Disk Controller Theory Of Operation - NorthStar HORIZON Manual

DISK CONTROLLER THEORY OF OPERATION
The North Star double density disk controller is an integrated
comJination of hardware and software. It is designed
spe2ifically to provide a complete, compact and economical disk
drive controller for use with S-100 bus 8080 or Z80 microcomputer
systems.
DISK CONTROLLER HARDWARE
The disk controller is implemented from medium and small scale
TTL integrated circuits and PROM memory. The entire controller
fits on a single S"x10" printed circuit card. The block diagram
below shows the general organization of the disk controller.
1. Address Buffers and Select Logic: The sixteen address lines
are received with Schmitt" trigger buffers (7D, 10D, lID) to
provide noise immunity. The high order g address lines are
used by the board select PROM (DSEL, IlC) to determine if the
current memory reference is addressing the controller board.
The low order eight address lines are used to present data and
control information to the controller. The use of these bits
is determined by the outputs of the board select PROM.
2. Write Data Logic: If the controller is issued a write data
byte command, then the low order eight address bits are
interpreted as data to write and are gated into the write
shift register (6D). The controller will place the CPU in a
wait state until the shift register is empty and only then
clock the new data into the shift register. write encoding
(using the FM encoding method for single density and MFM
encoding for double density) and write precompensation are
controlled by the logic at locations 3C, 4C, and part of SC.
3. Read Data Logic: Raw data from the disk is standardized by
the RD-DAT-OS one-shot (SA). A phase-locked loop (lA, 4A, and
parts of IB, SA) tracks the trailing edge of RD-DAT-OS to
match the frequency of the incoming data. The phase-locked
loop output drives the data separator (3B, 4D), whose outputs,
DATA-WINDOW (clock) and WIND-DATA (data), go to the 8-bit read
data shift register (lSD). The read shift register outputs
are multiplexed with the status bits (12D, 13D, 140) and
driven onto the Data Input Bus (100, lID). When reading, if
double density data is encountered, the double density
indicator (001, 4B) is set.
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North Star HORIZON Computer