https://devzone.nordicsemi.com/f/nordic-q-a/18532/nrf51822-gzll-in-device-first-ack-not-recive-need-second-sendhi,In the host, if(nrf_gzll_get_tx_fifo_packet_count(pipe) == 0 ) { nrf_gzll_add_packet_to_tx_fifo(...); } In the devcice, every 5 min send packets to host ,but ack always lost.en-USTelligent Community 13Thu, 22 Dec 2016 10:15:09 GMThttps://devzone.nordicsemi.com/thread/71535?ContentTypeID=1Thu, 22 Dec 2016 10:15:09 GMT137ad170-7792-4731-bb38-c0d22fbe4515:252e7fb3-6c56-41b9-9715-74d69fc0e18cHung Bui<p>I would suggest to test with shorter interval than 5minute. As you can see in the <a href="http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.sdk5.v12.2.0/gzll_02_user_guide.html?cp=4_0_1_5_0">doc</a> at &quot;Example on failing package transaction&quot;, depends on what you configured using nrf_gzll_set_max_tx_attempts() the gazell stack will try to retransmit until it reach the max attempt, and then throw you tx_failed.</p> <p>Please try to test with the gzll_ack_payload example in the SDK.</p><div style="clear:both;"></div>