diff --git a/bin/mbus-serial-request-data-multi-reply.c b/bin/mbus-serial-request-data-multi-reply.c
index bb9e793..e044c09 100755
--- a/bin/mbus-serial-request-data-multi-reply.c
+++ b/bin/mbus-serial-request-data-multi-reply.c
@@ -103,18 +103,17 @@ main(int argc, char **argv)
     device   = argv[c];
     addr_str = argv[c+1];
 
-
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_serial(device)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-serial-request-data.c b/bin/mbus-serial-request-data.c
index 9b94c83..1152a04 100755
--- a/bin/mbus-serial-request-data.c
+++ b/bin/mbus-serial-request-data.c
@@ -64,17 +64,17 @@ main(int argc, char **argv)
         return 0;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_serial(device)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-serial-scan-secondary.c b/bin/mbus-serial-scan-secondary.c
index 28e1dbe..894ccf2 100755
--- a/bin/mbus-serial-scan-secondary.c
+++ b/bin/mbus-serial-scan-secondary.c
@@ -116,12 +116,6 @@ main(int argc, char **argv)
         return 0;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if (addr_mask == NULL)
     {
         fprintf(stderr, "Failed to allocate address mask.\n");
@@ -141,6 +135,12 @@ main(int argc, char **argv)
         free(addr_mask);
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-serial-scan.c b/bin/mbus-serial-scan.c
index 8a16538..8a935c9 100755
--- a/bin/mbus-serial-scan.c
+++ b/bin/mbus-serial-scan.c
@@ -108,17 +108,17 @@ main(int argc, char **argv)
         return 0;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_serial(device)) == NULL)
     {
         fprintf(stderr,"Scan failed: Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-tcp-application-reset.c b/bin/mbus-tcp-application-reset.c
index 106efab..54ca46c 100755
--- a/bin/mbus-tcp-application-reset.c
+++ b/bin/mbus-tcp-application-reset.c
@@ -68,17 +68,17 @@ main(int argc, char **argv)
         return 1;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_tcp(host, port)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-tcp-raw-send.c b/bin/mbus-tcp-raw-send.c
index fc9ad65..a324378 100644
--- a/bin/mbus-tcp-raw-send.c
+++ b/bin/mbus-tcp-raw-send.c
@@ -79,17 +79,17 @@ main(int argc, char **argv)
         return 1;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_tcp(host, port)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-tcp-request-data-multi-reply.c b/bin/mbus-tcp-request-data-multi-reply.c
index fa38a35..8cc1d2e 100755
--- a/bin/mbus-tcp-request-data-multi-reply.c
+++ b/bin/mbus-tcp-request-data-multi-reply.c
@@ -105,17 +105,17 @@ main(int argc, char **argv)
         return 1;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_tcp(host, port)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-tcp-request-data.c b/bin/mbus-tcp-request-data.c
index c0093f8..91ff0c0 100755
--- a/bin/mbus-tcp-request-data.c
+++ b/bin/mbus-tcp-request-data.c
@@ -59,17 +59,17 @@ main(int argc, char **argv)
         return 1;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_tcp(host, port)) == NULL)
     {
         fprintf(stderr, "Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/bin/mbus-tcp-scan.c b/bin/mbus-tcp-scan.c
index a7ea01e..70ed102 100755
--- a/bin/mbus-tcp-scan.c
+++ b/bin/mbus-tcp-scan.c
@@ -94,17 +94,17 @@ main(int argc, char **argv)
         return 1;
     }
 
-    if (debug)
-    {
-        mbus_register_send_event(&mbus_dump_send_event);
-        mbus_register_recv_event(&mbus_dump_recv_event);
-    }
-
     if ((handle = mbus_context_tcp(host, port)) == NULL)
     {
         fprintf(stderr,"Scan failed: Could not initialize M-Bus context: %s\n",  mbus_error_str());
         return 1;
     }
+    
+    if (debug)
+    {
+        mbus_register_send_event(handle, &mbus_dump_send_event);
+        mbus_register_recv_event(handle, &mbus_dump_recv_event);
+    }
 
     if (mbus_connect(handle) == -1)
     {
diff --git a/mbus/mbus-protocol-aux.c b/mbus/mbus-protocol-aux.c
index 6f96d87..be7312d 100755
--- a/mbus/mbus-protocol-aux.c
+++ b/mbus/mbus-protocol-aux.c
@@ -707,25 +707,40 @@ mbus_variable_vif fixed_table[] = {
     { 0xFFFF, 0.0, "", "" },
 };
 
-void (*_mbus_scan_progress)(mbus_handle * handle, const char *mask) = NULL;
-void (*_mbus_found_event)(mbus_handle * handle, mbus_frame *frame) = NULL;
+//------------------------------------------------------------------------------
+/// Register a function for receive events.
+//------------------------------------------------------------------------------
+void
+mbus_register_recv_event(mbus_handle * handle, void (*event)(unsigned char src_type, const char *buff, size_t len))
+{
+    handle->recv_event = event;
+}
+
+//------------------------------------------------------------------------------
+/// Register a function for send events.
+//------------------------------------------------------------------------------
+void
+mbus_register_send_event(mbus_handle * handle, void (*event)(unsigned char src_type, const char *buff, size_t len))
+{
+    handle->send_event = event;
+}
 
 //------------------------------------------------------------------------------
 /// Register a function for the scan progress.
 //------------------------------------------------------------------------------
 void
-mbus_register_scan_progress(void (*event)(mbus_handle * handle, const char *mask))
+mbus_register_scan_progress(mbus_handle * handle, void (*event)(mbus_handle * handle, const char *mask))
 {
-    _mbus_scan_progress = event;
+    handle->scan_progress = event;
 }
 
 //------------------------------------------------------------------------------
 /// Register a function for the found events.
 //------------------------------------------------------------------------------
 void
-mbus_register_found_event(void (*event)(mbus_handle * handle, mbus_frame *frame))
+mbus_register_found_event(mbus_handle * handle, void (*event)(mbus_handle * handle, mbus_frame *frame))
 {
-    _mbus_found_event = event;
+    handle->found_event = event;
 }
 
 int mbus_fixed_normalize(int medium_unit, long medium_value, char **unit_out, double *value_out, char **quantity_out)
@@ -1424,6 +1439,10 @@ mbus_context_serial(const char *device)
     handle->recv = mbus_serial_recv_frame;
     handle->send = mbus_serial_send_frame;
     handle->free_auxdata = mbus_serial_data_free;
+    handle->recv_event = NULL;
+    handle->send_event = NULL;
+    handle->scan_progress = NULL;
+    handle->found_event = NULL;
 
     if ((serial_data->device = strdup(device)) == NULL)
     {
@@ -1467,6 +1486,10 @@ mbus_context_tcp(const char *host, uint16_t port)
     handle->recv = mbus_tcp_recv_frame;
     handle->send = mbus_tcp_send_frame;
     handle->free_auxdata = mbus_tcp_data_free;
+    handle->recv_event = NULL;
+    handle->send_event = NULL;
+    handle->scan_progress = NULL;
+    handle->found_event = NULL;
 
     tcp_data->port = port;
     if ((tcp_data->host = strdup(host)) == NULL)
@@ -1811,6 +1834,57 @@ mbus_send_request_frame(mbus_handle * handle, int address)
     return retval;
 }
 
+//------------------------------------------------------------------------------
+// send a user data packet from master to slave
+//------------------------------------------------------------------------------
+int
+mbus_send_user_data_frame(mbus_handle * handle, int address, const unsigned char *data, size_t data_size)
+{
+    int retval = 0;
+    mbus_frame *frame;
+
+    if (mbus_is_primary_address(address) == 0)
+    {
+        MBUS_ERROR("%s: invalid address %d\n", __PRETTY_FUNCTION__, address);
+        return -1;
+    }
+
+    if (data == NULL)
+    {
+        MBUS_ERROR("%s: Invalid data\n", __PRETTY_FUNCTION__);
+        return -1;
+    }
+
+    if ((data_size > MBUS_FRAME_DATA_LENGTH) || (data_size == 0))
+    {
+        MBUS_ERROR("%s: illegal data_size %d\n", __PRETTY_FUNCTION__, data_size);
+        return -1;
+    }
+
+    frame = mbus_frame_new(MBUS_FRAME_TYPE_LONG);
+
+    if (frame == NULL)
+    {
+        MBUS_ERROR("%s: failed to allocate mbus frame.\n", __PRETTY_FUNCTION__);
+        return -1;
+    }
+
+    frame->control = MBUS_CONTROL_MASK_SND_UD | MBUS_CONTROL_MASK_DIR_M2S;
+    frame->address = address;
+    frame->control_information = MBUS_CONTROL_INFO_DATA_SEND;
+    frame->data_size = data_size;
+    memcpy(frame->data, data, data_size);
+
+    if (mbus_send_frame(handle, frame) == -1)
+    {
+        MBUS_ERROR("%s: failed to send mbus frame.\n", __PRETTY_FUNCTION__);
+        retval = -1;
+    }
+
+    mbus_frame_free(frame);
+    return retval;
+}
+
 //------------------------------------------------------------------------------
 // send a request from master to slave and collect the reply (replies)
 // from the slave.
@@ -2138,9 +2212,9 @@ mbus_probe_secondary_address(mbus_handle *handle, const char *mask, char *matchi
 
             snprintf(matching_addr, 17, "%s", addr);
 
-            if (_mbus_found_event)
+            if (handle->found_event)
             {
-                _mbus_found_event(handle,&reply);
+                handle->found_event(handle,&reply);
             }
 
             return MBUS_PROBE_SINGLE;
@@ -2284,14 +2358,14 @@ mbus_scan_2nd_address_range(mbus_handle * handle, int pos, char *addr_mask)
     {
         mask[pos] = '0'+i;
 
-        if (_mbus_scan_progress)
-            _mbus_scan_progress(handle,mask);
+        if (handle->scan_progress)
+            handle->scan_progress(handle,mask);
 
         probe_ret = mbus_probe_secondary_address(handle, mask, matching_mask);
 
         if (probe_ret == MBUS_PROBE_SINGLE)
         {
-            if (!_mbus_found_event)
+            if (!handle->found_event)
             {
                 printf("Found a device on secondary address %s [using address mask %s]\n", matching_mask, mask);
             }
diff --git a/mbus/mbus-protocol-aux.h b/mbus/mbus-protocol-aux.h
index c2a255a..2cb61ec 100755
--- a/mbus/mbus-protocol-aux.h
+++ b/mbus/mbus-protocol-aux.h
@@ -93,6 +93,10 @@ typedef struct _mbus_handle {
     int (*send) (struct _mbus_handle *handle, mbus_frame *frame);
     int (*recv) (struct _mbus_handle *handle, mbus_frame *frame);
     void (*free_auxdata) (struct _mbus_handle *handle);
+    void (*recv_event) (unsigned char src_type, const char *buff, size_t len);
+    void (*send_event) (unsigned char src_type, const char *buff, size_t len);
+    void (*scan_progress) (struct _mbus_handle *handle, const char *mask);
+    void (*found_event) (struct _mbus_handle *handle, mbus_frame *frame);    
     void *auxdata;
 } mbus_handle;
 
@@ -148,17 +152,16 @@ typedef enum _mbus_context_option {
     MBUS_OPTION_PURGE_FIRST_FRAME  /**< option controls the echo cancelation for mbus_recv_frame */
 } mbus_context_option;
 
-/**
- * Event callback functions
- */
-extern void (*_mbus_scan_progress)(mbus_handle * handle, const char *mask);
-extern void (*_mbus_found_event)(mbus_handle * handle, mbus_frame *frame);
-
 /**
  * Event register functions
  */
-void mbus_register_scan_progress(void (*event)(mbus_handle * handle, const char *mask));
-void mbus_register_found_event(void (*event)(mbus_handle * handle, mbus_frame *frame));
+//
+// Event register functions
+//
+void mbus_register_recv_event(mbus_handle *handle, void (*event)(unsigned char src_type, const char *buff, size_t len));
+void mbus_register_send_event(mbus_handle *handle, void (*event)(unsigned char src_type, const char *buff, size_t len));
+void mbus_register_scan_progress(mbus_handle *handle, void (*event)(mbus_handle *handle, const char *mask));
+void mbus_register_found_event(mbus_handle *handle, void (*event)(mbus_handle *handle, mbus_frame *frame));
 
 /**
  * Allocate and initialize M-Bus serial context.
@@ -287,6 +290,18 @@ int mbus_send_switch_baudrate_frame(mbus_handle * handle, int address, long baud
  */
 int mbus_send_request_frame(mbus_handle * handle, int address);
 
+/**
+ * Sends user data frame (SND_UD) to given slave using "unified" handle
+ *
+ * @param handle    Initialized handle
+ * @param address   Address (0-255)
+ * @param data      User data
+ * @param data_size Byte count of user data
+ *
+ * @return Zero when successful.
+ */
+int mbus_send_user_data_frame(mbus_handle * handle, int address, const unsigned char *data, size_t data_size);
+
 /**
  * Sends a request and read replies until no more records available
  * or limit is reached.
diff --git a/mbus/mbus-protocol.c b/mbus/mbus-protocol.c
index ab85a0f..8ec3130 100755
--- a/mbus/mbus-protocol.c
+++ b/mbus/mbus-protocol.c
@@ -26,12 +26,6 @@ static char error_str[512];
 //------------------------------------------------------------------------------
 static mbus_slave_data slave_data[MBUS_MAX_PRIMARY_SLAVES];
 
-//
-// init event callback
-//
-void (*_mbus_recv_event)(unsigned char src_type, const char *buff, size_t len) = NULL;
-void (*_mbus_send_event)(unsigned char src_type, const char *buff, size_t len) = NULL;
-
 //
 //  trace callbacks
 //
@@ -47,24 +41,6 @@ mbus_dump_send_event(unsigned char src_type, const char *buff, size_t len)
     mbus_hex_dump("SEND", buff, len);
 }
 
-//------------------------------------------------------------------------------
-/// Register a function for receive events.
-//------------------------------------------------------------------------------
-void
-mbus_register_recv_event(void (*event)(unsigned char src_type, const char *buff, size_t len))
-{
-    _mbus_recv_event = event;
-}
-
-//------------------------------------------------------------------------------
-/// Register a function for send events.
-//------------------------------------------------------------------------------
-void
-mbus_register_send_event(void (*event)(unsigned char src_type, const char *buff, size_t len))
-{
-    _mbus_send_event = event;
-}
-
 //------------------------------------------------------------------------------
 /// Return a string that contains an the latest error message.
 //------------------------------------------------------------------------------
diff --git a/mbus/mbus-protocol.h b/mbus/mbus-protocol.h
index cbc5c7b..8c8edde 100755
--- a/mbus/mbus-protocol.h
+++ b/mbus/mbus-protocol.h
@@ -70,6 +70,8 @@ extern "C" {
 //
 //
 
+#define MBUS_FRAME_DATA_LENGTH 252
+
 typedef struct _mbus_frame {
 
     unsigned char start1;
@@ -83,7 +85,7 @@ typedef struct _mbus_frame {
     unsigned char checksum;
     unsigned char stop;
 
-    unsigned char   data[252];
+    unsigned char   data[MBUS_FRAME_DATA_LENGTH];
     size_t data_size;
 
     int type;
@@ -511,18 +513,9 @@ typedef struct _mbus_data_secondary_address {
 //
 // Event callback functions
 //
-extern void (*_mbus_recv_event)(unsigned char src_type, const char *buff, size_t len);
-extern void (*_mbus_send_event)(unsigned char src_type, const char *buff, size_t len);
-
 void mbus_dump_recv_event(unsigned char src_type, const char *buff, size_t len);
 void mbus_dump_send_event(unsigned char src_type, const char *buff, size_t len);
 
-//
-// Event register functions
-//
-void mbus_register_recv_event(void (*event)(unsigned char src_type, const char *buff, size_t len));
-void mbus_register_send_event(void (*event)(unsigned char src_type, const char *buff, size_t len));
-
 //
 // variable length records
 //
diff --git a/mbus/mbus-serial.c b/mbus/mbus-serial.c
index d120f8b..8949d02 100755
--- a/mbus/mbus-serial.c
+++ b/mbus/mbus-serial.c
@@ -255,8 +255,8 @@ mbus_serial_send_frame(mbus_handle *handle, mbus_frame *frame)
         //
         // call the send event function, if the callback function is registered
         //
-        if (_mbus_send_event)
-                _mbus_send_event(MBUS_HANDLE_TYPE_SERIAL, buff, len);
+        if (handle->send_event)
+                handle->send_event(MBUS_HANDLE_TYPE_SERIAL, buff, len);
     }
     else
     {
@@ -353,8 +353,8 @@ mbus_serial_recv_frame(mbus_handle *handle, mbus_frame *frame)
     //
     // call the receive event function, if the callback function is registered
     //
-    if (_mbus_recv_event)
-        _mbus_recv_event(MBUS_HANDLE_TYPE_SERIAL, buff, len);
+    if (handle->recv_event)
+        handle->recv_event(MBUS_HANDLE_TYPE_SERIAL, buff, len);
 
     if (remaining != 0)
     {
diff --git a/mbus/mbus-tcp.c b/mbus/mbus-tcp.c
index ebf0c65..81043db 100755
--- a/mbus/mbus-tcp.c
+++ b/mbus/mbus-tcp.c
@@ -159,8 +159,8 @@ mbus_tcp_send_frame(mbus_handle *handle, mbus_frame *frame)
         //
         // call the send event function, if the callback function is registered
         //
-        if (_mbus_send_event)
-            _mbus_send_event(MBUS_HANDLE_TYPE_TCP, buff, len);
+        if (handle->send_event)
+            handle->send_event(MBUS_HANDLE_TYPE_TCP, buff, len);
     }
     else
     {
@@ -232,8 +232,8 @@ retry:
     //
     // call the receive event function, if the callback function is registered
     //
-    if (_mbus_recv_event)
-        _mbus_recv_event(MBUS_HANDLE_TYPE_TCP, buff, len);
+    if (handle->recv_event)
+        handle->recv_event(MBUS_HANDLE_TYPE_TCP, buff, len);
 
     if (remaining < 0) {
         mbus_error_str_set("M-Bus layer failed to parse data.");