I M P U L S E 4 2 5 S C S I ( D ) PLUS DEVELOP.
NO MORE PRODUCED Native| Translation
------+-----+-----+-----
Form 3.5"/HH Cylinders 1520| | |
Capacity form/unform 426/ MB Heads 9| | |
Seek time / track 14.0/ 4.0 ms Sector/track | | |
Controller SCSI2 DIFFERENTIAL Precompensation
Cache/Buffer 256 KB SEGMENTED Landing Zone
Data transfer rate 2.000 MB/S int Bytes/Sector 512
10.000 MB/S ext SYNC
Recording method RLL 1/7 operating | non-operating
-------------+--------------
Supply voltage 5/12 V Temperature *C 4 55 | -40 65
Power: sleep W Humidity % 8 85 | 5 95
standby W Altitude km -0.061 3.000| -0.061 12.000
idle 11.5 W Shock g 10 | 60
seek 12.8 W Rotation RPM 3606
read/write W Acoustic dBA 45
spin-up W ECC Bit 48,WITH 11BIT BURST COR
MTBF h 150000
Warranty Month
Lift/Lock/Park YES Certificates CSA,FCC,SDA,TUV,UL1950,VDE
**********************************************************************
L A Y O U T
**********************************************************************
QUANTUM PRODRIVE 425S PRODUCT MANUAL 81100377-01, FEBRUARY 1992
+----------------------------------------------------------+
| A2 A1 A0 |
| JP1 +-+-+-++-+-+-+ RP12++|XX
| +-+-+-++-+-+-+ |||XX
| SS EP WS |||XX
| ++|XX
| RP11++|XX SCSI
| Terminators|||XX J2
| |||.X
| ++|XX
| RP10++|XX
| |||XX
| |||XX
| ++|XX
| |XX
| | 1
++ |XX
++ LED |XX J3
+----------------------------------------------------------+ 1
**********************************************************************
J U M P E R S
**********************************************************************
QUANTUM PRODRIVE 425S PRODUCT MANUAL 81100377-01, FEBRUARY 1992
Jumper Setting
==============
x = Jumper set at factory
Jumper Options
--------------
+---------+-----------+-------------------------------------------+
| Jumper | Setting | Description |
+---------+-----------+-------------------------------------------+
| SS | OPEN x | Self-seek test disabled |
| | CLOSED | Self-seek test enabled |
+---------+-----------+-------------------------------------------+
| EP | CLOSED x | Parity checking enabled |
| | OPEN | Parity checking disabled |
+---------+-----------+-------------------------------------------+
| WS | OPEN x | Wait/Spin disabled |
| | CLOSED | Wait/Spin enabled |
+---------+-----------+-------------------------------------------+
| A2 | CLOSED x | A0, A1, and A2 are used in combination to |
| A1 | CLOSED x | determine the drive's SCSI ID. The default|
| A0 | OPEN x | value is six. |
+---------+-----------+-------------------------------------------+
SS Self-Seek Jumper
--------------------
Quantum ships the ProDrive 425S hard disk drive from the factory
with no Self-Seek (SS) jumper installed- that is, with the self-seek
test disabled. To enable the Self-Seek test, install a the SS jumper.
When the SS jumper is installed, the drive initiates a self-seek
test. During the self-seek test, the drive can execute any command
that does not require a disk access. When the self-seek test begins,
the drive-activity LED (Light-Emitting Diode) lights and remains on
as long as the test proceeds without error. The drive executes seeks
in butterfly pattern. Each butterfly seek pattern starts and ends
with a seek to cylinder 256, where the drive verifies the sector ID.
If the vertification detects a seek error, the self-seek test
terminates immediately and the LED flashes continously until the SS
jumper is removed. When the SS jumper is removed, the drive completes
the seek pattern in progress and turns off the LED. The drive
attempts no error recovery and is ready to execute any command.
EP Enable Parity Jumper
------------------------
The Enable Parity (EP) jumper controls parity checking across the
SCSI bus. Installing the EP jumper enables parity checking. Quantum
ships ProDrive 425S hard disk drive from the factory with the
EP jumper installed. Removing the EP jumper disables parity checking.
The drive still generates parity, but does not perform parity check-
ing. When the jumper is changed, the new setting takes effect at the
next power on.
WS Wait/Spin Jumper
--------------------
The Wait/Spin (WS) jumper controls the Wait/Spin option. Quantum
ships the ProDrive 425S hard disk drive from the factory with
no WS jumper installed - that is, with the Wait/spin option disabled.
With the Wait/Spin option disabled at power-on, the motor starts
spinning automatically and the drive is ready within 20 seconds. To
enable Wait/Spin option, install the WS jumper.
With the WS jumper installed at power-on, the motor does not start to
spin until the host sends a START/STOP UNIT command. This delay
permits power sequencing, preventing the system power supply from be-
ing overloaded by simultaneous demands for peak start current from
multiple devices. Once the drive receives the START/STOP UNIT
command, it is ready within 20 seconds.
A2/A1/A0 SCSI-Bus Device Identification
----------------------------------------
The SCSI bus can support up to eight devices, including the host
system. The identification number of each device determines its
priority level. Prior to installing the ProDrive 425S hard disk
drive, you must choose its SCSI-device identification number.
Jumpers installed across three pair of jumper pins - A2, A1 and A0 -
in various combinations permit the assignment of the drive's
identification number. These three pairs of jumper together comprise
a 3-bit binary number, in which A2 is the most-significant bit and A0
is the least significant bit. With a jumper installed, a pair of pins
represents a 1; with no jumper installed a 0. Thus, with jumpers
installed across all of the pins, the device identification is 7;
without jumpers installed, the device identification is 0.
Quantum ships the ProDrive 425S hard disk drive from the factory
with the A2 and A1 jumpers installed - that is, the drive's default
SCSI-device identification is 6. If the jumpers are changed, the new
device identification takes effect at the next power on.
+----------+-----------------------+
| SCSI ID | Jumpers |
| | A2 | A1 | A0 |
+----------+-------+-------+-------+
| 0 | OPEN | OPEN | OPEN |
| 1 | OPEN | OPEN | CLOSED|
| 2 | OPEN | CLOSED| OPEN |
| 3 | OPEN | CLOSED| CLOSED|
| 4 | CLOSED| OPEN | OPEN |
| 5 | CLOSED| OPEN | CLOSED|
|x 6 | CLOSED| CLOSED| OPEN |
| 7 | CLOSED| CLOSED| CLOSED|
+----------+-------+-------+-------+
The SCSI bus supports up to eigth devices, including the host system.
A device's identification number determines its priority and must be
unique in the system.
Driver Terminators
------------------
On the ProDrive 425S hard disk drive, three resistor networks
installed in sockets on the PCB terminate the SCSI bus. Quantum ships
the ProDrive 425S hard disk drive from the factory with the
terminating resistors installed. On a SCSI bus, only the first device
- usually the host - and the last device can contain terminating
resistors. When installing the ProDrive 425S hard disk drive
in any other position on the bus, remove the terminating resistors.
When using the ProDrive 425S with a differential SCSI Adapter Kit
where the TERM PWR line is used to provide power to external
terminating resistors, remove the termination resistors from the hard
disk drive, and add an i-line termination resistor pack between the
differential SCSI adapter kit 50 pin SCSI connector and the 50-pin
connector on the SCSI flat cable.
**********************************************************************
I N S T A L L
**********************************************************************
QUANTUM PRODRIVE 425S PRODUCT MANUAL 81100377-01, FEBRUARY 1992
Notes on Installation
=====================
Orientation
The ProDrive 425S can be mounted in ANY orientation.
#6-32 UNC screws are recommended for mounting.
Note
With the exception of the faceplate and integral mounting bracket,
clearance between the drive and any other surface must be a minimum
of 0.10 inch.
CAUTION: The PCB is very close to the mounting holes. Do not exceed
the specified length for the mounting screws. The specified screw
length allows full use of the mounting-hole threads, while avoiding
damaging or placing unwanted stress on the PCB. To avoid stripping
the mounting-hole threads, the maximum torque applied to the screws
must not exceed 8 inch-pounds.
Ventilation
-----------
The ProDrive 425S hard disk drive operates without a cooling
fan, provided the ambient air temperature does not exceed 131*F
(55*C). Any user-designed cabinet should provide adequate air
circulation to prevent the maximum temperature from being exceeded.
J3 DC Power Connector
---------------------
The DC power connector (J3) is a 4-pin Ranoda Molex connector 8981-
4SM3 or equivalent, mounted on the back edge of the Printed-Circuit
Board (PCB).
+----------+
|4 3 2 1|
+----------+
Recommended connectors and their numbers:
4-pin connector: AMP P/N 1-480424-0 or equivalent
Loose-piece contacts: AMP P/N 60619-4 or equivalent
Strip contacts: AMP P/N 61117-4 or equivalent
pin 1 +12 VDC
pin 2 +12 Volts Return (Ground)
pin 3 + 5 Volts Return (Ground)
pin 4 + 5 VDC
Note:
Pins 2 and 3 are connected on the drive.
J2 SCSI Connector
------------------
The SCSI cable connector (J2) on the ProDrive 425S hard disk drive
is a 50-pin universal header.
Note:
A key slot on the header prevents the incorrect installation of the
header connector on the SCSI cable. To prevent the header connector
on the SCSI cable from being plugged in backwards, use only a keyed
header connector.
Recommended header connectors and their part numbers:
50-pin connector (without strain relief) AMP P/N 1-746195-2 or
equivalent
50-pin connector (with strain relief) AMP P/N 1-499508-2 or
equivalent
LED Operation
-------------
Whenever the drive initiates the execution of a command, a green LED
on the faceplate lights, and remains lit until the execution of the
command is complete, disconnectec, or aborted.
Air Filtration
--------------
ProDrive Series hard disk drives are Winchster-type drives. The heads
fly very close to the media surface, with a nominal flying height of
6 microinches. Therefore, it is essential that the air circulating
within the drive be kept free of paticles. Quantum assembles the
drive in a Class-100, purified air environment, then seals the drive
with a metal cover. When the drive is in use, the rotation of the
disks forces the air inside the drive through an internal filter.
SCSI Electrical Characteristics
-------------------------------
All signals are asserted, or true, at the low level, and use open-
collector drivers terminated at each end of the cable with 220 ohms
to +5 volts (nominal) and 330 ohms to ground. On the ProDrive 425S
hard disk drive, resistor networks in sockets terminate the lines.
Note
To connect multiple devices on a SCSI bus, remove the terminating
resistors on all but the two devices at the cable ends.
Each signal driven by the ProDrive 425S hard disk drive has
the following output characteristics:
- True (signal assertion) = 0.0 to 0.5 volts DC at 48 milliamperes
(maximum)
- False (signal deassertion) = 2.5 to 5.25 volts DC
Each signal from the host to the ProDrive 425S hard disk
drive must have the following characteristics, as measured at the
drive:
- True (signal assertion) = 0.0 to 0.8 volts DC
- False (signal deassertion) = 2.5 to 5.25 volts DC
The ProDrive 425S hard disk drive uses a standard 50-pin SCSI-bus
connector (J2) - providing a single-ended, non shielded connection.
The maximum cable length is 20 ft (6 meters). Odd pins are grounded
with the exception of Pin 25, which is not connected.
--------------------------------------------------------------------
DIFFERENTIAL SCSI ADAPTER KIT
-----------------------------
The adapter kit consists of a printed-circuit board (PCB) assembly
with flex circuit, a plastic connector protector, and retainer clip.
+-+------+
8-pin |x| |
Header+-+--+ |Flex Cable
| |
| |
+--+ |
| +--+
| |
+-----------------------------+---+
Drive +----+|XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXDrive SCSI Connector
Power |XXXX||XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX |
Connector++++++|+-------------------------------+|
|||| ||::::::::::::::::*::::::::::::::||Differential SCSI
+--++++-++-------------------------------+|Connector
| xxx+| |
| xxx-+ DIFFERENTIAL PCB |
| |
| XXXXXX System Power Connector |
+-----------------------------------------+
Precautions
-----------
The installation of the differential SCSI adapter kit is simple and
straightforward. However, before installation you should be aware of
the following precautions.
1. Static Electricity. The integrated circuits used on the
differential adapter and the disk drive are sensitive to static
electricity. Just prior to handling the adapter board, be sure to
discharge the static electricity from your body by wearing a
properly grounded wrist strap or touching a grounded object (a
metal water pipe, bare metal part of the system chasis or the
adapter's disk drive housing). Failure to take this precaution,
may damage the drive or the adapter PCB circuitry.
2. Flex Cable Damage. The adapter board comes with a flex cable. The
flex cable is used to connect the adapter kit to the drive. It
should be handled gently.
3. Disk Drive Damage. Disk drives should be handled with care. Be
careful to not subject the drive unnecessary shock or roughness.
This will ensure that you do not cause damage to the drive or its
data.
4. Compatibility. Only the following Quantum drives and adapters
should be used together:
Drive Adapter Kit
ProDrive 425S D_425
Adapter PCB Installation
------------------------
Push the adapter SCSI connector into the mating connector on the
drive. Next, push the 4-pin power connector of the adapter into the
mating connector on the drive.
Removal fo Terminating Resistors
--------------------------------
Remove resistor packs RP10, RP11, RP12.
Flex Cable Installation
-----------------------
------------------------------------------------------+
8-pin |
CONNECT FLEX CABLE**** JP1+-+-++-+-+-+ ++|XX
HERE! **** +-+-+-++-+-+-+ |||XX
|||XX
A flex cable connects the differential SCSI adapter to the PCB. The
free end of the flex cable contains an 8-pin female header. Secure
the flex cable to the underside of the PCB by gently bending the
cable and aligning the four guide holes in the cable with the four
outer pins of RP12, then push the cable so that the pins go through
the holes snugly. This can be accomplished by using a pair of
tweezers or similar tool. Plug the header at the end of the flex
cable should be facing upwards (the flex cable side goes onto the
header pin first). In order to keep the flex cable routed properly,
use the pressure-sensitive adhesive strips attached to the flex
cable. (Before attaching the flex cable to the drive remove the
protective covering on the pressure-sensitive adhesive strips.)
Secure the flex cable to the tops of the two large ICs (U21, U10).
Connector Protector and Retainer Clip Attachment
------------------------------------------------
With the adapter board securely connected to the drive, and with the
flex cable installed, cover the soldered ends of the upper two rows
of connector pins on the adapter PCB with the plastic protector
(groove side facing out). Next, attach the retainer clip (it holds
the protector securely to the adapter PCB). To do this, first hook
the left end of the clip into the 50-pin connector. Then, holding the
clip near the right end with needle-nose pliers, "wrap" the clip
around the connector protector and hook the right end of the clip
into the end of the connector.
Drive Cable Connection
----------------------
The system differential SCSI cable and power cables can now be
connected. Insert them into the appropriate connectors on the
adapter.
This complete the installation. The drive is now ready to receive
power and to operate as a differential SCSI device.
Differential Adapter Board
--------------------------
The differential SCSI adapter board consists of a small PCB that
translates all SCSI-bus signals coming from the drive single-ended to
differential, and translates signals to the drive from differential
to single-ended.
The adapter board contains two 50-pin connectors. The single-ended
female SCSI connector (P1) mates to the matching 50-pin connector on
the drive. The differential male SCSI connector (J2) provides for
connection to a differential host system.
System power is relayed to the drive by means of two 4-pin power
connectors. P3 is a 4-pin female connector that attaches to the
power connector on the drive, and J3 is a 4-pin male connector that
receives power from the system.
A flexible cable connects 5 proprietary control and 3 drive ID lines
from the PCB on the main drive to the differential adapter board
through connector P6.
Cable Requirements
------------------
Ordinary 50-pin flat cable (non-twisted pair) is not recommended for
use in a differential SCSI system due to excessive crosstalk at very
short distances. Either twisted-pair flat cable or 25 twisted-pair
regular cable (non-flat) having the following characteristics is
recommended:
Characteristic impedance: 90 to 132 ohms
Signal attenuation: 0.095 dB maximum per meter @ 5 MHz
Pair to pair propagation delta: 0.20 ns maximum per meter
DC resistance: 0.230 ohms maximum per meter
Total cable length: 25 meters maximum
Stub length: 0.2 meters maximum
Mixing cable types on the same SCSI bus - such as twisted-flat to
twisted-round twisted-flat - is specifically discouraged due to the
resultant impedance discontinuities and signal reflections.
Difference Fron Single-Ended Operation
--------------------------------------
The differential signals found in the operation of the differential
SCSI adapter differ from normal single-ended SCSI signals. The
signals in a differential SCSI-bus can assume three different states
- true, false, and released. A signal is false when +SIGNAL is
asserted (high) and -SIGNAL is deasserted (low). A signal is false
when +SIGNAL is deasserted (low) and -SIGNAL is asserted (high). A
signal is released when both +SIGNAL and -SIGNAL are neither
asserted nor deasserted (tri-state).
Also, in a differential SCSI-bus, there are no "OR-tied" signals, as
there are in a single-ended SCSI bus. Furthermore, under no
circumstances may a device drive any signal to a state other than
that to which it is already being driven by another device.
NOTE
Going from single-ended to differential causes no degradation in data
transfer rates.
Interface Characteristics
-------------------------
All active signals on the differential SCSI interface consist of two
lines designated +SIGNAL and -SIGNAL, where SIGNAL is the name of a
particular line. A signal is true when +SIGNAL is a minimum of 200
millivolts more positive than -SIGNAL. A signal is false when -SIGNAL
is a minimum of 200 millivolts more positive than +SIGNAL. All
signals should be terminated at the end of the cable with a 150 ohm
resistor between +SIGNAL and -SIGNAL, a 330 ohm resistor between
-SIGNAL and +5V (TERMPWR), and a 330 ohm resistor between +SIGNAL and
ground.
The differential adapter board has no provision for internal
termination. The DIFFSENS signal on the connector is used solely for
the purpose of automatically disabling the differential drivers
should a single-ended cable be inadvertently connected to the differ-
ential adapter board.
Each pair of differential signals driven by the adapter board has the
following characteristics:
Signal assertion: 0 to 1.7 VDC @ 55 mA (maximum)
Signal deassertion: 2.7 to 5.25 VDC @ 55 mA (maximum)
Differential voltage: 1.0 VDC (min.) with a common mode voltage
range of -7 to +12 VDC
Also, all outputs conform to the EIA RS-485-1983.1 standard for
balanced digital multipoint communication.
Each pair of differential signals driven by the host system must have
the following characteristics:
Signal assertion: 0 to 1.7 VDC
Signal deassertion: 2.7 to 5.25 VDC
Differential voltage: 1.0 VDC (min.) with a common mode voltage
range of -7 to +12 VDC
**********************************************************************
F E A T U R E S
**********************************************************************
QUANTUM PRODRIVE 425S PRODUCT MANUAL 81100377-01, FEBRUARY 1992
General Description
-------------------
The Quantum ProDrive Series is a broad family of high-performance,
3 1/2-inch form factor hard drives manufactured to meet the highest
product-quality standards. These drives use nonremovable rigid disks
as their storage media, have formatted capacities that range from
40 to 426 megabytes, and are available with various interfaces.
Indroduction
------------
The ProDrive 425S intelligent hard disk drive features the Small
Computer System Interface 2 (SCSI-2) and use powerful SCSI commands
to optimize system performance. Because the drives manage defects and
error recovery internally, these operations are fully transparent to
the user.
NOTE
As defined by Quantum a megabyte is 1,000,000 bytes.
Reliability
-----------
Mean Time Between Failures (MTBF): 150,000 Power On Hours (POH),
Preventive Maintenance (PM): Not required
Mean Time To Repair (MTTR): 30 minutes
Start/Stop: 10,000 cycles (minimum)
Seek Times/Miscellaneous Times:
------------------------------
Times | Typical | Nominal | Maximum |
msec. | Nominal | Condition | Worst-Case |
| Condition | | Condition |
-----------------------+-----------+-----------+------------+
Single Track Seek | 4 | 5 | 5 |
Average Seek | 14 | 15.5 | 16.5 |
Read-on Arrival | 13 | 14.5 | 15.5 |
Third-Stroke Seek | 15 | 16.5 | 17.5 |
Full-Stroke Seek | 27 | 30 | 33 |
Average Rotational | | | |
Latency | 8.3 | --- | --- |
Sequential Head Switch | 4 | --- | --- |
Power-Up Time | 20 | 20 | 20 |
-----------------------+-----------+-----------+------------+
Seek time is the time required for the actuator to seek and settle on
track. Quoted seek times include head settling time, but not command
overhead or rotational latency time.
Seek time is measured by averaging 1000 seeks of the indicated
length. Average seek time is the average time of 1000 random seeks.
When a seek error occurs, recovery for that seek may take up to two
seconds.
Media Quality
-------------
The ProDrive 425S hard disk drive features automatic defect
management, making the manual identification of defects unneccessary.
Defect management is completely transparent to the user.
Track Format
------------
Zone |User Sec./Cyl. MB/Sec.
------------+-------------------------
Inner 7| 394 1.53
6| 439 1.70
5| 484 1.87
4| 529 2.05
3| 574 2.22
2| 610 2.36
1| 655 2.53
Outer 0| 700 2.71
Error Rates
-----------
A data error occurs whenever the drive fails to read or write a
sector of data correctly. An error detected when reading may be
correctable or uncorrectable. When a data error occurs, if the data
can be transferred successfully using the drive's default error-
recovery algorithm, the data error is recoverable; otherwise, it is
unrecoverable. The miscorrection rate indicate the frequency with
which the drive transfers incorrect read data, due to faulty ECC
correction using the default error-recovery algoritm.
If a data error recurs on each of eight retries, it is a read error;
otherwise it is a soft error. Hard errors generally occur due to
media defects.
Defect Detection
----------------
At sector level, any discrepancy between recorded data and recovered
data constitutes a data error. In high-density digital recording
systems, it is necessary to use an error-detection and correction
scheme to enhance the performance and increase the reliability of the
system.
Data errors are either soft - correctable on retries and not readily
repeatable - or hard - repeatable with high probability. The
signal-to-noise ratio of the system affects the occurrence of soft
errors, which represent marginal conditions of the media, heads,
read/write circuitry. Hard errors occur due to defects, pits,
scratches, or thin spots in the media. The drive can detect and skip
defective sectors on the media. Thus, the drive will not use
defective sectors for storage.
Factory-Found Defects
---------------------
At the factory, Quantum scan the disks for defective sectors and
deallocates sectors containing defects prior to shipment. The drive
records a primary defect list that contains the locations of all
factory-found defective sectors in a reserved cylinder on the disk.
The defects are spared in-line. In-line sparing skips each factory-
found defects and shifts subsequent sectors on the track to
sequential sectors. Two sectors at the end of each cylinder are
spares, or replacement sectors. The drive allocates spare sectors as
required to maintain the proper number of blocks per cylinder.
Acoustics
---------
Idle Mode: 45 dBA maximum, at 1 foot in any direction
Power Sequencing
----------------
If power is applied or removed during drive operation, no data will
be lost or damaged, with the powwible exception that data may be lost
in the sector being written at the time of the power loss. During
power-up or power-down, the drive can sustain transient voltages be-
tween +10% and -100% of the nominal voltage, and can sustain the
shorting out or opening up to either voltage or return line.
Power Reset Limit
-----------------
The following power resets limits are measured at the drive power
connector:
+5V 4.65 V
+12V 10.50 V
While powering up, the drive remains reset, or inactive, until the
voltages applied exceed both power reset limits. The drive becomes
reset when either supply voltage drops below the lower threshold. The
minimum hysteresis is 50 mV.
Automatic Actuator Lock
-----------------------
To ensure data integrity and prevent damage during shipment, all
ProDrive Series hard disk drives feature a dedicated landing zone and
Quantum's patented AIRLOCK. AIRLOCK, locks the headstack in the
landing zone. It consists of an airvane mounted near the perimeter of
the disk stack and a locking arm that restrains the actuator arm
assembly when the disks are not rotating.
When DC power is applied to the motor and the disk stack rotates, the
rotation generates an airflow on the surface of the disk. As the flow
of air across the airvane increases with disk ratation, the locking
arm pivots away from the actuator arm, enabling the headstack to move
out of the landing zone. When DC power is removed from the motor, a
return mechanism automatically pulls the actuator into the landing
zone, where the AIRLOCK holds it in place.
DisCache
--------
Quantum's proprietary, look-ahead, caching scheme - provides faster
access to data on the disk by anticipating host-system requests for
data. DisCache prefetches data into a three segment cache in high-
speed RAM (Random-Access Memory). When the host requests data pre-
fetched to the cache, the drive accesses the cache, rather than the
disk. The ProDrive 425S hard disk drive uses an advanced continuous-
prefetch algorithm. When the drive receives a READ command that re-
quests data currently in the cache, it continues to prefetch data.
The 64 kilobyte segment containing the oldest data is the segment
that gets updated with new read data.
