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Barracuda 9 Product Manual, Rev. C 11 4.3 Start/stop time Disabling the Motor Start option causes the drive to become ready within 30 seconds after DC power is applied at nominal voltage. This means the motor starts as soon as power is applied. If a recoverable error condition is detected during the start sequence, the drive will execute a recovery procedure and may cause the time to become ready to be as much as 60 seconds. During this time the drive responds to some commands over the SCSI interface. Stop time is less than 30 seconds (maximum) from removal of DC power. Enabling the Motor Start option causes the internal controller to accept the commands listed in the SCSI Interface Product Manual (77738479) less than 3 seconds after applying DC power. After receiving the Motor Start command, the drive becomes ready for normal operations within 30 seconds (excluding an error recovery procedure). The Motor Start command can also be used to command the drive to stop the spindle in less than 30 seconds (see the Start/Stop command information in the SCSI Interface Product Manual). There is no power control switch on the drive. 4.4 Prefetch/multi-segmented cache control The drive provides prefetch (read look-ahead) and multi-segmented cache control algorithms that in many cases can enhance system performance. "Cache" as used herein refers to the drive buffer storage space when it is used in "cache" operations. To select prefetch and cache features the host sends the Mode Select command with the proper values in the applicable bytes in Mode Page 08h (see SCSI Interface Product Manual). Prefetch and cache operation are independent features from the standpoint that each is enabled and disabled independently via the Mode Select command. However, in actual operation the prefetch feature overlaps cache operation somewhat as is noted in Section 4.5.1 and 4.5.2. All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Table 9. 4.5 Cache operation Of the 512 Kbytes physical buffer space, approximately 442 Kbytes can be used as a cache. If you have the 2,048 Kbyte buffer space option, approximately 1,915 Kbytes can be used as a cache. The cache can be divided into logical segments (Mode Select page 08h, byte 13) from which data is read and to which data is written. The drive keeps track of the logical block addresses of the data stored in each segment of the cache. If the cache is enabled (see RCD bit = 0 in Mode page 08h, byte 2, bit 0 in the SCSI Interface Product Manual), data requested by the host with a read command is retrieved from the cache, if possible, before any disc access is initiated. If cache operation is not enabled, the buffer (still segmented with required number of segments) is still used, but only as circular buffer segments during disc medium read operations (disregarding Prefetch operation for the moment). That is, the drive does not check in the buffer segments for the requested read data, but goes directly to the medium to retrieve it. The retrieved data merely passes through some buffer segment on the way to the host. On a cache "miss," all data transfers to the host are in accordance with "buffer-full" ratio rules. On a cache "hit," the drive ignores the "buffer-full" ratio rules. See explanations associated with Mode page 02h (disconnect/reconnect control) in the SCSI Interface Product Manual. The following is a simplified description of a read operation with cache operation enabled: Case A - Read command is received and the first logical block is already in the cache. 1. Drive transfers to the initiator the first logical block requested plus all subsequent contiguous logical blocks that are already in the cache. This data may be in multiple segments. 2. When a requested logical block is reached that is not in any segment, the drive fetches it and any remaining requested logical block addresses from the disc and puts them in a segment of the cache. The drive transfers the remaining requested logical blocks from the cache to the initiator in accordance with the "buffer-full" ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h (see the SCSI Interface Product Manual). 3. The drive prefetches additional logical blocks contiguous to those transferred in step 2 above and stores them in the segment. The drive stops filling the segment when the maximum prefetch value has been transferred (see the SCSI Interface Product Manual).

Section	Page
Barracuda 9 Disc Drive	1
ST19171N/W/WD/WC/DC	1
Product Manual, Volume 1	1
Barracuda 9 Disc Drive	3
ST19171N/W/WD/WC/DC	3
Product Manual, Volume 1	3
Revision status summary sheet	5
Contents	7
1.0 Scope 1	7
2.0 Applicable standards and reference documentati...	7
2.1 Standards 3	7
2.1.1 Electromagnetic compatibility 3	7
2.1.2 Electromagnetic susceptibility 3	7
2.2 Electromagnetic compliance 3	7
2.3 Reference documents 4	7
3.0 General description 5	7
3.1 Standard features 6	7
3.2 Media characteristics 7	7
3.3 Performance 7	7
3.4 Reliability 7	7
3.5 Unformatted and formatted capacities 7	7
3.6 Programmable drive capacity 7	7
3.7 Factory installed accessories 7	7
3.8 Options (factory installed) 8	7
3.9 Accessories (user installed) 8	7
4.0 Performance characteristics 9	7
4.1 Internal drive characteristics (transparent to...	7
4.2 SCSI seek performance characteristics (visible...	7
4.2.1 Access time 9	7
4.2.2 Format command execution time 9	7
4.2.3 General performance characteristics 9	7
4.3 Start/stop time 11	7
4.4 Prefetch/multi-segmented cache control 11	7
4.5 Cache operation 11	7
4.5.1 Caching write data 12	7
4.5.2 Prefetch operation 12	7
5.0 Reliability specifications 15	7
5.1 Error rates 15	7
5.1.1 Environmental interference 15	7
5.1.2 Read errors 15	7
5.1.3 Write errors 15	7
5.1.4 Seek errors 16	7
5.2 Reliability and service 16	7
5.2.1 Mean time between failure 16	7
5.2.2 Preventive maintenance 16	7
5.2.3 Service life 16	7
5.2.4 Service philosophy 16	7
5.2.5 Service tools 16	7
5.2.6 Hot plugging Barracuda 9 disc drives 17	7
5.2.7 S.M.A.R.T. 17	7
5.2.8 Product warranty 18	7
6.0 Physical/electrical specifications 21	7
6.1 AC power requirements 21	7
6.2 DC power requirements 21	7
6.2.1 Conducted noise immunity 22	7
6.2.2 Power sequencing 22	7
6.2.3 12 V current profile 22	7
6.3 Power dissipation 23	7
6.4 Environmental limits 23	7
6.4.1 Temperature 23	7
6.4.2 Relative humidity 27	7
6.4.3 Effective altitude (sea level) 27	8
6.4.4 Shock and vibration 27	8
6.4.5 Air cleanliness 29	8
6.4.6 Acoustics 29	8
6.4.7 Electromagnetic susceptibility 29	8
6.5 Mechanical specifications 30	8
7.0 Defect and error management 33	8
7.1 Drive internal defects 33	8
7.2 Drive error recovery procedures 33	8
7.3 SCSI systems errors 34	8
8.0 Installation 35	8
8.1 Drive ID/option select header 35	8
8.1.1 Notes for Figures 9, 10, and 11 39	8
8.1.2 Function description 40	8
8.2 Drive orientation 41	8
8.3 Cooling 41	8
8.3.1 Air flow 41	8
8.4 Drive mounting 42	8
8.5 Grounding 42	8
9.0 Interface requirements 43	8
9.1 General description 43	8
9.2 SCSI interface messages supported 43	8
9.3 SCSI interface commands supported 44	8
9.3.1 Inquiry data 47	8
9.3.2 Mode Sense data 47	8
9.4 SCSI bus conditions and miscellaneous features...	8
9.5 Synchronous data transfer 52	8
9.5.1 Synchronous data transfer periods supported ...	8
9.5.2 REQ/ACK offset 52	8
9.6 Physical interface 52	8
9.6.1 DC cable and connector 52	8
9.6.2 SCSI interface physical description 55	8
9.6.3 SCSI interface cable requirements 55	8
9.6.4 Mating connectors 57	8
9.7 Electrical description 69	8
9.7.1 Single-ended drivers/receivers 69	8
9.7.2 Differential drivers/receivers 70	8
9.8 Terminator requirements 72	8
9.9 Terminator power 72	8
9.10 Disc drive SCSI timing 73	8
10.0 Seagate technical support services 75	8
List of Figures	9
Figure 1. Barracuda 9 disc drive (ST19171N drive s...	9
Figure 2. Barracuda 9 family drive 6	9
Figure 3. Typical Barracuda 9 drive +5 V and +12 V...	9
Figure 4a. Location of PCB components listed in Ta...	9
Figure 4b. Location of PCB components listed in Ta...	9
Figure 5. Recommended mounting 28	9
Figure 6. Mounting configuration dimensions for “N...	9
Figure 7. Mounting configuration dimensions for “W...	9
Figure 8. Mounting configuration dimensions for “W...	9
Figure 9. ST19171N option select jumper connectors...	9
Figure 10. ST19171W/WD option select jumper connec...	9
Figure 11. ST19171WC/DC option select jumper conne...	9
Figure 12. Suggested air flow 41	9
Figure 13. Physical interface for “N” model drives...	9
Figure 14. Physical interface for “W” and “WD” mod...	9
Figure 15. Physical interface for “WC” and “DC” mo...	9
Figure 16. SCSI daisy-chain interface cabling 59	9
Figure 17. Non-shielded 50-pin SCSI device connect...	9
Figure 18. Non-shielded 68-pin SCSI device connect...	9
Figure 19. Non-shielded 80-pin SCSI connector, use...	9
Figure 20. Single-ended transmitters and receivers...	9
Figure 21. Typical differential I/O line transmitt...	9
1.0 Scope	11
Figure 1. Barracuda 9 disc drive (ST19171N drive s...	11
2.0 Applicable standards and reference documentati...	13
2.1 Standards	13
2.1.1 Electromagnetic compatibility	13
2.1.2 Electromagnetic susceptibility	13
2.2 Electromagnetic compliance	13
Electromagnetic compliance for the European Union	13
Australian C-Tick	14
2.3 Reference documents	14
3.0 General description	15
Table 1: Drive model number vs. differentiating fe...	15
Figure 2. Barracuda 9 family drive	16
3.1 Standard features	16
3.2 Media characteristics	17
3.3 Performance	17
Note.	17
[1] Some host adapter companies support the term “...	17
3.4 Reliability	17
3.5 Unformatted and formatted capacities	17
Notes.	17
[1] Standard OEM models are formatted to have 512-...	17
[2] The number of data tracks per sparing zone and...	17
3.6 Programmable drive capacity	17
3.7 Factory installed accessories	17
3.8 Options (factory installed)	18
3.9 Accessories (user installed)	18
4.0 Performance characteristics	19
4.1 Internal drive characteristics (transparent to...	19
[1] Rounded off values	19
4.2 SCSI seek performance characteristics (visible...	19
4.2.1 Access time [7]	19
4.2.2 Format command execution time	19
4.2.3 General performance characteristics	19
Notes for Sections 4.2.	20
[1] Execution time is measured from receipt of the...	20
[2] Typical access times are measured under nomina...	20
[3] Assumes no errors and no sector reallocations....	20
[4] Rate measured from the start of the first sect...	20
[5] Assumes system ability to support the rates li...	20
[6] Assumes system ability to support the rates li...	20
[7] Access time = controller overhead + average se...	20
4.3 Start/stop time	21
4.4 Prefetch/multi-segmented cache control	21
4.5 Cache operation	21
1. Drive transfers to the initiator the first logi...	21
2. When a requested logical block is reached that ...	21
3. The drive prefetches additional logical blocks ...	21
1. The drive fetches the requested logical blocks ...	22
2. The drive prefetches additional logical blocks ...	22
4.5.1 Caching write data	22
4.5.2 Prefetch operation	22
5.0 Reliability specifications	25
Note.	25
[1] Error rate specified with automatic retries an...	25
5.1 Error rates	25
5.1.1 Environmental interference	25
5.1.2 Read errors	25
5.1.3 Write errors	25
5.1.4 Seek errors	26
5.2 Reliability and service	26
5.2.1 Mean time between failure	26
5.2.2 Preventive maintenance	26
5.2.3 Service life	26
5.2.4 Service philosophy	26
5.2.5 Service tools	26
5.2.6 Hot plugging Barracuda 9 disc drives	27
1. Configure the drive with no connection between ...	27
2. Ensure that all SCSI devices on the bus have re...	27
3. Eliminate all I/O processes for the drive.	27
4. Wait until the drive motor and discs have come ...	27
5. Insert or remove the drive after meeting the fo...	27
a. If you are inserting the drive, connect its pow...	27
b. If you are removing the device, maintain its po...	27
c. You may simultaneously switch the power to the ...	27
d. Ensure that the drive carrier discharges all st...	27
5.2.7 S.M.A.R.T.	27
Controlling S.M.A.R.T.	27
Performance impact	28
Reporting control	28
Determining rate	28
Predictive failures	28
5.2.8 Product warranty	28
Shipping	28
Product repair and return information	29
6.0 Physical/electrical specifications	31
6.1 AC power requirements	31
6.2 DC power requirements	31
Table 2: DC power requirements	31
[1] Measured with average reading DC ammeter. Inst...	31
[2] A –10% droop is permissible during initial sta...	31
[3] See +12V current profile in Figure 3.	31
[4] This condition occurs when the Motor Start Opt...	31
[5] See Section 6.2.1 “Conducted Noise Immunity.” ...	31
[6] At power-up, the motor current regulator limit...	31
[7] Operating condition is defined as a third-stro...	31
[8] No terminator power. See Section 9.9.	31
[9] Track following at track 0.	32
[10] Seeking is defined as a third-stroke seek at ...	32
[11] Read track is defined as repeat reads of trac...	32
1. Minimum current loading for each supply voltage...	32
2. The +5 and +12 volt supplies shall employ separ...	32
3. Where power is provided to multiple drives from...	32
6.2.1 Conducted noise immunity	32
6.2.2 Power sequencing	32
6.2.3 12 V current profile	32
Figure 3. Typical Barracuda 9 drive +5 V and +12 V...	33
6.3 Power dissipation	33
ST19171N/W/WC	33
ST19171WD/DC	33
6.4 Environmental limits	33
6.4.1 Temperature	33
a. Operating	33
Table 3: PCB and HDA temperatures	34
Notes.	34
[1] Section 8.3.1 describes the air-flow patterns ...	34
[2] The temperatures in Column 1 are calculated an...	34
[3] Measure HDA temperature at point labeled “HDA ...	34
[4] PCB mounted integrated circuit case.	34
Figure 4a. Location of PCB components listed in Ta...	35
Figure 4b. Location of PCB components listed in Ta...	36
b. Non-operating	37
6.4.2 Relative humidity	37
a. Operating	37
b. Non-operating/transit	37
6.4.3 Effective altitude (sea level)	37
a. Operating	37
b. Non-operating	37
6.4.4 Shock and vibration	37
6.4.4.1 Shock	37
a. Operating—normal	37
b. Operating—abnormal	37
c. Non-operating	37
d. Packaged	38
Figure 5. Recommended mounting	38
6.4.4.2 Vibration	38
a. Operating—normal	38
b. Operating—abnormal	38
c. Non-operating	39
6.4.5 Air cleanliness	39
6.4.6 Acoustics	39
6.4.7 Electromagnetic susceptibility	39
6.5 Mechanical specifications	40
Figure 6. Mounting configuration dimensions for “N...	40
Figure 7. Mounting configuration dimensions for “W...	41
Figure 8. Mounting configuration dimensions for “W...	42
7.0 Defect and error management	43
7.1 Drive internal defects	43
7.2 Drive error recovery procedures	43
Table 4: Read and write retry count maximum recove...	44
[1] Setting these retry counts to a value below th...	44
7.3 SCSI systems errors	44
8.0 Installation	45
Configure drive options	45
Formatting	45
8.1 Drive ID/option select header	45
Figure 9. ST19171N option select jumper connectors...	46
Figure 10. ST19171W/WD option select jumper connec...	47
Figure 11. ST19171WC/DC option select jumper conne...	48
8.1.1 Notes for Figures 9, 10, and 11	49
[1] Notes explaining the functions of the various ...	49
[2] The PCB on “N,” “WC,” and “DC” model drives do...	49
[3] Reserved useage. Do not install any jumpers.	49
[4] Table 5 summarizes the configuration selection...	49
[5] These signals are also on 80 pin J1. See Table...	49
[6] Voltage supplied by the drive.	49
Table 5: Drive configuration selections summary	49
Notes for Table 5 []:	49
[a] Use either J6 or J1-Aux, but not both.	49
[b] I/O connector J1 plugs directly into host. No ...	49
[c] The host can drive a remotely located Drive Ac...	49
[d] Use either J1 or J6, but not both.	49
[e] The drive reads the ID (asserted low) from J1-...	49
[f] The host selects drive ID through J1.	49
8.1.2 Function description	50
8.2 Drive orientation	51
8.3 Cooling	51
8.3.1 Air flow	51
Figure 12. Suggested air flow	51
8.4 Drive mounting	52
8.5 Grounding	52
9.0 Interface requirements	53
9.1 General description	53
9.2 SCSI interface messages supported	53
Table 6: SCSI messages supported by Barracuda 9 fa...	53
Notes.	53
[1] Extended message (refer to the SCSI Interface ...	53
[2] Not applicable to “N” models.	53
9.3 SCSI interface commands supported	54
Table 7: Commands supported by Barracuda 9 family ...	54
[1] The drive can format to any even number of byt...	56
[2] Table 9 shows how individual bits are set that...	56
[3] Warning: A power loss during flash programming...	56
9.3.1 Inquiry data	57
Table 8: Barracuda 9 family drive Standard Inquiry...	57
Notes.	57
9.3.1.1 Inquiry Vital Product Data pages	57
9.3.2 Mode Sense data	57
Definitions:	59
Table 9: Mode Sense data, ST19171 default values (...	59
[1] Though byte 12, bit 7 is shown as changeable, ...	60
[2] Default and saved values for page 10h, bytes 6...	60
9.4 SCSI bus conditions and miscellaneous features...	61
Table 10: SCSI bus conditions and other miscellane...	61
9.5 Synchronous data transfer	62
9.5.1 Synchronous data transfer periods supported	62
Table 11: Synchronous data transfer periods suppor...	62
[1] Fast-20 (Ultra SCSI) transfer rates.	62
9.5.2 REQ/ACK offset	62
9.6 Physical interface	62
9.6.1 DC cable and connector	62
Figure 13. Physical interface for “N” model drives...	63
Figure 14. Physical interface for “W” and “WD” mod...	64
Figure 15. Physical interface for “WC” and “DC” mo...	64
9.6.2 SCSI interface physical description	65
9.6.3 SCSI interface cable requirements	65
9.6.3.1 Single-ended I/O circuits (“N,” “W,” and “...	66
Table 12: Cable characteristics for single-ended c...	66
Notes:	66
[1] The spacing of devices on the mainline SCSI bu...	66
9.6.3.2 Differential I/O circuits (“WD” and “DC” m...	66
9.6.4 Mating connectors	67
9.6.4.1 Mating connectors for ST19171N model	67
[1] See Figure 16.	67
9.6.4.2 Mating connectors for ST19171W and ST19171...	67
[1] See Figure 16.	67
9.6.4.3 Mating connectors for ST19171WC and ST1917...	67
[1] Closed-end type 50-pin connector used. Install...	69
[2] Open-end type (in-line application) connector ...	69
[3] Host need not be on the end of the daisy chain...	69
[4] Total interface cable length must not exceed t...	69
[5] SCSI ID7 has highest arbitration priority, ID ...	69
[6] Last drive on daisy chain.	69
[7] Open-end type 68-pin connector used. Terminato...	69
Figure 16. SCSI daisy-chain interface cabling	69
Figure 17. Non-shielded 50-pin SCSI device connect...	70
Figure 18. Non-shielded 68-pin SCSI device connect...	71
Figure 19. Non-shielded 80-pin SCSI connector, use...	72
Table 13: Single-ended signal/contact assignments ...	73
Table 14: Single-ended wide cable assignments for ...	74
Table 15: Wide differential P cable assignments (n...	75
Notes [ ]: See page following Table 17.	75
Table 16: Wide, single connector, single-ended sig...	76
Notes [ ]: See page following Table 17.	76
Table 17: Wide, single connector, differential sig...	77
Notes [ ]: See page following this Table.	77
Notes [] for Tables 13 through 17:	78
[1] See Section 9.7.1 for detailed electrical char...	78
[2] The conductor number refers to the conductor p...	78
[3] Connector contacts are on 0.050 inch (1.27 mm)...	78
[4] Front panel LED signal; indicates drive activi...	78
[5] Asserted by host to enable Motor Start option ...	78
[6] Asserted by host to enable Delayed Motor Start...	78
[7] Binary code on A3, A2, A1, and A0 asserted by ...	78
[8] GND provides a means for differential devices ...	78
[9] Signals [4] through [7] are used in place of i...	78
[10] “NC” means no connection.	78
[11] The conductor number refers to the conductor ...	78
[12] Connector contacts are on 0.100 inch (2.54 mm...	78
[13] 8 bit devices which are connected to the 16 d...	78
9.7 Electrical description	79
9.7.1 Single-ended drivers/receivers	79
Transmitter characteristics	79
Receiver characteristics	79
Figure 20. Single-ended transmitters and receivers...	79
Notes.	79
[1] Part of active terminator circuits. Non-remova...	79
[2] ANSI SCSI compatible circuits.	79
[3] Total interface cable length should not exceed...	79
[4] Source of drive terminator power is an active ...	79
[5] Interface signal levels and logical sense at t...	79
9.7.2 Differential drivers/receivers	80
Notes.	81
[1] Positive logic enables transmitter (+5 V = ass...	81
[2] Negative logic signal (0 V = asserted).	81
[3] Total interface cable length should not exceed...	81
[4] I/O line terminators. If SCSI device is a Seag...	81
[5] Arrangements for connecting terminator power t...	81
[6] SCSI I/O line (pin 21) disables I/O circuits i...	81
[7] SCSI I/O cable ground. See Tables 15 and 17.	81
Figure 21. Typical differential I/O line transmitt...	81
9.8 Terminator requirements	82
ST19171N and ST19171W drives	82
ST19171WC and ST19171DC drives	82
ST19171WD drives	82
9.9 Terminator power	82
ST19171N and ST19171W drives	82
1. Drive accepts terminator power through SCSI bus...	82
2. Drive supplies power to the SCSI bus.	82
3. Drive provides terminator power for optional in...	82
4. Drive provides power to its own terminators and...	82
ST19171WD drives	82
ST19171WC and ST19171DC drives	82
9.10 Disc drive SCSI timing	83
Table 18: Disc drive SCSI timing	83
Notes.	84
[1] See the SCSI Interface Product Manual, part...	84
[2] Maximum SCSI asynchronous interface transfer r...	84
[3] Synchronous Transfer Period is determined by n...	84
10.0 Seagate technical support services	85
Index	87
Numerics	87
A	87
B	87
C	87
D	87
E	88
F	88
G	88
H	88
I	89
J	89
L	89
M	89
N	90
O	90
P	90
Q	90
R	90
S	91
T	91
U	92
V	92
W	92
X	92

Match case Limit results 1 per page

Barracuda 9 Product Manual, Rev. C

4.3

Start/stop time

Disabling the Motor Start option causes the drive to become ready within 30 seconds after DC power is applied

at nominal voltage. This means the motor starts as soon as power is applied. If a recoverable error condition is

detected during the start sequence, the drive will execute a recovery procedure and may cause the time to

become ready to be as much as 60 seconds. During this time the drive responds to some commands over the

SCSI interface. Stop time is less than 30 seconds (maximum) from removal of DC power.

Enabling the Motor Start option causes the internal controller to accept the commands listed in the

SCSI Inter-

face Product Manual

(77738479) less than 3 seconds after applying DC power. After receiving the Motor Start

command, the drive becomes ready for normal operations within 30 seconds (excluding an error recovery pro-

cedure). The Motor Start command can also be used to command the drive to stop the spindle in less than 30

seconds (see the Start/Stop command information in the

SCSI Interface Product Manual).

There is no power control switch on the drive.

4.4

Prefetch/multi-segmented cache control

The drive provides prefetch (read look-ahead) and multi-segmented cache control algorithms that in many

cases can enhance system performance. “Cache” as used herein refers to the drive buffer storage space when

it is used in “cache” operations. To select prefetch and cache features the host sends the Mode Select com-

mand with the proper values in the applicable bytes in Mode Page 08h (see

SCSI Interface Product Manual

Prefetch and cache operation are independent features from the standpoint that each is enabled and disabled

independently via the Mode Select command. However, in actual operation the prefetch feature overlaps

cache operation somewhat as is noted in Section 4.5.1 and 4.5.2.

All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this

drive family are given in Table 9.

4.5

Cache operation

Of the 512 Kbytes physical buffer space, approximately 442 Kbytes can be used as a cache. If you have the

2,048 Kbyte buffer space option, approximately 1,915 Kbytes can be used as a cache. The cache can be

divided into logical segments (Mode Select page 08h, byte 13) from which data is read and to which data is

written.

The drive keeps track of the logical block addresses of the data stored in each segment of the cache. If the

cache is enabled (see RCD bit = 0 in Mode page 08h, byte 2, bit 0 in the

SCSI Interface Product Manual),

data

requested by the host with a read command is retrieved from the cache, if possible, before any disc access is

initiated. If cache operation is not enabled, the buffer (still segmented with required number of segments) is still

used, but only as circular buffer segments during disc medium read operations (disregarding Prefetch opera-

tion for the moment). That is, the drive does not check in the buffer segments for the requested read data, but

goes directly to the medium to retrieve it. The retrieved data merely passes through some buffer segment on

the way to the host. On a cache “miss,” all data transfers to the host are in accordance with “buffer-full” ratio

rules. On a cache “hit,” the drive ignores the “buffer-full” ratio rules. See explanations associated with Mode

page 02h (disconnect/reconnect control) in the

SCSI Interface Product Manual

The following is a simplified description of a read operation with cache operation enabled:

Case A -

Read command is received and the first logical block is already in the cache.

Drive transfers to the initiator the first logical block requested plus all subsequent contiguous logical blocks

that are already in the cache. This data may be in multiple segments.

When a requested logical block is reached that is not in any segment, the drive fetches it and any remain-

ing requested logical block addresses from the disc and puts them in a segment of the cache. The drive

transfers the remaining requested logical blocks from the cache to the initiator in accordance with the

“buffer-full” ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h (see the

SCSI Interface Product Manual).

The drive prefetches additional logical blocks contiguous to those transferred in step 2 above and stores

them in the segment. The drive stops filling the segment when the maximum prefetch value has been

transferred (see the

SCSI Interface Product Manual).