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Maximum NUS data length

Pero Krivic

Hello, a newcomer here. 

I've just went through your wonderful BLE service and characteristics tutorials and now I'm experimenting with the BLE UART, i.e. proprietary NUS. I'm using nRF52-DK and SDK 17.1.0.

I have some issues figuring maximum allowed data length. At first, the nRF Connect app at my smartphone refuses to send more than 20 characters. I searched a bit and found a couple of threads here in the devzone that mention that maximum data length is 20 bytes per single TX/RX action. Is that true? How is that determined, and why such a small value? I cant find any explanation about it in the infocenter or in the codebase. 

Now, if I wanted to send a longer string of data, what would be better approach, to use the NUS or move on to some other service?

Best, 

W.

  • +1

    Hi,

    maximum data length is 20 bytes per single TX/RX action. Is that true?

    Yes, that is the default, as defined in the Bluetooth spec. It can be negotiated higher(if both central and peripheral supports higher MTU) with a MTU Request/Response procedure.

    Now, if I wanted to send a longer string of data, what would be better approach, to use the NUS or move on to some other service?

    NUS is perfect for sending strings.

    my smartphone refuses to send more than 20 characters

    If you have a old phone, it might not support long MTU.

  • 0

    Hi, thanks for the answer. Do you mind pointing me to where to find out what MTU is (in the Nordic SDK sense) and how to use this negotiation?

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