying to reprogram the chip while it is connected to something else.– Nick Gammon ♦yesterday
If we take a look at this portion of the Arduino Uno schematic (whole schematic is located here):
... we see there are two 1K Ohm (1,000 Ohms) resistors (labeled RN4A and RN4B) between the USB to serial conversion chip on the left to the Atmel processor at center right. These allow the Rx (Receive) and Tx (Transmit) pins to be back driven by an externally connected device.
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That must be an old schematic, as the Uno now has a Atmega328P not an Atmega8. However the principle is the same.– Nick Gammon ♦yesterday
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Hmmm, it came right off of arduino.cc's web site. Oooh, that's what they meant by "Reference Design".– st2000yesterday
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1Mine has "Rev 3" on it, so you must have found an earlier revision. It's still a reference design though.– Nick Gammon ♦yesterday