diff -uNr rt_com-0.5.4/Makefile patched/Makefile
--- rt_com-0.5.4/Makefile	Mon Dec  4 04:36:24 2000
+++ patched/Makefile	Tue May  8 15:56:36 2001
@@ -16,7 +16,7 @@
 
 OBJS			= rt_com.o # rt_com_posix.o currently broken ;-(
 
-VERSION			= 0.5.4
+VERSION			= 0.5.5pre1
 
 
 CFLAGS			+= -D$(SYSTEM) -DVERSION=\"$(VERSION)\"
diff -uNr rt_com-0.5.4/rt_com.c patched/rt_com.c
--- rt_com-0.5.4/rt_com.c	Wed Nov 15 14:48:50 2000
+++ patched/rt_com.c	Tue May  8 16:12:39 2001
@@ -86,9 +86,12 @@
 
 /** Remaining free space of buffer
  *
- * @return amount of free space remaining in a buffer (input or output) */
+ * @return amount of free space remaining in a buffer (input or output)
+ *
+ * Note that, although the buffer is RT_COM_BUF_SIZ bytes long, it can
+ * only hold RT_COM_BUF_SIZ - 1 bytes. */
 #define rt_com_buff_free( head, tail ) \
-        ( RT_COM_BUF_SIZ - ( ( head - tail ) & ( RT_COM_BUF_SIZ - 1 ) ) )
+        ( RT_COM_BUF_SIZ - 1 - ( ( head - tail ) & ( RT_COM_BUF_SIZ - 1 ) ) )
 
 
 
@@ -470,6 +473,7 @@
     int buff,t;
     char loop = 4;
     char toFifo = 16; 
+    int rxd_bytes = 0;
  
     do {
 	/* get available data from port */
@@ -479,6 +483,7 @@
 	while( RT_COM_DATA_READY & sta ) {
 	    data = inb( B + RT_COM_RXB );
 	    rt_com_irq_put( p, data );
+	    rxd_bytes = 1;
 	    sta = inb( B + RT_COM_LSR );
 	    if( 0x1e & sta )
 		p->error = sta & 0x1e;
@@ -515,6 +520,12 @@
       /* Note: why is it done at most 4 times ? */ 
     } while( 1 != ( ( inb( B + RT_COM_IIR ) & 0x0f ) ) && ( --loop > 0 ) );
 
+    if (rxd_bytes) {
+	/* We received 1+ bytes. If user requested a callback, invoke it. */
+	if (p->callback)
+	    (*(p->callback)) (com);
+    }
+
 #if defined RTLINUX_V1 || defined RTAI
     return;
 #else
@@ -568,6 +579,7 @@
 	inb( base + RT_COM_MSR );
 
 	p->error = 0; //!! init. last error code
+	p->callback = 0;
            
 	if( 0 == baud ) {
 	    /* return */
@@ -596,6 +608,38 @@
 }
 
 
+/** Optional callback function.
+ *
+ * If the user supplies a callback function, it is called whenever
+ * characters are received.
+ *
+ * *** NOTE *** The callback function is called in the context of the
+ * interrupt handler!
+ *
+ * @param com	Port to use; corresponding to internal numbering scheme.
+ * @param fn	Callback function (0 = de-register callback function).
+ * @return	Pointer to previously-installed callback function.
+ *
+ * @author James Puttick
+ * @version 2000/02/08 */
+IRQ_CALLBACK_FN rt_com_set_callback (unsigned int com, IRQ_CALLBACK_FN fn)
+{
+    struct rt_com_struct *p;
+    IRQ_CALLBACK_FN
+	old_fn;
+
+    if( com >= RT_COM_CNT )
+	return( 0 );
+
+    p = &( rt_com_table[ com ] );
+
+    old_fn = p->callback;
+    p->callback = fn;
+
+    return( old_fn );
+}
+
+
 /** Port and irq setting for a specified COM.
  *
  * This must run in standard Linux context !
@@ -619,21 +663,22 @@
 		p->port = port;
 	    if( 0 != irq )
 		p->irq = irq;
-	    p->used = 1;
 	    if( -EBUSY == check_region( p->port, 8 ) ) {
 		return( -EBUSY );
 	    }
 	    request_region( p->port, 8, "rt_com" );
 	    rt_com_request_irq( p->irq, p->isr );
 	    rt_com_setup( com, 0, 0, 0, 0 );
+	    p->used = 1;
 	} else {
 	    if( 0 > port ) {
 		rt_com_setup( com, 0, 0, 0, 0 );
 		rt_com_free_irq( p->irq );
 		release_region( p->port, 8 );
 		p->used = 0;
-	    } else
+	    } else {
 		return( -EBUSY );
+	    }
 	}
     }
     return( 0 );
@@ -676,7 +721,7 @@
 	for( j=0; j<i; j++ ) {
 	    p = &( rt_com_table[ j ] );
 	    if( 0 < p->used ) {
-		free_RTirq( p->irq );
+		rt_com_free_irq( p->irq );
 		release_region( p->port, 8 );
 	    }   
 	}
diff -uNr rt_com-0.5.4/rt_com.h patched/rt_com.h
--- rt_com-0.5.4/rt_com.h	Thu May  4 19:11:57 2000
+++ patched/rt_com.h	Tue May  8 15:34:42 2001
@@ -29,6 +29,8 @@
 #define RT_COM_H
 
 
+typedef void (*IRQ_CALLBACK_FN) (unsigned int com_port);
+
 
 extern void cleanup_module( void );
 extern int  init_module( void );
@@ -45,6 +47,8 @@
 extern int  rt_com_read( unsigned int, char *, int );
 /* set up port */
 extern int  rt_com_setup( unsigned int, int, unsigned int, unsigned int, unsigned int );
+/* set call-back function for when data rx'd */
+extern IRQ_CALLBACK_FN rt_com_set_callback (unsigned int com, IRQ_CALLBACK_FN fn);
 /* port & irq setting for a specified com */
 extern int  rt_com_set_param( unsigned int com, int port, int irq );
 /* set functioning mode */
diff -uNr rt_com-0.5.4/rt_comP.h patched/rt_comP.h
--- rt_com-0.5.4/rt_comP.h	Tue May  2 19:14:11 2000
+++ patched/rt_comP.h	Tue May  8 15:33:50 2001
@@ -129,6 +129,7 @@
     int type;
     int ier;  /* copy of chip register */
     int mcr;  /* copy of the MCR internal register */
+    IRQ_CALLBACK_FN callback;	/* called when chars rx'd */
     struct rt_buf_struct ibuf;
     struct rt_buf_struct obuf;
 };