DATA FLOW INSIDE THE HP82161A DRIVE

    LOOP                                          TAPE
     :                                             : 
     :--->DDL5 (FORMAT)---------------->:-HEX 255->: 
     :                                             : 
     :--->DDL8 (Close Sector) --------->:--------->: 
     :--->DDL0-->:                      :--DDL0--->: 
     :--->DDL2-->: Buffer 0 (256 bytes) :--DDL2--->: 
     :           : ____________________ :          : 
     :<---DDT0<--:-____________________-:<-DDTO<---: 
     :<---DDT2<--:     ^        |    ^  :<-DDT2<---: 
     :                 |        |    | 
     :                 |--.   DDL9  DDL10
     :                 |  |    or    or
     :                 |  |   DDT5  DDT4
     :--->DDL1-->:     v  |     v    v 
     :           : ____________________
     :<---DDT1<--:-____________________: 
     :              Buffer 1 (256 bytes)
     :                    | 
     :                    | 
     :--->DDL3----------->| 
     :--->DDL4----------. | 
     :                  ___
     :<---DDT3<-------:-___:Position Pointer (3 bytes)
     :                  ||'--Byte#3:  Byte pntr:  (0-255)
     :                  |'---Byte#2:  Sector pntr.  (0-255)
     :                  '----Byte#1:  Track pntr.  (0 or 1)
     :  NOTE:  For other HP-IL drives byte#1 may be > than 1
     :  All drives pointers work as above, even if the
     :  physical # of tracks or sectors/track is different. 
     : 
     : 
     :         Drive Status & Identification
     : 
     :<---SST (Send Status Byte).  See pg.11 Owner's Manual
     :         Used by INSTAT function
     : 
     :<---SAI (Send Accessory Id byte) Byte value is 16. 
     :          This is the ONLY way to identify the drive
     :          Used by AID, FINDAID. 
     : 
     :           Other Drives: 
     :<---SDI (Send Device ID)  Example:  HP9114A
     :           Used by ID, FINDID. 
     : 
     :<--DDT7  Maximum Sector (End of Medium)
     : 
     :<--DDT6  Send Physical Attributes


         NOTE:The  drive  also responds to several  other  HP-IL
    messages.   These  messages can be used to clear the  drive,
    put it in a low power consumption  mode,  and  more.   These
    messages mostly concern  basic  features common to all HP-IL
    devices.  These  features  are  described in great detail in
    "THE HP-IL SYSTEM:  AN INTRODUCTORY GUIDE"  by  Gerry  Kane,
    Steve Harper & David Ushijima (Osborne/McGraw-Hill)