Prestel Emulation Module
		    ------------------------


			  Introduction
			  ============

The following briefly describes the Prestel terminal emulation as
implemented in MIRROR II. Some familiarity with the Prestel
videotext  service is assumed.

Your MIRROR II Master Disk contains three files which are
required  for the proper operation of the Prestel emulation:
MIRROR.CFG, VIEWDATA.MAF  and PRESTEL.MEF. The MIRROR.CFG file
must be on the same floppy disk  or hard disk directory as
MIRROR.EXE. MIRROR.CFG specifies the loading  of the MIRROR
Add-in, VIEWDATA.MAF, which contains the graphical components  of
the available videotext emulations. PRESTEL.MEF is the module
which  emulates the Prestel terminal. If these files are in their
proper  place at load time, MIRROR will allow you to emulate the
Prestel terminal  through the use of the EMulate command.



		       The EMulate Command
		       ===================

The EMulate command is used to load the Prestel emulation, e.g.,

		       emulate  "prestel"

The EMulate command can also be used to configure some aspects of
the Prestel emulation while MIRROR is loading it. MIRROR also
allows  you to change these aspects for Prestel after you have
have loaded  it. The specific aspects that you can change with
the EMulate command  are listed below. The syntax for the command
is:

     emulate  "prestel"[/vn1=value,vn2=value, ... vnn=value]

Available variables are identified by name. You assign a value to
any Variable Name that you wish to change from its default (or
current  setting, in the case that the emulation is already
loaded). The value  is either On, Off, or the number 2.

If Prestel is not specified in the EMulate command, the command 
assumes that Prestel is already loaded and that you wish to
change  the listed variables. This form is also handy for
changing options  from within a Script file.

The variables available are:

	AUTOSAVE=ON	Turns On the Auto-save option 
	AUTOSAVE=OFF	Turns Off the Auto-save option 
	AUTOSAVE=2	Sets the Auto-save option to Dynamic
	DOWNLOAD=ON	Enables Telesoftware downloading
	DOWNLOAD=OFF	Disables Telesoftware downloading
	LINEFEED=ON	Sets end of line to CR/LF 
	LINEFEED=OFF	Sets end of line to CR
	MONOCHROME=ON	Display graphics using only two colors
	MONOCHROME=OFF	Display graphics using four colors

Examples:

	1. This example loads the Prestel emulation and sets the Auto-save 
	   and Download variables for Prestel to Off. 

		emulate  "prestel"/autosave=off,download=off

	2. This command sets the linefeed variable to On for the already loaded 
	   Prestel emulation.

		em /linefeed=on



			 PRESTEL Screen
			 ==============

The following is a snapshot of a blank Prestel terminal screen
showing  the Prestel emulation menu and MIRROR's command line.
MIRROR's Prestel  menu displays its various commands and options
as well as some of  the settings. While in the Prestel emulation,
MIRROR allows you to  execute commands and set options by using
the "Alt" key and the key  for the command or option you wish to
use. For instance, pressing  the "Alt" key and the `W' key at the
same time will write the Prestel  page displayed in the left half
of the screen to the capture buffer  or file. The key used with
the "Alt" key (`W' in the example above)  can be entered as
either upper (W) or lower case (w). MIRROR commands  entered
while using the Prestel emulation will be displayed on MIRROR's 
command line at the bottom of the screen.



		  PRESTEL Commands and Settings
		  =============================

Alt-A = Auto-save ON/OFF/DYNAMIC
--------------------------------

Prestel allows you to capture data from the screen to the capture
buffer or capture file. The Alt-A option toggles the Automatic
save  option from OFF to ON to DYNAMIC. When ON, the Prestel
textual data  on the left half of the screen will be written to
the capture file  or buffer each time a clear screen code is
received from the Prestel  host. This option can be used when you
wish to capture the data portion  of the display for editing or
saving. In the DYNAMIC mode, all data  received (including
attribute setting characters) will be captured.  The DYNAMIC mode
allows the user to capture a sequence of frames that  may be
played back from the disk at a later time using the PLayback 
command. The current status of the Auto-save option is displayed
near  the bottom of the menu.

NOTE: Capture must be turned on using MIRROR's CApture command
for  the Auto-save option to have any effect.


Alt-W = Write page to capture file
----------------------------------

The Alt-W command allows you to capture a snapshot of the current
screen by placing it in the capture buffer. This is similar to
the  ON mode of the Auto-save feature, except that you must
indicate when  the snapshot is to be taken. When using Alt-W,
Auto-save should be  Off and MIRROR's CApture should be On.

NOTE: In order to capture data using either the Alt-A feature or
the  Alt-W command, MIRROR's capture mode must also be enabled
with the  CAPTURE command.


Alt-G = Graphics display mode
-----------------------------

The Alt-G command changes the display into graphics mode so that
the  Prestel mosaics and double height characters can be properly
displayed.  Depending on the type of video display hardware in
your computer,  MIRROR may be unable to properly display all of
the colors used by  an actual Prestel terminal (See Alt-C, change
color palette; also  see SCreen G).


Alt-T = Text display mode
-------------------------

The Alt-T command sets the display mode from graphics back  to
text where all eight colors will be displayed. Mosaics  will be
represented by a single period (.) on the screen. In the text 
mode, the left half of the screen contains the Prestel text, and
the  right half of the screen is the function menu for Prestel.


Alt-R = Reveal concealed data
-----------------------------

The Alt-R command emulates the REVEAL key on an actual Prestel
terminal.  Its purpose is to cause concealed characters to be
displayed.  Once revealed, characters cannot be concealed again.


Alt-C = Change color palette
----------------------------

The Alt-C command allows the user to switch between color
palettes  while in the graphics display mode. This will cause all
colors to  be re-displayed with the new palette.


Alt-Y = Monochrome ON/OFF
-------------------------

The Alt-Y command toggles the monochrome flag ON and OFF.  When
the  monochrome flag is ON, the graphics screen is created using
only two  colors (typically white and black).  MIRROR will adjust
the colors  requested by Prestel such that the display will be
readable.  This  option is useful on systems with monochrome
monitors or LCD displays.


Alt-S = Take snapshot of text
-----------------------------

The Alt-S command causes the text displayed in the Prestel text
screen  to be copied to the right half of the screen, overwriting
the menu.  This allows you to take a snapshot of a Prestel text
screen so that  it may be referenced later. The following command
will re-display  the menu, overwriting the last snapshot screen.
This is  similar to MIRROR's snapshot command; however, the
screen  saved by the Alt-S command is on-screen even while new
information  is being displayed on the left half of the screen.


Alt-M = Re-display this menu
----------------------------

The Alt-M command re-displays the Prestel menu on the right half
of  the text screen. This allows you to recover the menu if you
have overwritten  it with the snapshot command (see Alt-S above).


Alt-D = ENABLED/DISABLED Telesoftware
-------------------------------------

The Alt-D option toggles the Telesoftware ENABLE value displayed
near  the bottom of the menu. This option allows you to
automatically initiate  a Telesoftware download. If the function
is ENABLED, the receipt of  a Telesoftware start of block
sequence (|A) will cause MIRROR to capture  a file downloaded
from the Prestel host using the Telesoftware verification 
protocol. As the file is downloaded, frame number information is
displayed.  If DISABLED, the characters are simply displayed in
the text screen  without interpretation. See the next section for
more on the Telesoftware  downloading.


Alt-L = End of line CR or CR/LF
-------------------------------

This option specifies whether the end of line code in a
Telesoftware  download is translated to a Carriage Return (used
by some operating  systems or for some binary files) or a
Carriage Return/Line Feed pair  (used by MS-DOS and CP/M to
indicate end of line in a text file). This command acts like a
toggle switch.



		Telesoftware Downloading Support
		================================

MIRROR supports the specification of the UK Council for
Educational  Technology for downloading files from a Prestel
host. Except for the  enabling the Telesoftware protocol,
interaction by you is unnecessary  during the downloading
process. By default, the Telesoftware protocol  is ENABLED. When
this feature is enabled, MIRROR will automatically  detect the
start of block sequence, interpret the remaining characters  in
the block, verify the accuracy of the transmission of the data, 
write the data to the file specified by the host and request the
next  block. This process will continue until the downloading is
complete.  The downloaded file is written to the current disk
drive and directory  using the name included in the header block
by the host. Note that  any file currently on the disk, with the
same name, will be overwritten.  You  may change the disk drive
and directory specifications by using MIRROR's  DIr and CDir
commands.

During a Telesoftware protocol transmission, you will see a
message  on the bottom line of the text screen indicating the
name of the file  being downloaded and the message that any
keyboard key can be used  to abort the transfer. Also, the
Prestel header line will be updated  upon the completion of each
block, indicating which frame is currently  being received.

As indicated on the bottom line during the transfer, any key may
be  used to abort the Telesoftware download. If you abort the
transfer,  the Telesoftware status will automatically be set to
DISABLED.  To re-try the transfer, you must first re-enable the
protocol by pressing  Alt-D.



	   Uploading Files Into Prestel Mailbox Frames
	   ===========================================

MIRROR allows you to automatically upload disk files into Prestel
mailbox frames. Text is uploaded with the words at the end of
lines  wrapped to the next line if they will not fit. Since
Prestel does  not allow a single message to contain more than one
screen full of  text, MIRROR automatically splits large files
into multiple messages. During this upload process, MIRROR's
LWait and TUrnaround parameters  are not honored at the end of
each line, but at Prestel's end  of frame indicator (#). At the
end of frame indicator, any characters  that need to be sent to
the Prestel host to place you at the text  portion of the next
mailbox frame must be included in the MIRROR's  TUrnaround
string. For example, in Prestel's mailbox frame #77,  you would

press the #,
		 wait for a prompt saying "Press 1 to send ... ",
press the 1,
		 wait for a prompt saying "Press # to continue",
press *00,
		 wait for the screen to repaint, and
press the #
		 to keep the current user number.

The string to automatically achieve this sequence would be
"_~1~*00~_".  Note  that in MIRROR's TUrnaround and REply
commands, the underscore  character ( _ ) is used to represent
the hash character (#) when using  Prestel. This TUrnaround
string may be up to 20 characters in length  and the tilde
character (~) is used as a one second delay. MIRROR  continues to
send text in this method until the file to be uploaded  is empty.



		      The PLayback Command
		      ====================

When the Prestel emulation is loaded into MIRROR, an additional
command  is available to the you. This command allows you to
playback a frame  that was previously captured using the dynamic
Auto-save  mode. The syntax of the command is:

			   PLayback fn

where fn is the name of the file that was saved using Auto-save. 
During the playback you may press control-S to pause (any key
will  then restart) or the attention key (normally ESC) to abort.
The user  may also issue commands listed on the Prestel menu,
such as Alt-G,  to switch the playback into graphics mode.


		     SCreen Command Addition
		     =======================

The SCreen command has an additional option when the Prestel
emulation  is loaded. The command SCreen Graphics (which can be
abbreviated  to  sc g) causes MIRROR to switch to the graphics
screen. This is  the same as pressing the Alt-G key, except that
the SCreen command  may be executed from a Script file. The
MIRROR command SCreen Terminal  (e.g., sc t) will switch back to
the text screen.



		      Prestel Keyboard Map
		      ====================

When using the Prestel emulation, the following keyboard 
assignments have been made:

	IBM-PC Keyboard		Transmitted Data
	---------------		----------------
	Up Arrow			^K
	Left Arrow			^H
	Right Arrow			^I
	Down Arrow			^J
	Del				*
	Keypad Period			*
	Keypad Plus			#
	PgUp				*#
	PgDn				#
	Keypad Enter with Num-Lock on	#  
	   (on IBM extended keyboard)