MAGNETIC TAPE CONTROLLER Common  functions  performed  by  the  magnetic  tape control section are as follows: l l l l l l Receives   data   and   commands   (external functions) from one or more central processors (computers) Provides  timing  and  control  signals  to  one  or more tape transports for read, write, search, and rewind operations Reformats computer words received from the central   processor   into   frame-size   bytes, generates  parity  bits  for  each  frame,  and transmits frame data to the write heads (write operation) Receives frames of data from the tape transport read heads, checks for parity errors, and formats data  into  computer  words  for  the  central processor (read operation) Receives status signals from one or more tape transports and monitors tape movement Transmits data and unit status (status words)  to the central processor MAGNETIC TAPE UNIT OPERATIONS The magnetic tape unit is capable of performing several operations such as read (forward and reverse), write, and rewind. Operations can be performed online or  offline. MTU Online Operations Typical operations performed by the MTU while under computer control (online) include the following: .  Read l Write l Space file . Rewind MTU READ/WRITE OPERATIONS.— Read and write densities on most MTUs are selectable. The density selected depends on the operational mode selected, the installed options, and the number of tracks used to record and read data. The NRZI recording format is the most common recording method  used  for  seven-  and  nine-track operations up to densities of 800 fpi. For 1600 fpi, nine-track operations, PE recording/reading is used. MTU READ/WRITE TAPE SPEED.— All read operations, including search and space file, and write operations are performed at a tape speed of 120 inches per second (ips). Data may be read in the forward or reverse directions, but it may be written only in the forward direction. MTU WRITE PROTECTION.— Protection of master tapes is provided by the use of a write-enabling ring and sensing circuitry in the MTTs. Only tapes with a write-enabling ring installed, as shown in figure 9-15, may be used in the write operation. Tapes without a write-enabling  ring  are  protected  from  the  write operation. MTU SEARCH.—  In the search operation, the controlling computer provides a single word code known as the search key to the MTU. The MTU then performs a record-by-record search of the tape on the MTT selected. The first data word of each record is compared to the search key. If they match, a  find record  condition  exists;  the  record  is  then  read normally and transmitted to the controlling computer. If no match is found, then the unit will continue searching records until EOT or BOT is detected. SPACE FILE.— The space file operation is used to find a tape mark, which should indicate the start of a file.  The  selected  MTT  moves  tape,  under  the  direction of the MTU, until a tape mark is detected. Tape motion is then stopped with the read head over the interrecord gap following the tape mark. Tape marks are not normally found in front of the first file on the tape. Figure  9-15.—A  write-enabling  ring. 9-12