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Modbus RTU protocol over "BLE UART"

I used the Nordic EVAL model nRF52-DK which uses the "nRF52832" chip.
The EVAL has been programmed as "Pheripheral" with the application "BLE over UART" made available by Nordic itself.
The "BLE over UART" profile contains the SoftDevice inside: "s132_nrf52_6.1.0_softdevice.hex".
The RX / TX signals of the UART have been connected to the serial port of an external device.
The BLE client is a PC application interfaced with the VCP of the "nRF51-Dongle" dongle.
The data frames to be exchanged have a maximum size of 512 bytes (stardard modbus RTU). The ATT_MTU = 20.
What is the reliability of BLE communication in the case of buffer fragmentation in packets of 20 bytes each ?
Is there a guarantee that the packets are always sent sequentially without the loss of packets ?
Which recommended solution to use for data exchange with such large buffers with modbus RTU ?

Best regards.
Thank you.

Demetrio Magrin

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