IN-105










                   Setting Densities for Magnetic Tapes in RSX-11M
                   _______ _________ ___ ________ _____ __ _______


                                    David M. Berg
                                   Mark Pyatetsky

                                    January 1984





               This  document  discusses  the   setting   of   magtape
               densities  for  the  various  tape systems supported by
               RSX-11M.































          Keywords:  RSX-11M, PDP-11

          Setting Densities for Magnetic Tapes in RSX-11M            Page 2


          This document discusses the  setting  of  magtape  densities  for
          various  tape systems as supported by RSX-11M Version 4.1.  There
          are four magtape systems in general use at Fermilab.  These are:

               1.  TU10 transport with  TM11  controller,  referred  to  as
                   TU10.
                   ____

               2.  TJU16 transport and controller, referred to as TU16.
                                                                  ____

               3.  STC transport and controller, referred to as STC.
                                                                ___

               4.  STC transport with Aviv coupler, referred to as Aviv.
                                                                   ____


          The following table shows  the  appropriate  RSX  device  driver,
          available densities, and default density for each system:

                         |   TU10   |   TU16   |   STC    |   Aviv   |
               ----------+----------+----------+----------+----------+
               device    |   MT:    |   MM:    |   MJ:    |   MT:    |
               driver    |          |          |          |          |
               ----------+----------+----------+----------+----------+
               available |   800    | 800 or   | 1600 or  | 1600 or  |
               density   |   bpi    | 1600 bpi | 6250 bpi | 6250 bpi |
               ----------+----------+----------+----------+----------+
               default   |   800    |   800    |   1600   |   6250   |
               density   |   bpi    |   bpi    |   bpi    |   bpi    |
               ----------+----------+----------+----------+----------+


          The default density applies after power-up, hardware or  software
          re-boot,  or  whenever  an RSX command or utility is used without
          specifying a density.  The commands and utilities which refer  to
          magtapes all set the density to the default when no other density
          is specified.

          All RSX commands and utilities support the TU10 and TU16, with  a
          density  switch  for  the latter, e.g., specify /DENS=1600 to get
          1600 bpi.  The STC is also supported with a density switch, but a
          translation must be made:  specify /DENS=800 to get 1600 bpi, and
          /DENS=1600 to get  6250  bpi.   The  Aviv  is  supported  by  RSX
          commands and utilities at 6250 bpi only.

          When writing magtape, the density must be set before the data are
          written.   For  the  TU16, the density must also be set correctly
          before reading.  But for the STC or Aviv, the drive automatically
          senses  the  density when the tape is read.  Thus, for example, a
          1600 bpi tape may be read with the Aviv, although the  system  is
          unaware that the density is not the default.

          The RSX commands and utilities referred  to  above  include  INI,
          MOU,  BRU,  DSC,  and  FLX;  for all of these, the magtape volume
          must not be mounted when the command  is  entered.   Because  the
          utility  PIP can access mounted volumes only, the magtape density
          must be specified by the MOU command.

          Setting Densities for Magnetic Tapes in RSX-11M            Page 3


          User programs can likewise access volumes either mounted  or  not
          mounted.   Standard  Fortran I/O, for instance, requires that the
          magtape  volume  be  mounted.   Otherwise,  the  tape  drive   is
          considered  a  unit-record device and is accessible only with QIO
          directives;  the Data Acquisition  System  uses  this  method  to
          write tape.

          A new command, DENS, is available for systems distributed by  the
          Fermilab  Computing  Dept.  DENS should be used only with the STC
                                      ____ ______ __ ____ ____ ____ ___ ___
          or Aviv, and only when the volume is not mounted.  If it is  used
          __ _____ ___ ____ ____ ___ ______ __ ___ ________
          with mounted volumes, the results are unpredictable.

          To set the density of a device, enter

               >DENS ddnn: dens

          where "ddnn:" is the physical device name and  unit  number,  and
          "dens" specifies the density;  the space between the arguments is
          optional.  The device name must be MT, MM, or  MJ;   the  density
          must  be  either 1600 or 6250.  Specify the appropriate number to
          get the corresponding density, e.g., specify  6250  to  get  6250
          bpi.

          If the density argument is omitted, the DENS  command  types  the
          current  value  of the density for the specified device.  It also
          senses whether a loaded tape is positioned at load  point  (BOT),
          and types a message if it is.  For example,

               >DENS MT:
               DENS = 6250
               TAPE IS AT (BOT)


          It is possible to use the MM: driver with the STC in place of the
          MJ:  driver.  When this is done, however, the density is fixed at
          1600 bpi:   neither  the  density  switch  on  RSX  commands  and
          utilities  nor  the  DENS  command  have  any  real effect on the
          density even though they may appear to be working normally.