i want to do simple data transfer and why blueetooth serial terminal was not be able to detect the device?

Hello,

You could use the BLE NUS peripheral and central examples to do such a transfer between your devices.
The examples will relay anything they receive over UART to their connected peer over BLE, and the peer will output the messages received over BLE out on its UART, and visa versa.
Test this according to the example documentation, and let me know if you encounter any issues or questions.

Best regards,
Karl

i go through the nsu code but which api was used to transfer the data?

err_code = sd_ble_gap_tx_power_set(BLE_GAP_TX_POWER_ROLE_ADV, m_advertising.adv_handle, tx_power_level);

if tx power set to 4dbm it increases the transmission range right?

Yes, compared to +0dBm it will increase the transmission range.

but it was not increased in my case?

-40dBm, -20dBm, -16dBm, -12dBm, -8dBm, -4dBm, 0dBm, +3dBm and +4dBm.
* In addition, on some chips following values are supported: +2dBm, +5dBm, +6dBm, +7dBm and +8dBm.

can i use any of the above transmission power levels for nrf52840 development kit?

venkatesha kj said:

but it was not increased in my case?

It is not guaranteed - it depends on a lot of external factors in the environement.

venkatesha kj said:

can i use any of the above transmission power levels for nrf52840 development kit?

You can use the values specified as valid for the TXPOWER register in the documentation for the SoC you are working with, which I linked earlier.

Best regards,
Karl

venkatesha kj said:

but it was not increased in my case?

It is not guaranteed - it depends on a lot of external factors in the environement.

venkatesha kj said:

can i use any of the above transmission power levels for nrf52840 development kit?

You can use the values specified as valid for the TXPOWER register in the documentation for the SoC you are working with, which I linked earlier.

Best regards,
Karl