diff --git a/src/libboard_artiq/src/cxp_downconn.rs b/src/libboard_artiq/src/cxp_downconn.rs
index 3d6af4f..7b982ad 100644
--- a/src/libboard_artiq/src/cxp_downconn.rs
+++ b/src/libboard_artiq/src/cxp_downconn.rs
@@ -43,16 +43,22 @@ pub fn loopback_testing(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEE
 
         cxp_proto::downconn_debug_send_trig_ack(channel);
 
+        const MAX_PACKET: usize = 128;
+        const DATA_MAXSIZE: usize = /*max size*/MAX_PACKET - /*Tag*/4 - /*Op code & length*/4 - /*addr*/4 - /*CRC*/4 ;
+        let data: u32 = 0x11;
+        let mut data_slice: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
+        data_slice[..4].clone_from_slice(&data.to_be_bytes());
         cxp_proto::downconn_debug_send(
             channel,
-            &cxp_proto::Packet::CtrlRead {
-                addr: 0x00,
+            &cxp_proto::Packet::CtrlAckLoopback {
+                ackcode: 0x00,
                 length: 0x04,
+                data: data_slice,
             },
         )
         .expect("loopback gtx tx error");
 
-        timer.delay_us(200); // wait packet has arrive at RX async fifo
+        timer.delay_us(1000); // wait packet has arrive at RX async fifo
         (CXP[channel].downconn_tx_stb_write)(0);
 
         info!("trig ack = {}", (CXP[channel].downconn_trigger_ack_read)());
@@ -63,7 +69,7 @@ pub fn loopback_testing(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEE
         info!("test error = {}", (CXP[channel].downconn_test_error_read)());
         info!("packet type = {:#06X}", (CXP[channel].downconn_packet_type_read)());
 
-        cxp_proto::receive(channel);
+        cxp_proto::receive(channel).expect("loopback gtx rx error");
         // DEBUG: print loopback packets
         const LEN: usize = 20;
         let mut pak_arr: [u32; LEN] = [0; LEN];