diff --git a/src/libboard_artiq/src/cxp_downconn.rs b/src/libboard_artiq/src/cxp_downconn.rs
index c6c165a..2ae44b8 100644
--- a/src/libboard_artiq/src/cxp_downconn.rs
+++ b/src/libboard_artiq/src/cxp_downconn.rs
@@ -43,24 +43,26 @@ pub fn loopback_testing(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEE
 
         cxp_proto::downconn_debug_send_trig_ack(channel);
 
-        const SEND_TIMES: usize = 4;
-        const DATA_MAXSIZE: usize = 48;
-        let mut data_slice: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
+        const DATA_MAXSIZE: usize = 253;
+        let data_size = 4; // no. of bytes
 
-        for i in 0..SEND_TIMES {
-            let data: u32 = i as u32;
-            data_slice[..4].clone_from_slice(&data.to_be_bytes());
-            cxp_proto::downconn_debug_send(
-                channel,
-                &cxp_proto::UpConnPacket::CtrlAckLoopback {
-                    ackcode: 0x00,
-                    length: 0x04,
-                    data: data_slice,
-                },
-            )
-            .expect("loopback gtx tx error");
-            timer.delay_us(10);
-        }
+        let data: u32 = 0xDADA as u32;
+        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::UpConnPacket::Event {
+                conn_id: 0x1234_5678_u32,
+                packet_tag: 0x69_u8,
+                length: data_size + 3,
+                event_size: data_size,
+                namespace: 0x02_u8,
+                event_id: 0x00_6969u16,
+                timestamp: 0x1234_5678u64,
+                data: data_slice,
+            },
+        )
+        .expect("loopback gtx tx error");
 
         timer.delay_us(1000); // wait packet has arrive at RX async fifo
         (CXP[channel].downconn_tx_stb_write)(0);
@@ -73,9 +75,7 @@ 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)());
 
-        for _ in 0..SEND_TIMES {
-            cxp_proto::receive(channel).expect("loopback gtx rx error");
-        }
+        cxp_proto::receive(channel).expect("loopback gtx rx error");
         // cxp_proto::downconn_debug_mem_print(channel);
 
         // DEBUG: print loopback packets