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

Select Reod Modified from Position (RMP) Command: Select RMP is an immediatecommand. A Select RMP command is executed by recording the Read Modifiedcondition and returning Device End.The commands following the Select RMP command should be a chained Writecommand followed by a chained Read Modified command. The Write Commandcontains only four bytes (WCC, SBA xx) to set the buffer address. If the sequenceis other than as described, the command will not be accepted, and Channel End,Device End, Unit Check, Operation Check will be sent to the host.Upon receipt of the Write command, the IBM 7171 will perform the Read Modifiedfrom Position preparation, and return Device End to the host when the datastream is completed. The Read Modified command is then executed.Select Reod Buffer from Position (RBP) Command: Select RBP is an immediatecommand. A Select RBP command is executed by recording the Read Buffer conditionand returning Device End.The commands following the Select RBP command should be a chained Writecommand followed by a chained Read Buffer command. The Write command containsonly four bytes (WCC, SBA xx) to set the buffer address. If the sequence isother than as described, the command will not be accepted, and Channel End,Device End, Unit Check, Operation Check will be sent to the host.Upon receipt of the Write command, the IBM 7171 will perform the Read Bufferfrom Position preparation, and return Device End to the host when the data streamis completed. The Read Buffer command is then executed.Select Write Command: Select WRT is an immediate command. A Select WRTcommand is executed by returning Device End to the host. If the chainedcommand following the Select WRT is not a Write command, then Channel End,Device End, Unit Check, Operation Check will be sent to the host.Erose All Unprotected Command: This command performs five functions at theaddressed device:1. Clears all unprotected buffer character locations to nulls.2. Resets to 0 the MDT bit for each unprotected field.3. Always unlocks the keyboard.4. Resets the AID byte.5. Repositions the cursor to the first character location in the first unprotectedfield of the buffer. If no unprotected fields exist, the cursor is positioned tobuffer location O.Erase All Unprotected is an immediate type command. Upon acceptance of thiscommand, the IBM 7171 becomes Busy and sends Channel End initial status to thechannel. Upon successful completion of this command, the control unit sendsDevice End status asynchronously to the channel and then becomes Not Busy.Chapter 6. IBM 7171 II 0 Interface to the Host System 6-1 7