Quantum Q250 User Manual

The microprocessor strobes an address into the 5080 on the falling
edge of ALE (address latch enable). If the 5080 is selected, -lORD
strobes data from the MAD bus; -IOWR strobes data to the MAD bus.
DICEY controls a DMA read or write to buffer RAM over the BDATAO-7
(buffered data) bus:
o When -DMA I/O is asserted, data flow is from RAM to the
5080. -DMA CLK strobes the data from the bus to the DMA
to HOST register.
o When -DMA I/O is deasserted, data flow is from the 5080 to
RAM. -DMAIENB turns on the 5080 drivers, placing data from
the HOST to DMA register onto the bus.
Handshaking between DICEY and the 5080 is controlled by -DMA REQ (to
DICEY) and -DMA ACK (from DICEY). DICEY handles addressing of the
buffer RAM without intervention by the 5080. Timing for the 5080
arbitration logic is set by the 12 MHz clock from FYLO.
2.3.5
Servo Control, AMC and Servo AGC
Before reading this section, read the summary description in section
2.2.2. Follow this explanation on Figure 2-4, a simplified block
diagram of the drive. Figure 2-3 shows the disk format including
details of the servo signals in the wedges.
Seeking
While a drive is seeking, the read circuit reads the servo track
numbers from the data in the servo bursts, thus determining the head
position. As each wedge passes under the moving head, the AMC mea-
sures the peak amplitudes of the burst's preambles, which depend upon
how nearly the bursts are centered under the moving head. The micro-
processor attempts to use the servo track number from the burst with
the highest amplitude, because the head is over the servo track cor-
responding to that burst. But if the checksum bits indicate that the
data is invalid, data from the next largest amplitude burst is used.
If data from all bursts in a wedge is invalid, the microprocessor
infers the head position from previous data, and then reads data from
the bursts in the next wedge.
Track Following
While track following, the track, head and sector numbers from each
wedge are stored in RAM as they are read. Also, the AMC measures the
servo-burst amplitudes in order to center the head over the track:
when the head is centered, the head equally overlaps the servo bursts
written on each side of the data track--the microprocessor positions
2-23

Quantum Q250 Specifications

General

Brand	Quantum
Model	Q250
Category	Storage
Language	English