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
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
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
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