+In general, the <tt>failback</tt> handler is invoked when serious
+unrecoverable errors occur when communicating with the target.
+In this case the <sf/IrTcl/ system shuts down the connection.
+The <tt>failback</tt> handler might inspect the <tt>failInfo</tt>
+setting to determine the cause of the failure; it returns
+two elements. The first is an error integer; the second is an
+english representation of the error. The error codes and
+the corresponding messages are:
+
+<descrip>
+<tag><tt>0</tt></tag>ok
+<tag><tt>1</tt></tag>connect failed
+<tag><tt>2</tt></tag>connection closed
+<tag><tt>3</tt></tag>connection closed
+<tag><tt>4</tt></tag>failed to decode incoming APDU
+<tag><tt>5</tt></tag>unknown APDU
+</descrip>
+
+Note: in case 3 the connection was closed during read a read operation
+whereas in case 4 it was closed during a write operation.
+