Product Features
Introduction
This chapter provides an overall functional description, major assembly descriptions, and a
block diagram description.
Functional
Description
3
This section provides a functional description of
the
RP
C2486A/88A/90A SCSI disk drives.
Disk
Format
The head/disk assembly (RDA) contains the magnetic disks (see Figure 3-1). Table
3-1
lists
the
physical allocation of the cylinders. Each physical sector can store 512 bytes of user
data
in
the
standard format. The user can choose
to
format the drives using other physical sector
sizes (from
180-744 bytes in increments of 2 bytes). The user can choose logical block sizes
which are
1,
2,
4, or 8 times as large as the physical sector size.
Sector Format
The smallest directly addressable storage area on a
data
surface is a sector. Accessing a sector
is
accomplished when the controller specifies the address of
the
cylinder, head, and sector.
The formatted sector bytes are allocated as shown in figure 3-2.
.
Addressing
Structure
All
addressing between the disk drive and the host is logical.
The
drive's embedded controller
converts the logical block address into the appropriate physical address (Le. cylinder, head,
sector), allowing for any sparing operations
that
have been performed. To support logical
blocks larger
than
the physical sector size, the drive automatically blocks and deblocks the
physical sectors into the currently specified logical block size.
'
Error
Correction
Code
The disk drives use a Reed-Solomon error correction code (ECC) for detection and correction
of
data
errors. During a write operation,
the
ECC function generates
18
bytes of
ECC
information, and writes
the
information into the ECC field as
the
sector is written. During a
read operation, the controller generates an 18-byte code from
the
data
field being read, and
compares
it
to
the
ECC field created during the write operation.
If
the
I8-byte code differs
from
the
ECC field, a
data
error is detected and the ECC field is used
to
correct
the
data.
The ECC function is enabled or disabled via the DCR (Disable Correction)
bit
in
parameter
page 01R of the MODE SELECT command. When enabled,
the
ECC algorithm divides a
DRAFT
Product
Features
3·1
1/18/93
09:58
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