diff --git a/src/libboard_artiq/src/cxp_upconn.rs b/src/libboard_artiq/src/cxp_upconn.rs
index f1710b5..f343151 100644
--- a/src/libboard_artiq/src/cxp_upconn.rs
+++ b/src/libboard_artiq/src/cxp_upconn.rs
@@ -75,6 +75,7 @@ pub enum Packet {
 impl Packet {
     pub fn write_to<W>(&self, writer: &mut W) -> Result<(), Error>
     where W: Write {
+        // CoaXpress use big endian
         match self {
             Packet::ControlU32Reg(cmd) => match cmd {
                 Command::Read { addr } => {