ESB EVENT_RX_RECEIVED on ACK

Question

Hello, 
 I am trying to establish a bi-directional communication between two nRF52840-DK through ESB protocol. As a first step I am using the ESB example available in the SDK 15.3.0, setting one unit as TX and the other as RX. I expected to be able to catch the ACK message in the event handler of the TX (and get the ACK payload), but debugging the code with a breakpoint in the case NRF_ESBEVENT_RX_RECEIVED, I see that that part of code is never reached. However, TX messages are correct and the case NRF_ESB_EVENT_TX_SUCCESS in the TX side is regularly reached. 
 Reading all the ESB documentation I understood that the configuration of the example enables the automatic ACK, so bi-directional communication should be active. Can you please tell me where I am wrong? 
 Thank you and best regards, 
 Enrico

Joakim Jakobsen · Answer

Hi. 
 Just to clarify - you won't get the NRF_ESB_EVENT_RX_RECEIVED event on the TX-device when the message is sent and ACK'ed. The TX-device will send the message and then listen for an ACK from the RX-device. In the native examples the ACK sent from the RX-device is empty, so when the ACK is received you will get the NRF_ESB_EVENT_TX_SUCCESS event on the TX-device. 
 If you add a payload to the ACK, you should first receive the NRF_ESB_EVENT_TX_SUCCESS, then the NRF_ESB_EVENT_RX_RECEIVED. 
 Best regards, Joakim