The Free On-line Dictionary of Computing (30 December 2018):

TYMNET

    A United States-wide commercial computer
   network, created by Tymshare, Inc. some time before 1970,
   and used for remote login and file transfer.  The network
   public went live in November 1971.

   In its original implementation, it consisted of fairly simple
   circuit-oriented nodes, whose circuits were created by
   central network supervisors writing into the appropriate
   nodes' "permuter tables".  The supervisors also performed
   login validations as well as circuit management.  Circuits
   were character oriented and the network was oriented toward
   interactive character-by-character full-duplex
   communications circuits.

   The network had more than one supervisor running, but only one
   was active, the others being put to sleep with "sleeping pill"
   messages.  If the active supervisor went down, all the others
   would wake up and battle for control of the network.  After
   the battle, the supervisor with the highest pre-set priority
   would dominate, and the network would then again be controlled
   by only one supervisor.  (During the takeover battle, the net
   consisted of subsets of itself across which new circuits could
   not be built).  Existing circuits were not affected by
   supervisor switches.

   There was a clever scheme to switch the echoing function
   between the local node and the host based on whether or not a
   special character had been typed by the user.  Data transfers
   were also possible via "auxiliary circuits".

   The Tymshare hosts (which ran customer code) were SDS 940,
   DEC PDP-10, and eventually IBM 370 computers.  Xerox
   XDS 940 might have been used if Xerox, who bought the design
   for the SDS 940 from Scientific Data Systems, had ever built
   any.

   The switches were originally Varian Data Machines 620i.  The
   Interdata 8/32 was never used because the performance was
   disappointing.  The TYMNET Engine, based loosely on the
   Interdata 7/32, was developed instead to replace the Varian
   620i.  In the early 1990s, newer "Turbo" nodes based on the
   Motorola 68000 began to replace the 7/32s.  These were later
   replaced with SPARCs.

   PDP-10s supported (and still do in 1999) cross-platform
   development and billing.

   Tymshare, Inc. originally wrote and implemented TYMNET to
   provide nationwide access for their time-sharing customers.

   La Roy Tymes booted up the public TYMNET in November of 1971
   and, as of March 2002, it had been running ever since without
   a single system crash.

   TYMNET was the largest commercial network in the United States
   in its heyday, with nodes in every major US city and a few
   overseas as well.  Tymshare acquired a French subsidiary,
   SLIGOS, and had TYMNET nodes in Paris, France.

   Tymshare sold the TYMNET network software to TRW, who
   created their own private network (which was not called
   TYMNET).  In about 1979, TYMNET Inc. was spun off from
   Tymshare, Inc. to continue administration and development of
   the network.

   TYMNET outlived its parent company Tymshare and was acquired
   by MCI.  As of May 1994 they still ran three DEC KL-10s
   under TYMCOM-X, although they planned to decommission them
   soon.

   The original creators of TYMNET included: Ann Hardy, Norm
   Hardy, Bill Frantz.  La Roy Tymes (who always insisted that
   his name was NOT the source of the name) wrote the first
   supervisor which ran on the 940.  Joe Rinde made many
   significant technical and marketing contributions.  La Roy
   wrote most of the code of the network proper.  Several others
   wrote code in support of development and administration.  Just
   recently (1999) La Roy, on contract, wrote a version of the
   supervisor to run on SPARC hardware.

   The name TYMNET was suggested by Vigril Swearingen in a weekly
   meeting between Tymshare technical and marketing staff in
   about 1970.

   (http://cap-lore.com/ETH.html).

   [E-mail from La Roy Tymes]

   (2002-11-26)