Connecting to the VTAM network

You use the /START DC command to establish communications between IMS and VTAM®. The /START DC command tells VTAM to pass queued logon requests to IMS for all terminals defined to VTAM as belonging to IMS.

Before establishing a session with IMS for any terminal, verify that the following conditions exist:

  • VTAM and the Network Control Program (NCP) are active.
  • Controllers and logical units are active. If they are not automatically activated by VTAM, the VTAM network operator can use the VTAM VARY command to activate them.
  • Controllers are powered on, appropriately configured, initialized, and activated for VTAM and NCP.
Recommendation: Start VTAM before starting the IMS control region.

You also need to use the /START DC command in either of the following situations:

  • If VTAM is stopped and restarted while the IMS control region is running
  • If you terminate communications between IMS and VTAM using the /STOP DC command

The /START DC command activates the following processes:

  • Initiating IMS Transaction Manager processing
  • Opening the VTAM access method control block (ACB)
  • Enabling the IMS VTAM logon exit routine
  • Opening the primary and secondary VTAM master terminals if these terminals were not opened during IMS initialization (due to the specification of VACBOPN=DELAY in the DFSDCxxx IMS.PROCLIB member)

If VTAM is active when IMS starts, the IMS VTAM ACB is opened; otherwise, the ACB is opened by the /START DC command. Any logon requests received by VTAM before the IMS /START DC command, but after the IMS VTAM ACB has been opened, are enqueued by VTAM until the /START DC command completes. If the queuing of logon requests by VTAM causes operational problems, the specification of VACOPN=DELAY in the DFSDCxxx PROCLIB member can be used to delay the opening of the VTAM ACB. When the master terminals are generated to use VTAM, the master terminal operator will be unable to enter either the /NRE or /ERE commands (including /ERE BACKUP) or the /START DC command. These commands can be issued using automatic restart (AUTO=Y) or automated operations.

Before you can establish a session between IMS and a logical unit or node, the logical unit or node must be active, connected, enabled, and available to VTAM and started in IMS. You can activate logical units or nodes when VTAM starts using VTAM start options, or a VTAM operator can activate them using the VTAM VARY command.

Normally after you start IMS, all VTAM terminals have a started status. If you stop a VTAM terminal with an IMS /STOP command, the terminal remains stopped after an IMS warm start or emergency restart. You must use the /START NODE command before issuing the /OPNDST (open destination) command to reactivate the terminal.

You must establish sessions between terminals and IMS before anyone can use them to transmit data. You can initiate sessions in several ways:

  • VTAM can automatically log them on (based on the VTAM terminal definition).
  • The end user can log on.
  • You can issue an IMS /OPNDST command (if they are not automatically logged on).
  • You can issue a VTAM VARY command.