Defining physical terminals

When implementing ETO, to achieve your desired VTAM® terminal network you need to be aware of certain requirements and aspects of how ETO and VTAM work together as you plan for and define the physical terminals in the network.

Performing the following actions can ensure that you achieve your desired VTAM terminal network:
  • Assess how often IMS application programs depend on specific terminal characteristics.
  • Check the accuracy of each VTAM terminal definition. For each dynamic terminal, ETO builds a terminal structure that relies on the VTAM definition for the characteristics (such as the LU type, screen size, and model) for that terminal.

    Terminal characteristics that are specified in your IMS system definition might differ from (and override) those in the VTAM definitions. If these terminal characteristics in the IMS system definition are compatible with those of the actual terminal, the discrepancy is not apparent. 1

  • Either provide specific node-name logon descriptors or use the Logon exit routine (DFSLGNX0) for terminals that are not adequately defined using the default logon descriptor.
  • Code one of the following two exit routines to determine the device type:
    • The Logon exit routine (DFSLGNX0) can determine the device type by examining the default logon descriptor.
    • The Signon exit routine (DFSSGNX0) can determine the device type later in the process by examining the terminal control blocks.
When receiving a request to establish a terminal session, IMS relies on the following information from VTAM session parameters:
  • UNITYPE
    The unit type of the node that is attempting to log on. IMS determines the UNITYPE by using the following fields:
    • The LUTYPE field of the CINIT 2 request provides part of the UNITYPE information. The LUTYPE value is usually found in the first byte of the PSERVIC operand of the MODEENT macro, which is used to generate the VTAM mode table entry that is used for a logon. The following figure shows the fields of the PSERVIC operand in the VTAM MODEENT macro.
      Figure 1. VTAM MODEENT macro PSERVIC operand fields that IMS uses
      PSERVIC=X'02(LU type)00000000001850(default screen type: 24X80) 0000(alternate screen size: not defined)7E(presentation space size control: fixed default)00(device type: applicable to LU type 0 only)'

      The following table shows the mapping of the LUTYPE value to the IMS UNITYPE.

      Table 1. Mapping for VTAM LUTYPE value to IMS UNITYPE
      VTAM LUTYPE IMS UNITYPE
      LUTYPE X'06' LUTYPE6
      LUTYPE X'02' SLUTYPE2
      LUTYPE X'01' SLUTYPE1 (default) or NTO
      LUTYPE X'00' 3270, SLUTYPEP, or 3600/Finance
    • If the LUTYPE field is X'00' (indicating that the terminal is 3270 non-SNA, Finance, or SLU P), IMS must check the transmission services profile specification in the TS field of the CINIT request. The TS value is usually found in the TSPROF operand of the MODEENT macro that is used to generate the VTAM mode table. This LUTYPE value must match the value in the logon descriptor that IMS selects for a logon.
      The following table shows the mapping of the TSPROF specification to the IMS UNITYPE.
      Table 2. Mapping for TSPROF specification to IMS UNITYPE
      TSPROF specification IMS UNITYPE
      X'02' or X'03' 3270
      X'04' SLUTYPEP (default) or 3600/Finance
  • Input RU size

    The input RU (request unit) size in the BIND must be less than or equal to the RECANY buffer size for the IMS (also required for static terminals).

  • Output RU size

    The output RU size in the BIND must be greater than or equal to the OUTBUF size that IMS determines for the terminal from the selected logon descriptor. This parameter is also required for static terminals.

  • Screen size and model number
    For non-SNA 3270 and SLUTYPE2 devices, IMS retrieves both screen size (row and column) and model number from the BIND:
    • For static terminals, the screen size is the value that was specified in the system definition.
    • For dynamic terminals, IMS determines the screen size from the VTAM definition or from the Logon exit routine (DFSLGNX0).

      Recommendation:  Until the ETO feature was available, IMS ignored the screen size and model number values in the BIND, because the IMS system definition held this information. Therefore, check to ensure this definition is accurate.

      If you determine that a VTAM definition is inaccurate, you can use the Logon exit routine (DFSLGNX0) to override the VTAM-provided screen size and model number. For example, use a terminal naming convention or MODETAB definition convention. The Logon exit routine can also assign USER=NODE as a name, when appropriate.

      IMS uses the values in the PSERVIC operand of the MODEENT macro, which is used to generate the VTAM mode table entry that is used for a logon. MFS formats must be available for all screen sizes that IMS dynamically builds.

    Restriction: IMS does not use the 3270 Read Partition Query (RPQ) command to determine the screen size from the device controller.

For more information on the Logon exit routine (DFSLGNX0), see IMS Version 15.5 Exit Routines.

1 If the actual terminal characteristics do not match those in the IMS definition or the VTAM definition, it is possible that the terminal can function with IMS.
2 CINIT is a network services request sent from a system services control point (SSCP) to a logical unit (LU), asking that LU to establish a session with another LU and to act as the primary end of the session.