diff --git a/src/libboard_artiq/src/cxp_proto.rs b/src/libboard_artiq/src/cxp_proto.rs
index 5289332..64fc357 100644
--- a/src/libboard_artiq/src/cxp_proto.rs
+++ b/src/libboard_artiq/src/cxp_proto.rs
@@ -1,6 +1,6 @@
 use core::slice;
 
-use core_io::{Error as IoError, Write};
+use core_io::{Error as IoError, Read, Write};
 use crc::crc32;
 use io::Cursor;
 use libboard_zynq::println;
@@ -27,6 +27,7 @@ impl From<IoError> for Error {
 }
 
 pub enum Packet {
+    // Upconn packet
     CtrlRead {
         addr: u32,
         length: u8,
@@ -51,9 +52,55 @@ pub enum Packet {
         packet_tag: u8,
     },
     TestPacket,
+
+    // Downconn packet
+    CtrlAck {
+        ackcode: u8,
+        length: u8,
+        data: [u8; DATA_MAXSIZE],
+    },
+    CtrlAckWithTag {
+        tag: u8,
+        ackcode: u8,
+        length: u8,
+        data: [u8; DATA_MAXSIZE],
+    },
+    CtrlAckNoData {
+        ackcode: u8,
+    },
+    CtrlAckNoDataWithTag {
+        tag: u8,
+        ackcode: u8,
+    },
+
+    // DEBUG: Loopback message
+    CtrlAckLoopback {
+        ackcode: u8,
+        length: u8,
+        data: [u8; DATA_MAXSIZE],
+    },
+}
+
+fn read_word(reader: &mut Cursor<&mut [u8]>) -> Result<u32, Error> {
+    let mut bytes = [0; 4];
+    reader.read(&mut bytes)?;
+    Ok(u32::from_be_bytes(bytes))
 }
 
 impl Packet {
+    pub fn read_from(reader: &mut Cursor<&mut [u8]>, packet_type: u8) -> Result<(), Error> {
+        match packet_type {
+            0x03 => {
+                let ackcode4x = read_word(reader);
+                println!("ackcode4x = {:#010X}", ackcode4x.unwrap());
+                let len = read_word(reader);
+                println!("len = {:#010X}", len.unwrap());
+            }
+            0x06 => {}
+            _ => {}
+        }
+        Ok(())
+    }
     pub fn write_to(&self, writer: &mut Cursor<&mut [u8]>) -> Result<(), Error> {
         // CoaXpress use big endian
         match *self {
@@ -90,7 +137,20 @@ impl Packet {
                 writer.write(&[0x08; 4])?;
                 writer.write(&[packet_tag; 4])?;
             }
-            _ => {}
+
+            // DEBUG: Loopback message
+            Packet::CtrlAckLoopback { ackcode, length, data } => {
+                writer.write(&[0x03; 4])?;
+                writer.write(&[ackcode; 4])?;
+
+                if ackcode == 0x00 || ackcode == 0x04 {
+                    writer.write(&[0x00, 0x00, 0x00, length])?;
+                    writer.write(&data[0..length as usize])?;
+                    let checksum = crc32::checksum_ieee(&writer.get_ref()[4..writer.position()]);
+                    writer.write(&(!checksum).to_le_bytes())?;
+                }
+            }
+            _ => return Err(Error::UnknownPacket(0)),
         }
         // Section 9.2.2.2 (CXP-001-2021)
         // Only Control packet need CRC32 appended in the end of the packet
@@ -113,10 +173,13 @@ impl Packet {
 pub fn receive(channel: usize) -> Result<(), Error> {
     unsafe {
         // let ptr = CXP_LOOPBACK_MEM[0].base as *mut u32;
-        let ptr = CXP_RX_MEM[0].base as *mut u32;
-        // let mut reader = Cursor::new(slice::from_raw_parts_mut(ptr as *mut u8, MEM_LEN));
+        let ptr = CXP_RX_MEM[channel].base as *mut u32;
+        let mut reader = Cursor::new(slice::from_raw_parts_mut(ptr as *mut u8, MEM_LEN));
+        let packet_type = (CXP[channel].downconn_packet_type_read)();
 
-        print_packet(slice::from_raw_parts_mut(ptr as *mut u8, MEM_LEN));
+        Packet::read_from(&mut reader, packet_type);
+
+        // print_packet(slice::from_raw_parts_mut(ptr as *mut u8, MEM_LEN));
     }
     Ok(())
 }