Reading from a 3270 terminal

There are two basic read commands for the 3270, read buffer and read modified.

As noted in the previous section, the AID indicates which action or attention key causes transmission. The cursor address indicates where the cursor was at the time of transmission. CICS® stores this information in the EIB, at EIBAID and EIBCPOSN, on the completion of any RECEIVE command.

The read buffer command brings in the entire buffer following the read header, and the receiving program is responsible for extracting the information it wants based on position. It is intended primarily for diagnostic and other special purposes, and CICS uses it in executing a RECEIVE command only if the BUFFER option is specified. CICS never uses read buffer to read unsolicited terminal input, so the BUFFER option cannot be used on the first RECEIVE of a transaction initiated in this way.

With read modified, the command that CICS normally uses, much less data is transmitted. For the short read keys (CLEAR, CNCL and PAs), only the read header comes in. For other attention keys (ENTER and PFs), the fields on the screen that were changed (those with the MDT on, to be precise) follow the read header. We describe the format in the next section. When transmission occurs because of a trigger field, light pen detect or cursor select, the amount and format of the information is slightly different; these special formats are described in Support for special hardware . Input from a program attention key on an SCS printer is also an exception; see SCS input for a description of that data stream.