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I have this application where I want to send commands from the PC to an Arduino, remotely, and receive an answer back.

The hardware configuration I'm using to achieve that is the following:

PC <--> Arduino with an Xbee <--> Another Arduino with another Xbee

It's just half-duplex communication. Everything seems easy, but there is a problem.

Each Arduino has an Xbee Shield with an Xbee module connected to it. Reading the documentation of the ArduinoXbeeShield, an excerpt caught my eye:

"With the jumpers in the Xbee position (i.e. on the two pins towards theinterior of the board), the DOUT pin of the Xbee module is connected to the RX pin of the microcontroller; and DIN is connected to TX. Note that the RX and TX pins of the microcontroller are still connected to the TX and RX pins (respectively) of the FTDI chip - data sent from the microcontroller will be transmitted to the computer via USB as well as being sent wirelessly by the Xbee module. The microcontroller, however, will only be able to receive data from the Xbee module, not over USB from the computer.

With the jumpers in the USB position (i.e. on the two pins nearest the edge of the board), the DOUT pin the Xbee module is connected to the RX pin of the FTDI chip, and DIN on the Xbee module is connected to the TX pin of the FTDI chip. This means that the Xbee module can communicate directly with the computer - however, this only works if the microcontroller has been removed from the Arduino board. If the microcontroller is left in the Arduino board, it will be able to talk to the computer normally via USB, but neither the computer nor the microcontroller will be able to talk to the Xbee module."

That means I will not be able to build what I want with this configuration? I already tried, but didn't work.

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Basically with your shield you have two options:

  1. Communicate between the MCU and the XBee at the cost of not being able to communicate between the PC and the MCU.
  2. Communicate between the PC and the XBee at the cost of having to remove the MCU from the board completely.

Neither of which are particularly ideal. The first option is completely out since that rules out all PC communication. The second option turns the Arduino into just a USB->TTL Serial adapter - and a rather expensive one at that (you can get them on eBay for like $2).

What you want is to re-wire the system using jumper wires (messy, I know) so that the TX and RX pins of the XBee connect to other pins on your Uno then use SoftwareSerial to communicate with the XBee. That will leave the Arduino's TX and RX pins free for communicating with the PC.

Alternatively invest in a different Arduino that has more hardware UART interfaces, like a Mega2560.

  • With Mega I solve the problem completely? – user14480 Oct 31 '15 at 22:15
  • @stdio.h With a Mega you can manually wire the XBee to a different pair of UART pins instead of 0 and 1. That way you can communicate between the MCU and PC and the MCU and XBee. – Majenko Oct 31 '15 at 22:27

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