diff --git a/src/main.rs b/src/main.rs
index 417a407..f878ac6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -66,7 +66,7 @@ fn mopp(speed: u8, data: &[u8]) -> Vec<u8> {
         (b'\'', ".----."),
     ]);
 
-    let mut m = "01".to_owned();
+    let mut m = "01".to_owned(); // protocol version
     m.push_str(&format!("{:06b}", serial));
     m.push_str(&format!("{:06b}", speed));