The ordinary TN3270 protocol

The reason for this problem is that the unenhanced TN3270 protocol does not include a way for a host to inform a client that the host application screen is complete. (TN3270 implements a screen-oriented protocol, 3270 Data Stream, over a character-oriented connection, Telnet). Therefore, the host cannot send several blocks of data to the client and then say, "OK, the application screen is now complete – you can let the user enter data now." Instead, each block arrives without any indication about whether it is the last block for this application screen. From the client's point of view, something like the following events occur:
  1. A block of commands and data arrives. The client sets the input inhibit indicator, processes the block, and displays the new data on the specified parts of the session window. The client then clears the input inhibit indicator and waits.
  2. 30 milliseconds pass.
  3. Another block of commands and data arrives. The client processes the block as in step 1 above. This block causes a different part of the screen to be updated. The client waits.
  4. 50 milliseconds pass.
This process continues until the host has completely displayed a new host application data screen. The client still waits, not knowing that the host application screen is complete. (For more information, see How the macro runtime processes a macro screen).

This process does not present problems for a human operator, for various reasons that are not important here.

However, this process does present problems for the macro runtime during screen recognition. Recall that during screen recognition the macro runtime tries to match the application screen to one of the valid next macro screens every time the screen is updated and every time an OIA event occurs (see Re-doing the evaluation). Therefore the macro runtime might find a match before the screen is completely updated. For example, a String descriptor might state that recognition occurs if row 3 of the application screen contains the characters "ISPF Primary Option Menu". When the host has updated row 3 to contain these characters, then the macro runtime determines that a match has occurred, regardless of whether the host has finished updating the remainder of the application screen.