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Working with Telnet

Our company decided a few weeks ago to change from native SNA connections to Telnet connections (to the mainframe z/OS 1.4) The problem is now that with SNA our developers had the possibility to work with a 62 x 160 session. With Telnet now they are restricted to 27 x 132. Do you have any ideas how we could do a work around that so we are able to work again with 62x160, or is it really impossible without SNA?

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No, it is not impossible. The answer lies within the TELNET parms used in the TCP/IP TN3270 server. These parms can be customized by the system programmer responsible for TCP/IP.

Within the parms for the TN3270 server, VTAM logon mode (logmode) tables are defined. They may look like this:

TELNETDEVICE 3278-2-E NSX32702 ; 24 line screen
TELNETDEVICE 3279-2-E NSX32702 ; 24 line screen
TELNETDEVICE 3278-3-E NSX32703 ; 32 line screen
TELNETDEVICE 3279-3-E NSX32703 ; 32 line screen
TELNETDEVICE 3278-4-E NSX32704 ; 43 line screen
TELNETDEVICE 3279-4-E NSX32704 ; 43 line screen
TELNETDEVICE 3278-5-E NSX32705 ; 132 col screen
TELNETDEVICE 3279-5-E NSX32705 ; 132 col screen

The key is to add the proper logmode for an application defined alternate screen size to the various device type entries (add the DYNAMIC entry also):

TELNETDEVICE 3278-2-E D4C32XX3 ; 24 line screen
TELNETDEVICE 3279-2-E D4C32XX3 ; 24 line screen
TELNETDEVICE 3278-3-E D4C32XX3 ; 32 line screen
TELNETDEVICE 3279-3-E D4C32XX3 ; 32 line screen
TELNETDEVICE 3278-4-E D4C32XX3 ; 43 line screen
TELNETDEVICE 3279-4-E D4C32XX3 ; 43 line screen
TELNETDEVICE 3278-5-E D4C32XX3 ; 132 col screen
TELNETDEVICE 3279-5-E D4C32XX3 ; 132 col screen
TELNETDEVICE DYNAMIC ,D4C32XX3 ; dynamic

The other pieces of the puzzle that must also support a custom screen size are the emulation program (your TN3270E client), the VTAM session manager (if used) and of course the application. Since you were already using the custom screen size in an SNA environment, it sounds like all the pieces are already in place except the TCP/IP TELNET parms.

For more information on customizing TCP/IP on z/OS see: IP Configuration Reference

IP Configuration Guide

This was first published in March 2006

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