Display screen protection for finance stations

When a Finance station is defined with the NOBID option, IMS provides support similar to the screen protection function for the IBM® 3270 Information Display System.

IMS does not transmit two consecutive output messages to a display component without an intervening input message from the component. This gives the user the opportunity to view and respond to one message before another is displayed.

When IMS transmits an output message to a display component, it marks the component as protected, unavailable for output. IMS does not transmit another output message to it until an input message is received from that component. The input message can be an IMS transaction, a message switch, an IMS command, the VTAM® RTR command, or an MFS control request. Upon receipt of one of these, IMS changes the display component's status to unprotected.

When a workstation is defined with the BID option, consecutive messages are not transmitted to a station. Input from the workstation or a positive response to the BID command from IMS must be received after one transmission before the next message can be sent.

When a workstation uses MFS, the screen is protected on a physical-page basis. MFS control requests or input data are used to request additional screens of data.

If a workstation does not use MFS, the screen is protected on a message-by-message basis. The input of any IMS transaction, message switch, or command causes the screen to be unprotected. The VTAM ready-to-receive (RTR) command can be used to request the next output message. If no message is available, or if the node is in a status that does not allow output to be sent (output stopped or quiesced), IMS returns an exception DR1, followed by an error message indicating that no output is available.

If the RTR command is received during the transmission of MFS-paged output, IMS returns an exception DR1 response, followed by an error message indicating an invalid paging request, because IMS cannot determine the MFS control function to be performed.

After an exception response is received by IMS for a current IMS output message to a display component, the screen is unprotected. After an exception response is sent by IMS, any defined display component is automatically marked protected and unavailable for output.