NorthStar HORIZON Manual page 59

4. On Board PROM: Bootstrap software (8080/Z80 machine code) is
:;tored in an on-board 256 byte bipolar PROM (DPGM, 9D). This
PROM provides 256 bytes of memory. The 8 low order address
Ilits address the PROMs. The PROM outputs are driven onto the
Data Input Bus.
5. ':lock Circuit:
provides 2 MHz
"ircui try.
The crystal clock oscillator circuit (15B)
clock signals to synchronize the control
6.
~ead,
Write Control: A read or write command sequence begins
',ith detection of a sector pulse. A read or write command
Joust be issued to the controller within a 96 microsecond
"indow after the sector pulse. Writing data begins at the end
Ilf the window. The controller writes one byte of zeros.
"riting of subsequent data is controlled by the software.
"riting stops at the next sector pulse. Reading starts some
:ime after the end of the window so as to be in the middle of
:he written preamble. First the phase-locked loop is turned
'In by the read enable (RE)' signal. After locking to the
incoming data is achieved, the hunt enable (HE) signal causes
:he search for a sync character to begin. When a sync
'character is detected, the controller enters "body" mode.
When the software issues a read command to the controller, the
controller will put the CPU into the wait state until the read
3hift register is full.
7. :ommand Decode Logic: When a command address is issued to the
~ontroller, the 8 low order address bits are decoded to
jetermine which command actions are to be taken and which disk
~ontrols
should be performed. See the Disk Controller
:ommands section for details.
8. Counters: The counters (6A, 7A, eA, 9A) provide sector timing
and read and write command sequencing. The counter (15C) is
the sector position counter. The output of 9A provides pseudo
sector pulses in case no diskette is inserted in the drive or
the drive motor is off and also allows discrimination between
sector holes and the index hole. The counter (3D) is used to
count disk revolutions to determine when to cause automatic
motor off. The counters (9B) are used to count bits while
reading or writing data.
59 North Star HORIZON Computer