In order for Session Manager to update your display screen, TSO/E requires that the terminal keyboard remain locked. When the keyboard is locked, you cannot enter commands, and attention interruptions will not be processed. The CONTROL operand of the CHANGE.TERMINAL command lets you specify the number of seconds the keyboard is to be locked while a command is executing or while Session Manager is updating the screen. CONTROL can be any integer from 0 to 999.

If you want to wait for each TSO/E command or program to execute and have its output displayed before entering the next command, set CONTROL to a very high value. Note that attention interruptions will not be processed while the keyboard is locked.

If you want to enter TSO/E or Session Manager commands before the currently executing command has completed, set CONTROL to 0. The keyboard unlocks immediately after Session Manager rewrites the screen and you can then enter new commands. However, Session Manager only updates the screen after you have pressed the Enter key or any PF key. You must press the Enter key or a PF key every time you want new output from a command or program displayed.

Specifying CONTROL as a small number (10 to 20) is a compromise between the two extremes. A number in that range allows most TSO/E commands time to execute and have their output displayed, yet does not cause the terminal keyboard to be locked for long periods of time.

In the IBM-supplied default environment, the keyboard locks for a maximum of 15 seconds. The scrolling PF keys reset the CONTROL timer to 0 so that the keyboard immediately unlocks when you press one of them. PF key 12 (or 24) resets the CONTROL operand to the last non-zero value it had. This is done to allow you to scroll back and review previous information in a stream while a TSO/E command is executing without waiting for the command or program to complete.