BMS supports several special hardware features that allow the operator to enter and transmit input by selecting a field on the screen:
Trigger fields are a special hardware feature of certain types of terminal, such as the 8775. A field defined as a trigger field causes the terminal to transmit its contents if the operator moves the cursor out of the field when it is primed. The field gets primed when the operator moves the cursor into it and enters data or uses either the DELETE or ERASE EOF keys. It becomes unprimed after it causes transmission, or if the operator uses the ERASE INPUT key, or after a send to the terminal (if you are using partitions, the send must be to the partition that contains the trigger field to have this effect).
You define a field as a trigger field by setting the VALIDN extended attribute to a value of TRIGGER, either in the map or by program override.
Only the field itself is sent when a trigger field causes transmission; other fields are not sent, even if they have been modified. You can detect a transmission caused by a trigger field by checking the attention identifier, which has a value of X'7F'.
Terminals that support the validation feature buffer the keyboard, so that the operator can continue to enter data while the host is processing an earlier transmission. The program processing such inputs needs to respond quickly, so that the operator does not exceed the buffer capacity or enter a lot of data before an earlier error is diagnosed.
The customary procedure is for the program receiving the input to check the contents of the trigger field immediately. If correct, the program simply unlocks the keyboard to let the operator continue (a BMS SEND command containing the FREEKB option does this). If the field is in error, you may wish to discard the stored keystrokes, in addition to sending a diagnostic message. Any of the following actions does this:
See the IBM® 3270 Information Display System Data Stream Programmer’s Reference manual for more information about trigger fields.
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