We have a fully working java program that can connect to another computer and can send messages from this (both sides run java and we used SPP Server and Client).

However, when trying to connect to the NXT Brick and send a message via Bluetooth, we are able to pair with the brick and send a message but we have no idea if the NXT receives it.

When we tried to use Java SPPClient on the computer(we provide the ServerURL of NXT to java program) to send the message to NXT Brick running the RobotC, it doesn't work. On the java side, this error is given:

Failed to connect; [10064] A socket operation failed because the destination host was down.

Using RobotC the brick is waiting for a message to be sent and it is awaiting 3 integers to help the robot move.

  • that sounds like a firewall problem. Is the java program trying to access the internet?
    – tuskiomi
    Jul 20, 2016 at 15:21
  • @tuskiomi The socket connection error is for the bluetooth connection, not the internet. The firewall should not interfere in local bluetooth connections.
    – Nils Munch
    Sep 20, 2021 at 6:03

2 Answers 2


Have you updated the firmware? That may be a problem but it also may be that there is an error connecting using Bluetooth. “Destination host was down” means that the coding platform could not connect to the NXT.


I have a strong suspicion that your COM3 is connected to BlueTooth so your Python program can't connect.

I would go through the information in the NXT Bluetooth Developer Kit (available from the LEGO MINDSTORMS download page) to troubleshoot the process.

Beyond that, this sounds like a potential general BlueTooth issue, and not directly related to the NXT brick.

In the worst case, there might be a hardware issue with the brick itself. Lending another NXT brick and checking if it gives a different result should answer that question.

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