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+Network Working Group                                       A. Berggreen
+Request for Comments: 600              COMPUTER SYSTEMS LABORATORY--UCSB
+NIC: 20884                                                 November 1973
+
+
+         INTERFACING AN ILLINOIS PLASMA TERMINAL TO THE ARPANET
+
+INTRODUCTION
+
+   The PLATO IV System based at the University of Illinois at Urbana is
+   a highly sophisticated and very powerful approach to Computer Aided
+   Instruction.  The PLATO IV system makes use of a plasma display
+   terminal that is a unique device with capabilities not presently
+   found on computer terminals.  A number of ARPA supported projects
+   intend to use the plasma terminal on local connection to computer
+   resources or by long-distance connection to the PLATO IV System.
+
+   One problem in using the PLATO System from any appreciable distance,
+   is the communication costs involved (i.e. long-distance telephone
+   rates for many consecutive hours).  Also, use of the plasma terminal
+   in other applications is hampered since the communications scheme
+   employed in the PLATO System in non-standard.
+
+   One approach to reducing the communications cost is to use the
+   ARPANET for the long-distance connection, since the Network is
+   potentially one of the most reliable and cost effective means of
+   transmitting computer data.  This approach is reasonable the is a
+   Network node near the PLATO System, (the PDP-11/ANTS system at the
+   Center for Advanced Computation at the University of Illinois at
+   Urbana) and with the increasing number of TIPS and IMPS on the
+   ARPANET access is becomming easier ad more widespread.
+
+   The plasma terminals are designed to be connected directly to
+   telephone lines using Frequency Shift Keying (FSK) modulation.  Using
+   dedicated telephone lines, the plasma terminal may be run at a data
+   rate of 1200 bits/sec in full-duplex operation.  Using dial-up lines,
+   the terminal may be run with display information being received at
+   1200 bits/sec and data to the computer being transmitted at 120
+   bits/sec using a reverse chanel scheme.
+
+   The data and command words used by the plasma terminal differ for
+   input and output.  Input received from the computer arrives in 20-bit
+   words plus one start bit.  Data transmitted to the computer is sent
+   in 11-bit words plus one start bit.
+
+   In order to make the plasma terminal more generally applicable for
+   standard communication, and specifically adapted to the ARPANET
+   connection by way of a TIP, the terminal must be interfaced in such a
+
+
+
+Berggreen                                                       [Page 1]
+
+RFC 600                                                    November 1973
+
+
+   way as to communicate using Teletype-like codes.  In addition, if the
+   PLATO System is to be linked by way of the Network with no changes to
+   the system, then a special interface must be provided to allow the
+   Network to communicate with the PLATO System using the FSK
+   communication scheme.
+
+APPROACH
+
+   So that the plasma terminal would communicate like a Teletype when
+   tied to a TIP, and still be able to work with the PLATO System
+   through the Network, it was decided to build an interface that could
+   be operated in two modes.  There would be an "ASCII" mode to send and
+   receive Network oriented data (such as TIP log-on or running at some
+   arbitrary Network site); and a "PLATO System" mode to allow data,
+   imbedded in 8-bit codes, to pass transparently through the Network.
+
+   Since there is a possibility that when in the PLATO mode, re-
+   formatted codes can appear to be standard ACSII characters that will
+   be seized upon by intervening TIPs or HOSTs, the interface must
+   insure that no recognizable codes be sent.  For example, the @ is
+   recognized by a TIP as the beginning of a TIP command string.
+   Therefore the interface must either "double-up" this code (@@) or not
+   send it at all.
+
+   With the above requirement, and with other limitations, the proto-
+   type interface, now in use at UCSB, operates as follows:
+
+   1. In ASCII mode, the plasma terminal has been made to send and
+      receive 8-bit ASCII code.  In this mode, there is no graphics
+      capability.  The keyboard that is provided can only send 124
+      codes, therefore 4 seldom used ASCII codes have been excluded, and
+      certain ASCII characters cannot be displayed.
+
+   2. In PLATO mode, PLATO data is embedded in 8-bit codes.  The
+      capability of running the keyboard in ASCII mode while the display
+      remains in PLATO mode has also been provided.
+
+SUBSEQUENT WORK
+
+      After discusion, it became clear that the flexability of the
+      interface to do such things as emulate standard graphics
+      terminals, implement a cursor, and to respond to Network Graphics
+      Protocols, will be highly desirable.  So it has been decided that
+      the original hardware will be re-packaged using a micro-computer
+      with a ROM for the control program.  With the addition of more RAM
+      and/or ROM, the micro-computer will have the capability of being
+      programmed to allow the plasma terminal to do a wide variety of
+      tasks.  Work on developing this interface has begun at UCSB.
+
+
+
+Berggreen                                                       [Page 2]
+
+RFC 600                                                    November 1973
+
+
+      Figure 1 shows the planned version of plasma data format for
+      Network use.
+
+              PACKING SCHEME FOR PLASMA TERMINAL DATA
+
+                                               Data from Plasma
+                 |Msb|x|x|x|x|x|x|x|x|Lsb|P|  <----------------
+                   |  | | | | | | | |  |  *    Terminal
+                   |  | | | | \ \ \ \  \    Parity for Keyboard
+                   |  | | | |  \ \ \ \  \   data is regenerated
+                   |  | | | |   \ \ \ \  \  at the PLATO System
+                   |  | | | |    \ \ \ \  \ end.
+                   |  | | | |     \ \ \ \  \
+                   / / / / /       \ \ \ \  \
+                  / / / / /         \ \ \ \  \
+      Data to    | | | | |           | | | | |
+      <-------- |x|x|x|x|x|1|1|0|<--|x|x|x|x|x|1|0|0|
+      Network
+
+   For the second part of Figure 1, please view the PDF version of this
+   document.
+
+
+   NOTE: NO-OP codes are removed from the data stream at the PLATO
+   System end by the hardware.
+
+         [ This RFC was put into machine readable form for entry ]
+          [into the online RFC archives by Neil Philp 11/99]
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+Berggreen                                                       [Page 3]
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