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

      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
     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
     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.
     [E-mail from La Roy Tymes]

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