2.2 The IBM 7171 ASCII Device Attachment Control Unit - Index of

Setting Host Disconnect Methods4. The new input parse nodes are ready to be inserted into the parse. Point theTDT to the new beginning of the input parse sequence.ENTER: SW DCOO:1F22 1F40RESPONSE: OkNote: Although this allows the cent sign to be keyed in at the terminal, the centsign still is displayed as a backslash.A local 3270 terminal is always connected to its control unit by a coaxial cable. AnASCII terminal, however, can be remotely attached to the IBM 7171 over switchednetwork telephone lines. If such a connection were lost and the host applicationcannot be appropriately notified, then the next user to dial into the same phone linewould continue with the interrupted session. The IBM 7171 provides two methodsto signal line drop to different host access methods.Note: By default, the IBM 7171 uses the Asynchronous Status Method describedbelow with the VM disconnect. If the control unit will be used with a systemrunning another operating system, then the proper disconnect sequence MUST beset up by the installation. Failure to do so will cause unpredictable results.Setting Up the Asynchronous Status Method: An isolated Device End is a signal froma 3270 terminal that it has just powered on. This response is enough for VM to putthe session into a disconnected state. A BT AM application program can use theREADYQ keyword on the DCB to intercept the Device End, but many existingBT AM applications do not use READYQ. TCAM and VT AM both ignore isolatedDevice Ends. If this method is chosen, the IBM 7171 will allow anystatus/sense combination to be sent to the host.The word at X ' DCOO:E' is the location where the status/sense information isstored. To select this option, store the desired sense in the first byte and thedesired status in the second byte. If this word is zero, then this method is disabled.For example, if the host is running VM, then an isolated Device End is sufficient todisconnect the session. Therefore, store X ' 00' (no sense) at X' DCOO:E ' , andX' 04' (Device End status) at X 'DCOO:F '.Setting Up the Send Predefined Message to Host Method: When this method is used,the IBM 7171 will generate an Attention to the host when a line drop occurs. TheIBM 7171 will respond to the resulting read with the predefined message which ispointed to by X' DCOO:OOOA '. If X' 0000' is stored at X' DCOO:OOOA ' , then amessage will not be sent.The format of the message is as follows:• Length of the message to be sent (word),Message to be sent to the host.The message must be in the form of an EBCDIC datastream which could normallybe built by a Read Modified command, since interpretation is not performed.Typically, the message to be sent to the host is a Test Request header followed bythe word "LOGOFF." The 7171 contains this message in the correct format inROM, and so all that needs to be done is to store the address in X' DCOO:OOOA ' .Chapter 4. Customizing IBM 7171 Tables 4-65