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

Terminal Status Reporting: The host may request certain status information aboutthe terminal from the IBM 7171. If a status request order string is included anywherein a buffer written to the terminal, the IBM 7171 will be primed to respondto the next host Read Modified command with a status message. This messagebegins with a special AID byte (X'E4' for status 1, X' E5' reserved), followed bythe cursor address as usual. The remainder of the message is returned as follows:• Status 1 - begins with an SBA byte, followed by the address of the "mark," ifany, or screen address zero (x' 4040'). Then, as contents of the "field," amUltiple setup format string describing the current state of all host settableoptions is returned. The format of this string is such that, if appended to aWCC and sent to the terminal, it will re-establish the current status withrespect to all reported modes. For example, the following string might bereturned:HexChar.LabelHexChar.Label11C1502B5B5BB06CD5A9C583A2E597C189C4$ $ % N z E c s V P AiDSBAcont. belowE3424A454A4F54595E63686D72777C4040 ... 405B'T $(home,new line, & 28 column tabs)The string will always be 49 or 50 characters long, depending on whether"mark" is turned on, and will include all 28 tabs, with trailing x ' 40' charactersfor undefined entries.• Status 2 - ReservedStatus may be requested with or without attention. Thus the host program mayspecify that the IBM 7171 is to generate an attention immediately after receivingthe buffer containing the status request. The host may then respond with a ReadModified, and read the status response. Alternately, the host may perform theRead Modified chained to or immediately following the write containing the statusrequest, without requesting or waiting for an attention.Note: Status should not be requested while the keyboard is logically unlocked, askeyboard generated attentions and AID bytes may be confused with status reports.If the attention generating form of status request were used while the keyboardwere unlocked, attentions would occasionally be lost.Asynchronous Status Change Notification: The host program may receive notificationof changes in terminal status caused by the keyboard operator. When AsynchronousStatus Change Notification is enabled, any keyboard initiated change instatus will cause the two high order bits of the first address byte in the cursoraddress field of the next Read Modified buffer sent to the host to be set toB' 10 ...... ' (a value which is formally illegal and reserved, but which is seldomchecked). The host application may then request status and observe changes sincean earlier request.Host Initiated Line Drop: The special order string X' 2B5B4B' occurring anywherein an output buffer will cause the IBM 7171 to drop the communication line to theterminal after the entire buffer has been read out. This will have the effect ofhanging up the phone on a dialup connection. The IBM 7171 then re-enables theline to permit another user to dial up.Chapter 6. IBM 71711/0 Interface to the Host System 6-43