interfaces   with   each   disk   drive,   one   from   the microprocessor called the A CABLE and one from the disk control logic called the B CABLE. The two interfaces combine to provide all timing, control, and data lines needed for disk drive operation. THE A CABLE.— The A cable connects the disk drives to the processor input and output buses. The disk drives are daisy chained on the A cable and only the selected drive will respond to the microprocessor commands. The A cable is used for microprocessor control of the  drives.  The  microprocessor  passes  commands  to  the drives  using  three  command  lines  called  TAG  lines  and eight BUS OUT lines. The three-bit TAG CODE on the tag lines identifies the type of command while the bus out lines carry the command code or address data to the drives. Status data from the selected disk drive is passed over   eight   bus   in   lines   to   the   microprocessor. Additional sector mark and index signals are sent from the selected drive to the microprocessor. THE B CABLE.—  The B cable connects the individual disk drives with the disk control logic. Each disk drive has its own unique B cable. The B cable is used for read/write operations. The selected disk drive (A cable under microprocessor control) sends a MODULE ADDRESSED signal to the disk control logic indicating it has been selected. The selected drive provides a SEEK END signal indicating it has positioned the heads over the addressed cylinder and an INTERRUPT signal indicating the start of the addressed sector. Both the seek and sector addressing operations are controlled by the microprocessor over the A cable. Timing  for  the  read/write  operations  is  provided  by the SERVO CLOCK and READ or WRITE CLOCK signals. The servo clock originates from reading the servo track dibits on the servo surface of the disk pack. The servo clock provides the basic timing for the read/write operations. The read clock is generated by the disk drive during the read operation and is used to control the transfer of the serial read data from the drive to the disk control logic. The write clock is generated by the disk control logic during a write operation and is used to control the transfer of serial data over the bidirectional line to the disk drive. DISK CONTROL LOGIC.— The  disk  control logic is used during read/write operations. Its two major functions are (1) to convert the parallel 16-bit data  words  from  the  data  bus  into  a  serial nonreturn-to-zero (NRZ) pulse train (B cable) when writing to disk and (2) to convert the NRZ pulse train coming from the selected disk into parallel 16-bit words for output on the data bus during read operations. The   disk   control   logic   is   enabled   by   the microprocessor and provides requests to the DBCU for data transfer with buffer memory when reading or writing. Overall timing for read and write operations is provided by the SERVO CLOCK signal. The SEEK END and INTERRUPT signals (B cable) notify the disk control logic when to begin read/write operations. Data Bus Control Unit (DBCU) The data bus control unit (DBCU) controls the transfer of data from source to destination on the data bus.   The   microprocessor   defines   the   source, destination, and number of words to be transferred (buffer length) to the DBCU. The DBCU transfers the data a word at a time from the specified source to the specified destination until the transfer is complete. The DBCU contains a control file and a count file that contain the necessary information to control the data exchanges. The control and count files are loaded by the microprocessor definition commands. Once the files are loaded, the actual data transfers occur on a request basis. The requests for data bus transfers are handled  on  a  priority  basis.  The  highest  priority transfers are between the disk control logic and buffer memory  (read/write  operations).  Next  come  the processor input and output holding register requests and the lowest in priority are the input/output channel requests. CDS Channel Interface The  CDS  channel  interface  controls  all  data exchanges between the magnetic disk set and the CDS computer. The interface can be configured for up to four  16-bit  or  32-bit  parallel  input/output  channels. Basic I/O operations including external functions, interrupts,  and  input/output  data  transfers  are  controlled by the interface logic. DISK DRIVE UNIT The addressable disk drives (0, 1, 2, 3) contain the electromechanical  portions  of  the  magnetic  disk  set  and the read/write circuitry. The disk drive performs the actual   recording   and   reading   back   of   data   as commanded by the controller logic contained in the disk 10-19