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                        TOPS-20 Beware File


                           January 1981


                          VERSION 4(3345)


         Release 4  (Including  RP20  driver and  associated
	 modifications)





COPYRIGHT (C) 1976,1977,1978,1979,1980,1981 BY DIGITAL  EQUIPMENT
CORPORATION, MAYNARD, MASS.


THIS SOFTWARE IS FURNISHED UNDER A LICENSE AND MAY  BE  USED
AND COPIED ONLY IN ACCORDANCE WITH THE TERMS OF SUCH LICENSE
AND WITH THE INCLUSION OF THE ABOVE COPYRIGHT NOTICE.   THIS
SOFTWARE  OR ANY OTHER COPIES THEREOF MAY NOT BE PROVIDED OR
OTHERWISE MADE AVAILABLE TO ANY OTHER PERSON.  NO  TITLE  TO
AND OWNERSHIP OF THE SOFTWARE IS HEREBY TRANSFERRED.

THE INFORMATION  IN  THIS  SOFTWARE  IS  SUBJECT  TO  CHANGE
WITHOUT  NOTICE  AND SHOULD NOT BE CONSTRUED AS A COMMITMENT
BY DIGITAL EQUIPMENT CORPORATION.

DIGITAL ASSUMES NO RESPONSIBILITY FOR THE USE OR RELIABILITY
OF  ITS  SOFTWARE  ON  EQUIPMENT  WHICH  IS  NOT SUPPLIED BY
DIGITAL.

                                                      Page 2





1.0 INTRODUCTION

1.1 Function

This document supplements the TOPS-20  manuals  by  pointing
out  hints  and  potential  problems  discovered at the last
minute.



2.0 Hardware

2.1 RP20 8000 controller

If the attention button on a drive is pressed while there is
a  transfer  in  progress, the RP20 controller will hang and
the DX20 will respond to any command with  a  select  error.
This  is a problem in the RP20 controller microcode which we
are trying to get fixed.  In the  meantime,  NEVER  hit  the
attention  button  under  any circumstances.  Spin the drive
down and back up instead.  The way to  tell  that  the  RP20
controller  is  in this state is to look at the state of the
CHK 1 and CHK 2 lights inside the back  door.   If  both  of
these  lights are on, the controller is hung.  Reloading the
DX20 microcode seems to clear up this condition.

At times you may notice SYSERR reports that  indicate  error
class  1  (Unusual  device  status)  with  unit check in the
ending status byte but the sense bytes contain  all  zeroes.
This  combination  can  happen  legally and is not cause for
searching for a software error.

                                                      Page 3



3.0  FLOPPIES

3.1  Floppies A & B

To make room for the larger BOOT and MTBOOT files on these
floppies, some reorganization and shifting of files has been
done.  The MAP files have been shifted to FLOPPY-C.  Note
that floppies A & B still contain all files necessary to 
build a working front-end file system as described in the
TOPS-20 Installation Guide.

3.2  Floppy-C

This floppy now contains the MAP files for RSX20F.  These
files are not critical to installing a new system and may
be installed at any time after a working front-end file
system has been built.  The MAP files may be installed as follows:

This procedure assumes:

     1.  You have a running TOPS-20 V4 monitor

     2.  PS:  is mounted on DB0:


Mount floppy-C in floppy drive 0 (the left-hand  drive),  go
to  the  CTY  (the  system may remain under timesharing) and
type:
     ^\
     PAR>MCR MOU
     MOU>DX0:
       Mount Complete
     MOU>^Z
     ^\
     PAR>MCR PIP
     PIP>DB0:/NV=DX0:*.*
     PIP>^Z
     ^\
     PAR>MCR DMO
     DMO>DX0:
       Dismount Complete
     DMO>^Z

[This procedure is also documented in the TOPS-20 Software Installation
Guide, Chapter 4, Creating Front End File System.]

                                                      Page 4

4.0  SOFTWARE

4.1  DSKOP JSYS


Sites that do significant numbers of DSKOP JSYSs on drives that are
dual ported (either to other systems or to the console front end) may
occasionally find that the JSYS will fail with something like
340021,,0 returned in AC 1 after the call.  This is an aspect of the
way the monitor handles dual ported drives that have been seized by
the other port.  The key to determining that this is happening is that
the bits returned in AC 1 (which are the IORB status bits from the
operation) do not contain either device error (IS.DVE=1B10) or data
error (IS.DTE=1B9).  If you see this condition, simply retry the DSKOP.


5.0  MANUAL RELOAD OF DX20

Under certain powerfail conditions, the DX20 must be manually reloaded.
See DX20LD.DOC and TOPS-20 Operator's Guide, Part VI, Section 1.14 
for information concerning appropriate times to manually reload the DX20.