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TUCoPS :: Networks :: kermit_t.txt

How to use Kermit through a TAC

[ netinfo/kermit-tac-info.txt ]                                [ 7/92 BLV ]


There are four obstacles to performing Kermit file transfers through a TAC
(DDN Terminal Access Controller).  These are:

  1. The default TAC intercept character (used by the TAC to identify
     commands intended for it) is the "@", which is also used by Kermit.

  2. Flow control between the PC and the TAC must be turned on.

  3. The TAC has a small buffer (approximately 64K), which can overflow
     if the TAC is heavily loaded.

  4. The default mode for the TAC is a 7-bit ASCII data path.  This
     prevents the transfer of binary files (such as programs).


  1. According to Frank Da Cruz, original author of Kermit, the protocol
     Kermit uses may contain any visible character of the ASCII
     character set. This means that the intercept character on the TAC
     should be set to an invisible (non-printing) character. In the
     examples below, we have chosen to change the intercept character to
     CONTROL-Y -- indicated as ^Y. 

     Note: You create this character by holding down the CONTROL KEY and 
           pressing the Y key simultaneously.

     For example, to change the TAC intercept character to ^Y (ASCII
     code 25) type:

                             @i 25 <Return>

     You can use this command to change the TAC intercept character to any
     character in the ASCII character set.  Just substitute the decimal code
     of the chosen character for the "25" above.  A table of common intercept 
     characters is listed in the TAC User's Guide.

  2. If you've changed your intercept character to a ^Y, but your files still
     do not transfer successfully, try using flow control.  Turn on flow 
     control between the PC and the TAC by typing the commands:
                   ^Yd c a <Return>   ("device code ASCII")
                   ^Yf i s <Return>   ("flow input set")
                   ^Yf o s <Return>   ("flow output set")

     Note that the above example assumes you have changed the intercept 
     character to CONTROL-Y.  You may issue these commands before setting
     your intercept character if you wish.

  3. If you are still having problems transferring files, try decreasing the
     size of the Kermit packets to 60 characters -- smaller than the
     size of the TAC's buffer.

     You must do this for BOTH the PC Kermit and the host Kermit.  Although
     command syntax may vary between Kermit implementations, you will
     generally type something like this to set the packet size to 60:

                  set send packet-size 60 <Return>
                  set receive packet-size 60 <Return>

     Decreasing the packet size will increase the length of time required
     for file transfers.  Also, a packet size that works when the TAC is
     lightly loaded may not work when the load increases, so experiment
     until you find a combination that works for your environment.

  4. When the TAC receives what it considers an intercept character, even
     though it is embedded in a file, it interrupts the transfer process
     and tries to interpret whatever follows as a command to the TAC. 
     Therefore, to transfer binary files, you must put the TAC into binary
     mode.  Doing so disables the current intercept character.  

     To put the TAC in binary mode, first open a connection to your host.  
     After connecting to your host, put the TAC in binary mode by typing:

                  ^Yd c a <Return>  ("device code ASCII")
                  ^Yb o s <Return>  ("binary output set")
                  ^Yb i s <Return>  ("binary input set")

     Note, again, the example assumes the intercept character has been changed
     to the ^Y.

     TAC flow control is *NOT* compatible with TAC binary mode.  Therefore,
     if you've set flow control at the TAC, you must disable it before
     switching to binary mode.  To disable flow control, type the 

                            ^Yd c a
     command (assuming "^Y" is your intercept character).  After doing so,
     issue the commands to put the TAC in binary mode.

     The binary commands MUST be typed in the order shown above because the
     TAC will no longer recognize the intercept character once it's in binary
     mode, and it will ignore the "^yd c a" command.  Closing the connection 
     (by logging off your host) returns the TAC to non-binary mode, 
     re-enabling local control with "@" commands.

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