diff --git a/user/diagdummy/serial.c b/user/diagdummy/serial.c
index 59980d8..04d68ef 100644
--- a/user/diagdummy/serial.c
+++ b/user/diagdummy/serial.c
@@ -6,6 +6,8 @@
 #define SENDREADY 1
 #define RECVB     2
 #define RECVREADY 3
+#define LOCK      4
+#define UNLOCK    5
 
 void diagdummy_srpr(void) {
   char *str = "Hello world\n";
@@ -35,9 +37,13 @@ void diagdummy_srpr2(void) {
   Dev_t serialdev;
   dev_gethandle(&serialdev, "serialdev");
 
+  while(dev_cmd(&serialdev, LOCK, NULL, 0) != E_OK);
+
   for (;;) {
     while (!dev_cmd(&serialdev, SENDREADY, NULL, 0));
     dev_cmd(&serialdev, SENDB, str, 1);
     schedsleep(500);
   }
+
+  dev_cmd(&serialdev, UNLOCK, NULL, 0);
 }