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     The release 4 RSX20F front end is significantly  more  reliable
than  the  release 3A RSX20F.  In addition it makes possible the use
of modems and terminal like devices which did not work under release
3A.   However,  there are performance and reliability considerations
which must be made when selling and upgrading TOPS20 systems.

     The SPD for TOPS20 lists only Digital  terminals  as  supported
optional  hardware.   Modems  are  not  mentioned.   In  house,  our
engineering system is configured  for  128  asynchronous  terminals,
most  of  which  are high speed CRT's.  This configuration is highly
reliable.  However, configurations containing terminal like  devices
which   do   high  speed  input,  non  Bell  modems,  microprocessor
controlled switches, and many lines should be carefully examined for
potential problems.

     Following are the points to consider  when  examining  existing
systems for upgrades, or potential new systems.



     PLEASE CONSIDER:



     1) READ THE TOPS20 RELEASE 4 BEWARE FILE  FOR  DETAILS  ON  THE
UNIBUS REPEATER FCO.

     2) Auto baud detection is  now  available  through  1200  baud.
Release 3A could only auto baud through 300 baud.

     3) The use of Bell 212A modems has  been  successfully  tested.
At 1200 baud the Bell 212A did not work with release 3A.

     4) Low speed input (human typist) in  a  maximum  configuration
(128 lines) presents no known problems.  This configuration has been
in use in house for many months  with  no  problems.   A  number  of
problems  with  memory  management  have  been  solved  in release 4
reducing the occurences of buffer overflow crashes.

     5) High speed output using XON/XOFF control presents  no  known
problems with the restriction that split speed not be used.  An XOFF
sent by a device at low speed to control output  to  the  device  at
high  speed  can't  work.   For  non  split  speed  lines  at most 3
characters will be sent to the device following the  receipt  of  an
XOFF.   This  change  makes  the use of XON/XOFF controlled LA120's,
LA180's, VT100's and  buffer  limited  multiplexers  and  electronic
switches  work.   Note:   When doing XON/XOFF controlled output to a
VT100 in smooth scroll mode, the page length must be set to 0.  Near
page  ends  the  monitor cannot differentiate between an XON sent by
the VT100's processor and an XON typed  by  the  terminal  operator.
Because  of this, high speed output to a smooth scroll VT100 in page
mode may not stop at the end of a page even though the  bell  rings.
Also note that split speed cannot be used and that 4800 baud is just
as good as 9600 baud, but puts less of a strain on the front end.

                                                              Page 2
RSX20F changes from release 3A to release 4


     The release 3A RSX20F often  lost  characters  during  XON/XOFF
control of high speed output.

     6)The use of high speed output  using  XON/XOFF  control  in  a
maximum configuration has not been fully tested or simulated.  There
may be a problem with a configuration  containing  many  high  speed
output XON/XOFF controlled output terminals all running continuously
at full speed.

     7) High speed input with no XON/XOFF control.  If messages  are
sent  one  at  a  time  and  an  entire message fits in one TTY line
buffer, then there are no known problems.  In release  3A  the  line
buffer  was  64  characters  long.   In release 4 its 173 characters
long.  However, if more than  this  number  of  characters  is  sent
before  the  TOPS20  process can read them, they are discarded and a
bell  (control  G)  is  echoed.   This  technique  can  be  used  to
communicate with a TTY like device which is willing to use an ACKing
protocol to control data flow.  The DX software for word  processing
stations uses this technique.

     8)The use of high speed input with no  XON/XOFF  control  in  a
maximum  configuration  has  not  been  tested.  The performance and
reliability of such a configuration is not known.

     9) High speed input under XON/XOFF control may not work well at
high  speeds.  Characters may be lost.  Where it starts to fail is a
function of system load, and characteristics of the high speed input
device.   For  high  speeds  it  is suggested that the customer send
short messages one at a time to insure that no data is lost.

     10) The use of high speed input under XON/XOFF  control  for  a
maximum configuration should not be considered.




     If you have any questions  about  potential  configurations  or
problems  during  the  conversion  from  release  3A to release 4 of
RSX20F call the HOTLINE.