diff --git a/src/libboard_artiq/src/cxp_downconn.rs b/src/libboard_artiq/src/cxp_downconn.rs
index da16fd0..c6c165a 100644
--- a/src/libboard_artiq/src/cxp_downconn.rs
+++ b/src/libboard_artiq/src/cxp_downconn.rs
@@ -44,8 +44,7 @@ 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 MAX_PACKET: usize = 128;
-        const DATA_MAXSIZE: usize = /*max size*/MAX_PACKET - /*Tag*/4 - /*Op code & length*/4 - /*addr*/4 - /*CRC*/4 ;
+        const DATA_MAXSIZE: usize = 48;
         let mut data_slice: [u8; DATA_MAXSIZE] = [0; DATA_MAXSIZE];
 
         for i in 0..SEND_TIMES {
@@ -60,6 +59,7 @@ pub fn loopback_testing(channel: usize, timer: &mut GlobalTimer, speed: CXP_SPEE
                 },
             )
             .expect("loopback gtx tx error");
+            timer.delay_us(10);
         }
 
         timer.delay_us(1000); // wait packet has arrive at RX async fifo