https://devzone.nordicsemi.com/f/nordic-q-a/23475/error-0x3401-on-sending-advertising-packetsHello, I am currently working on project that uses BLE peripheral and BLE central for automatic sensor calibration on custom board. There is an initial connection where both central and peripheral connect and successfully communicate data on notificationen-USTelligent Community 13Mon, 24 Jul 2017 14:33:24 GMThttps://devzone.nordicsemi.com/thread/92212?ContentTypeID=1Mon, 24 Jul 2017 14:33:24 GMT137ad170-7792-4731-bb38-c0d22fbe4515:cbe6e703-daa9-4f3c-8263-66b860949206KasemC<p>Thanks endnode!</p> <p>I have debugged the problem to find that the problem was caused by not enabling the Notifications using the BLE central upon the second connection. This originated from the fact, that during service discovery, the discovered service type doesnt match the pressure service uuid type, thus failing to issue a Discovery Complete event where the notification is enabled.</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/92211?ContentTypeID=1Wed, 12 Jul 2017 23:16:57 GMT137ad170-7792-4731-bb38-c0d22fbe4515:98261664-d944-47d5-8508-fa2b692f1215endnode<p>Should be pretty easy with the proper documentation:</p> <ul> <li>If you are using S130 V2 as per your tag and you get 0x3401 return code when <a href="http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s130.api.v2.0.1/group___b_l_e___g_a_t_t_s___f_u_n_c_t_i_o_n_s.html?cp=3_7_2_1_0_2_4_2_3#ga313fe43c2e93267da668572e885945db">calling <code>sd_ble_gatts_hvx</code> function</a> (or actually any other) then it means <code>BLE_ERROR_GATTS_SYS_ATTR_MISSING</code>.</li> <li>According to further text in Nordic documentation it seems that you haven&#39;t <a href="http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s130.api.v2.0.1%2Fgroup___b_l_e___g_a_t_t_s___f_u_n_c_t_i_o_n_s.html&amp;anchor=ga577c788e199b1a2c0d19602b07ef2515">issued <code>sd_ble_gatts_sys_attr_set</code> function</a> which seems to be mandatory for every connection link where you run GATT Server and it is with previously bonded device.</li> <li>However it might also be caused by not enable Notifications on this particular GATT Server-Client connection (have in mind that these Server-Client things are completely independent so enabling Notify methods through writing to CCCD on one connection handle has no effect on any other). Are you sure that you are treating paired/bonded devices and GATT Server-Client contexts properly?</li> </ul><div style="clear:both;"></div>