use constant PACK_TEMPLATE = 'H4 b7 b1 b1 b1 b6 H2 A8 H4';
=comment 
'H4';# 16 bits     Msg ID                          
'b7';# 7 bits      Frag Seq                        
'b'; # 1 bit       More Frags flag                 
'b'; # 1 bit       Unit ID Present                 
'b'; # 1 bit       CRC Present                     
'b6'; # 6 bits Unused
'H2'; # 8 bits     Frag Length  
'A8'; # if present, unit ID is 8 chars
'H4'; # if present, CRC is a 16 bit word                      
=cut