The VTAM® logmode defines many characteristics of the session established between the Telnet LU representing the client and the host VTAM application. For example, the logmode defines response types, presentation style, and the type of LU Telnet is emulating. LU0 (non-SNA) and LU2 (SNA) represent terminal LU types. LU1 (SCS) and LU3 (3270 Data) represent printer LU types.
Telnet matches a VTAM logmode to each client as it connects based on the client device type, unless the user specifies a logmode on the USSMSG10 screen or logmode is configured in the USS table mapped to the connection. See z/OS Communications Server: IP Configuration Reference for default device type and logmode table information. The default terminal logmodes are non-SNA for TN3270 connections and SNA for TN3270E connections. At session request time, Telnet indicates to VTAM the appropriate logmode based on device type. The host VTAM application usually honors the request and binds the session using the requested logmode. However, depending on VTAM statements, the application can override the requested logmode and bind the session using different characteristics than Telnet requested. For this reason, some screen sizes might not work correctly even though the logmode defined in Telnet is correct. If the KEEPOPEN function is used to allow session initiation by the host application, the wanted logmode must be coded on the DLOGMOD parameter as part of the VTAM application definition statement that defines the Telnet LU. Otherwise, the application will choose its own logmode.
Telnet processes the ATTN KEY request differently for non-SNA and SNA sessions. For non-SNA sessions (BIND FM value 02), Telnet converts the ATTN KEY request to a '6C'x data byte and sends it to the application. For SNA sessions (BIND FM value 03), Telnet converts the ATTN KEY request into a SNA signal and sends it to the application as expedited data. Some clients send both an ATTN KEY function code and a '6C'x data byte to ensure the ATTN is seen by the application. Telnet converts the ATTN KEY function into either a '6C'x data byte or a SNA signal and also forwards the '6C'x data. Some applications give unexpected results or Telnet might appear to not support ATTN when two ATTNs are received. The SINGLEATTN parameter causes Telnet to drop the second ATTN if it immediately follows an ATTN. The SINGLEATTN and NOSINGLEATTN parameters can be coded at all three parameter block levels for different levels of granularity.
To change either the TN3270 or the TN3270E logmode for a device type, use the TELNETDEVICE parameter. Whenever Telnet initiates the session request, Telnet will request that the logmode specified on the TELNETDEVICE statement be used for the session. The application (the primary LU) does have the ability to override the requested logmode and use a completely different logmode. The TELNETDEVICE parameter can be coded in all three parameter block levels for different levels of granularity. Coding TELNETDEVICE in a PARMSGROUP that is mapped on the LUMAP-DEFAPPL-PMAP statement enables the logmode to be LU and application specific.
If the application initiates the session, the TELNETDEVICE logmode has no affect on the session. For example, printer sessions are initiated by the application unless a printer default application is specified. The LUMAP-KEEPOPEN parameter can be used to open a terminal ACB and wait for the primary application to initiate the session.
Transform Linemode connections can have a unique logmode by coding TELNETDEVICE with a device type of TRANSFORM. Any logmode used must not support extended graphics.
The special case logmode NONE can be specified indicating that Telnet should not send any logmode request when initiating the session.
In the examples that follow, the first line causes only the TN3270 logmode to change from the default to SNX32705. The second line causes both the TN3270 and TN3270E logmodes to change from their defaults to SNX32705 and SNX32702. The third line causes only the TN3270E logmode to change from the default to SNX32702.
TELNETDEVICE 3278-5-E SNX32705
TELNETDEVICE 3278-5-E SNX32705,SNX32702
TELNETDEVICE 3278-5-E ,SNX32702