Single channel, multiple head servo writing with embedded head identification

A write drive logic circuit is described. The write driver logic circuit comprises a set of write drivers, with each one of the set of write drivers having an input and an output, such that each write driver is responsive to an input signal applied to the input to provide a write output signal at the output as a function of the input signal. A set of memory devices (e.g. shift registers) is provided, one for each write driver. Each one of the shift registers stores unique head identification information and includes an output to controllably output a signal representative of the unique head identification information. Furthermore, each one of a set of multiplexers includes an output, a first input coupled to a corresponding one of the outputs of the set of shift registers to receive the signal representative of the unique identification information stored in the respective shift register and a second input coupled to a common write data line that transmits a signal representative of preselected information. The output of each one of the set of multiplexers is coupled to the input of a corresponding one of the set of write drivers such that one of the first and second inputs of the multiplexer is selectively applied to the input of the corresponding write driver.

FIELD OF THE INVENTION 
The present invention is directed to disk drives. More particularly, the 
present invention provides a single channel, multiple head servo writing 
system that embeds unique head identification information in servo 
sectors. 
BACKGROUND OF THE INVENTION 
Disk drives are commonly used in workstations, personal computers, laptops 
and other computer systems to store large amounts of data in a form that 
can be made readily available to a user. In general, a disk drive 
comprises a magnetic disk that is rotated by a spindle motor. The surface 
of the disk is divided into a series of data tracks. The data tracks are 
spaced radially from one another across a band having an inner diameter 
and an outer diameter. Each of the data tracks extends generally 
circumferentially around the disk and can store data in the form of 
magnetic transitions within the radial extent of the track on the disk 
surface. Typically, each data track is divided into a number of data 
sectors that store fixed sized data blocks. 
A head includes an interactive element, such as a magnetic transducer, that 
is used to sense the magnetic transitions to read data, or to conduct an 
electrical signal that causes a magnetic transition on the disk surface, 
to write data. 
The magnetic transducer includes a read/write gap that positions the active 
elements of the transducer at a position suitable for interaction with the 
magnetic transitions on the surface of the disk, as the disk rotates. 
In accordance with known disk drive design, the head is electrically 
coupled to a pre-amplifier. During a read operation, electrical signals 
transduced by the transducer from the magnetic transitions on the disk 
surface, are processed by the pre-amplifier and transmitted to a 
read/write channel in the disk drive for eventual transmission to a host 
computer using the disk drive to store data. During a write operation, 
electrical signals representative of data are received by the read/write 
channel from the host computer for transmission to the preamplifier. The 
pre-amplifier includes a write driver electrically coupled to the head 
transducer to transmit the signals corresponding to the data to the head. 
The head is responsive to the signals received from the write driver to 
conduct a current and thereby cause magnetic transitions on the disk 
surface corresponding to the data. 
As known in the art, the magnetic transducer is mounted by the head to a 
rotary actuator arm and is selectively positioned by the actuator arm over 
a preselected data track of the disk to either read data from or write 
data to the preselected data track of the disk, as the disk rotates below 
the transducer. The head structure includes a slider having an air bearing 
surface that causes the transducer to fly above the data tracks of the 
disk surface due to fluid currents caused by rotation of the disk. 
In modern high capacity disk drives, the spindle motor is arranged to mount 
a stack of axially aligned storage disks, with the storage disks in the 
stack being spaced from one another. The use of multiple disks increases 
the total disk surface available for the storage of data. A head stack 
assembly comprises a stack of actuator arms, each mounting a head or a 
pair of heads. The stack of actuator arms is arranged adjacent the slack 
of storage disks with each head being positioned by the respective 
actuator arm over the surface of a corresponding one of the disks. 
Two aspects of conventional disk drive design are position control of the 
heads and address headers for the data sectors recorded in the data 
tracks. The position control is used to accurately position a head over a 
data track for data read or write operations. Address headers are used to 
provide unique identification information for data stored in a particular 
data sector. 
Whenever data are either written to or read from a particular data track, 
the transducer gap of the corresponding head must be centered over the 
centerline of the magnetic transitions of the data track where the data 
are to be written or from where the data are to be read, to assure 
accurate transduction of the transitions representing data. If the head is 
off-center, the head may transduce transitions from an adjacent track. 
A servo system is typically used to control the position of the actuator 
arm to insure that the head is properly centered over the magnetic 
transitions during either a read or write operation. In a known servo 
system, servo position information is recorded on the disk surface itself, 
and periodically read by the head for use in controlling the position of 
the actuator arm. Such a servo arrangement is referred to as an embedded 
servo system. In modern disk drive architectures utilizing an embedded 
servo, each data track is divided into a number of data sectors for 
storing fixed sized data blocks, one per sector, as noted above. In 
addition, associated with the data sectors are a series of servo sectors 
that are generally equally spaced around the circumference of the data 
track. The servo sectors can be arranged between data sectors or arranged 
independently of the data sectors such that the servo sectors split data 
fields of the data sectors, as is well known. 
Each servo sector contains magnetic transitions that are arranged relative 
to a track centerline such that signals derived from the transitions can 
be used to determine head position. For example, the servo information can 
comprise two separate bursts of magnetic transitions, one recorded on one 
side of the track centerline and the other recorded on the opposite side 
of the track centerline. Whenever a head is over a servo sector, the head 
reads each of the servo bursts and the signals resulting from the 
transduction of the bursts are transmitted to, e.g., a microprocessor 
within the disk drive for processing. 
When the head is properly positioned over a track centerline, the head will 
straddle the two bursts, and the strength of the combined signals 
transduced form the burst on one side of the track centerline will equal 
the strength of the combined signals transduced form the burst on the 
other side of the track centerline. The microprocessor can be used to 
subtract one burst value form the other each time a servo sector is read 
by the head. When the result is zero, the microprocessor will know that 
the two signals are equal, indicating that the head is properly 
positioned. 
If the result is other than zero, then one signal is stronger than the 
other, indicating that the head is displaced from the track centerline and 
overlying one of the bursts more than the other. The magnitude and sign of 
the subtraction result can be used by the microprocessor to determine the 
direction and distance the head is displaced from the track centerline, 
and generate a control signal to move the actuator back towards the 
centerline. 
In a conventional disk drive design, each data sector of a data track is 
divided into a number of fields, including an address header field that 
contains magnetic transitions representing unique identification 
information for the specific data stored in the data sector. In this 
manner, the disk drive system can locate and verify the exact data sector 
for any particular block of data that a host computer may require, e.g., 
in a read operation. Among the information stored in an address header 
field is head identification information to uniquely identify the 
particular head of the head stack assembly that is transducing the 
magnetic transitions. During certain types of disk drive failures or error 
conditions, the electronics system in the disk drive is unable to identify 
which particular head is transducing magnetic transitions. The head 
identification information can then be read by the active head and used to 
determine which disk surface is being read. 
Overhead refers to portions of a disk surface that are used to store 
information necessary for the control of the disk drive. Space on a disk 
surface used to store control information is not available to store data, 
and thus reduces the storage capacity of the disk drive. The servo sectors 
and address headers discussed above are examples of overhead. One proposal 
for increasing the storage capacity of a disk drive is referred to as a 
headerless format. In a headerless format, the headers are removed from 
the data fields to reduce overhead and thereby free up additional space on 
the disk surfaces that can then be used to store data. The headers are 
stored in RAM memory available in the disk drive electronics system. 
Careful monitoring of clock signals is relied upon to associate the data 
fields on the disk surface with the complementary headers containing the 
unique identification information. 
In a headerless format, there is a risk that data cannot be located. For 
example, during a failure condition of the type discussed above, the 
electronics system of the disk drive would not be able to identify which 
head is active, and the lack of headers recorded on the disk surface 
leaves the electronics system without a source of unique identification 
information. A solution to this problem is to record the head 
identification information portion of the header within the servo sectors. 
One step in the process of manufacturing a disk drive is a servo writing 
operation when first installed, the disks are blank, and the servo writing 
operation involves the performance of a series of writes to all the disk 
surfaces to record the servo sectors. The most efficient form of servo 
writing involves simultaneous activation of the heads of the head stack 
assembly for parallel writing of servo patterns. Since the disks are 
stacked by the spindle motor in an axially aligned arrangement, the data 
tracks of the disks, as defined by servo position information in the servo 
sectors, can be aligned with one another. Ordinarily, the servo pattern 
used for any particular data track can be common in content to all servo 
sectors of a set of data tracks that are axially aligned with the 
particular data track. Thus, the heads of the head stack assembly can be 
activated in parallel, via an external pre-amp parallel write or an 
internal multi-head write, to simultaneously record servo patterns on the 
aligned set of data tracks, using common servo information input from a 
single servo write channel. 
However, in the headerless format described above, each head must record, 
in addition to the common servo pattern, head identification information 
that is unique to that head. Accordingly, a conventional parallel write is 
not feasible. A serial or staggered write does permit each head to record 
unique identification information, but results in a significant increase 
in servo write time since the servo sectors would be recorded one head at 
a time. Such an increase in servo write time can be seriously detrimental 
to a commercially viable disk drive mass production operation. Thus, there 
is a need for a servo write system that implements a parallel write 
operation for maximum efficiency in a disk drive manufacturing operation, 
while permitting the writing of unique identification information by each 
head. 
SUMMARY OF THE INVENTION 
The present invention provides a servo format and a pre-amplifier circuit 
that can write a common servo pattern with multiple heads of the head 
stack assembly, and also selectively control the heads, at certain times, 
to simultaneously, but separately, write identification information that 
is unique to each head. In accordance with the present invention, unique 
identification information is stored for each head in a manner whereby the 
unique identification information for any particular head can be 
controllably transmitted to that head for writing to a corresponding disk 
surface. In addition, a common write data line is coupled to the heads for 
simultaneous, parallel transmission of common servo information to all of 
the heads for simultaneous writing of servo sectors on the disk surfaces. 
Logic is added to the pre-amplifier to select, at any particular time 
during a servo write operation, either the write data line or the unique 
identification information for input to the heads. When the write data 
line is selected by the logic as an input to the heads, the common servo 
information is input to all of the heads for a simultaneous, parallel 
write of the servo sectors on the disk surfaces. When the unique 
identification information is selected by the logic as an input to the 
heads, the unique identification for each head is input to the respective 
head, and all the heads are activated simultaneously so that each head 
writes its corresponding unique identification information to at least one 
servo sector of the respective disk surface. 
The present invention achieves parallel operation of the heads for fast and 
efficient servo writing, while also accommodating a parallel recording of 
information that varies from head to head. A commercially viable disk 
drive mass production process can be implemented with a servo write 
operation utilizing the present invention to provide servo sectors with 
embedded information to uniquely identify the heads associated with the 
respective disk surfaces. This, in turn, makes a headerless format 
feasible since the unique identification information embedded in the servo 
sectors minimizes the possibility of misplaced data.

DETAILED DESCRIPTION 
Referring now to the drawings, and initially to FIG. 1, there is 
illustrated an example of a disk drive designated generally by the 
reference numeral 20. The disk drive 20 includes a plurality of storage 
disks 22a-d and a plurality of read/write heads 24a-h. Each of the storage 
disks 22a-d is provided with a plurality of data tracks to store user 
data. As illustrated in FIG. 1, one head is provided for each surface of 
each of the disks 22a-d such that data can be read from or written to the 
data tracks of all of the storage disks. It should be understood that the 
disk drive 20 is merely representative of a disk drive system utilizing 
the present invention and that the present invention can be implemented in 
a disk drive system including more or less storage disks. 
The storage disks 22a-d are mounted for rotation by a spindle motor 
arrangement 29, as is known in the art. Moreover, the read/write heads 
24a-h are supported by respective actuator arms 28a-h for controlled 
positioning over preselected radii of the storage disks 22a-d to enable 
the reading and writing of data from and to the data tracks. 
To that end, the actuator arms 28a-h are rotatably mounted on a pin 30 by a 
voice coil motor 32 operable to controllably rotate the actuator arms 
28a-h radially across the disk surfaces. 
Each of the read/write heads is mounted to a respective actuator am 28a-h 
by a flexure element (not shown) and comprises a magnetic transducer 25 
mounted to a slider 26 having an air bearing surface (not shown), all in a 
known manner. As typically utilized in disk drive systems, the sliders 26 
cause the magnetic transducers 25 of the read/write heads 24a-h to "fly" 
above the surfaces of the respective storage disks 22a-d for non-contact 
operation of the disk drive system, as discussed above. When not in use, 
the voice coil motor 32 rotates the actuator arms 28a-h during a contact 
stop operation, to position the read/write heads 24a-h over a respective 
landing zone 58 or 60, where the read/write heads 24a-h come to rest on 
the storage disk surfaces. As should be understood, each of the read/write 
heads 24a-h is at rest on a respective landing zone 58 or 60 at the 
commencement of a contact start operation. 
A printed circuit board (PCB) 34 is provided to mount control electronics 
for controlled operation of the spindle motor 29 and the voice coil motor 
32. The PCB 34 also includes read/write channel circuitry coupled to the 
read/write heads 24a-h, to control the transfer of data to and from the 
data tracks of the storage disks 22a-d. To that end, a pre-amplifier 31 is 
mounted adjacent the voice coil motor 32 to electrically couple the heads 
24a-h to the read/write channel circuitry. The pre-amplifier 31 includes 
an amplification stage to amplify electrical signals transduced by a head 
during a read operation, and a write driver arrangement to transmit a 
current to a head in a write operation. The manner for coupling the PCB 34 
to the various components of the disk drive is well known in the art. 
Referring now to FIG. 2, there is illustrated in schematic form the PCB 34 
and the electrical couplings between the control electronics on the PCB 34 
and the components of the disk drive system described above. A 
microprocessor 35 is coupled to each of a read/write control 36, spindle 
motor control 38, actuator control 40, ROM 42 and RAM 43. In modern disk 
drive designs, the microprocessor can comprise a digital signal processor 
(DSP). The microprocessor 35 sends data to and receives data from the 
storage disks 22a-d via the read/write control 36 and the read/write heads 
24a-h. 
The microprocessor 35 also operates according to instructions stored in the 
ROM 42 to generate and transmit control signals to each of the spindle 
motor control 38 and the actuator control 40. 
The spindle motor control 38 is responsive to the control signals received 
from the microprocessor 35 to generate and transmit a drive voltage to the 
spindle motor 29 to cause the storage disks 22a-d to rotate at an 
appropriate rotational velocity. 
Similarly, the actuator control 40 is responsive to the control signals 
received from the microprocessor 35 to generate and transmit a voltage to 
the voice coil motor 32 to controllably rotate the read/write heads 24a-h, 
via the actuator arms 28a-h, to preselected radial positions over the 
storage disks 22a-d. The magnitude and polarity of the voltage generated 
by the actuator control 40, as a function of the microprocessor control 
signals, determines the radial direction and speed of the read/write heads 
24a-h. 
When data to be written or read from one of the storage disks 22a-d are 
stored on a data track different from the current radial position of the 
read/write heads 24a-h, the microprocessor 35 determines the current 
radial position of the read/write heads 24a-h and the radial position of 
the data track where the read/write heads 24a-h are to be relocated. The 
microprocessor 35 then implements a seek operation wherein the control 
signals generated by the microprocessor 35 for the actuator control 40 
cause the voice coil motor 32 to move the read/write heads 24a-h from the 
current data track to a destination data track at the desired radial 
position. 
When the actuator has moved the read/write heads 24a-h to the destination 
data track, the pre-amplifier 31 is used to couple the head 24a-h over the 
specific data track to be written or read, to the read/write control 36, 
as is generally known in the art. The read/write control 36 includes a 
read channel that, in accordance with modern disk drive design, comprises 
an electronic circuit that detects information represented by magnetic 
transitions recorded on the disk surface within the radial extent of the 
selected data track. As described above, each data track is divided into a 
number of data sectors. 
During a read operation, electrical signals transduced by the head from the 
magnetic transitions of the data sectors are amplified by the 
pre-amplifier 31 and input to the read channel of the read/write control 
36 for processing. The RAM 43 can be used to buffer data read from or to 
be written to the data sectors of the storage disks 22a-d via the 
read/write control 36. The buffered data can be transferred to or from a 
host computer utilizing the disk drive for data storage. 
Referring now to FIGS. 3a-c, there is illustrated an exploded diagram 
showing the format and constituency of a representative data sector of one 
of the data tracks of the disks 22a-d, as used in a prior art, 
conventional disk drive. FIG. 3a represents a portion of a sequence of 
data sectors recorded in the form of magnetic transitions within the 
radial extent of the data track. The data sectors are labeled N.sub.-1, 
N.sub.0, N.sub.1 and N.sub.2. The sequence of data sectors extends around 
the entire circumferential length of the data track. 
FIG. 3.sub.b is an exploded view of the data sector N.sub.0. The data 
sector N.sub.0 is divided into a number of fields. The left most field 
comprises a servo field 100 that contains servo position information, as 
will be described in more detail in respect of FIG. 3c. The servo field 
100 is followed by a sync field 102 containing recorded magnetic 
transitions that are used to synchronize the read and write electronics of 
the read/write control 36 to the frequency of magnetic transitions 
recorded on the disk surface in the following field. 
Following the sync field 102 is an address header field 104 that contains 
magnetic transitions representing unique identification information for 
the specific data stored in the data sector N.sub.0. In this manner, the 
disk drive system can locate and verify the exact data sector for any 
particular block of data that a host computer may require in a read 
operation. Included in the address header field 104 is head identification 
information to specify which one of the heads 24a-h is actually active and 
transmitting signals to or from the pre-amplifier 31. Another sync field 
106 follows the address header field 104. 
Actual data are stored in the next data field 108, which is followed by an 
error correcting field 110. The error correcting field 110 includes 
magnetic transitions representing information that is redundant of the 
data recorded in the data field 108. The error correcting information is 
used by the read/write control 36 to detect and correct errors that may 
occur during a read operation, using known error correcting techniques. A 
sector gap 112 follows the error correcting field 110 to physically 
separate the data sector N.sub.0 from the following data sector N.sub.1. 
Referring now to FIG. 3c, each servo field 100 comprises position 
information that is used to control the radial position of the actuator 
arms 28a-h, e.g., during a read operation. A servo sync field 114 is used 
to synchronize the read and write electronics of the read/write control 36 
to the frequency of magnetic transitions representing position information 
within the servo field 100. A sector mark 116 is a recorded transition 
that is used by the read/write control electronics to determine the 
beginning of the data sector N.sub.0. The read/write control electronics 
uses the sector mark 116 to time the beginning of processing of electric 
signals transduced by the head over data sector N.sub.0. 
A Gray code field 118 follows the sector mark 116. The Gray code field 118 
contains coded information that indicates the track number where data 
sector N.sub.0 is located. This information is used to locate a particular 
data track during a seek operation, as described above, by providing a 
unique identification for each data track on the respective disk surface. 
The Gray code field 118 is followed by a fill field 120 to separate the 
Gray code from the remaining servo field information comprising a servo 
pattern including an A burst 122 and a B burst 124. 
As illustrated in FIG. 3c, the A and B bursts are arranged to straddle the 
centerline of the data sector N.sub.0, with the A burst 122 positioned 
above the centerline, as shown in the example of FIG. 3c, and the B burst 
124 positioned below the centerline. Each of the A and B bursts comprises 
a series of magnetic transitions of alternating north/south, south/north 
magnetic pole transitions which result in a series of electrical signals, 
when the transitions of the A and B bursts of the data sector N.sub.0 are 
transduced by a corresponding head 24a-h, as the disk rotates. 
The width of the head 24a-h positioned above the data sector N.sub.0 is 
approximately equal to the radial extent or width of the sector. As 
discussed, during a read or write operation, the head 24a-h must be 
centered over the sector N.sub.0 to properly transduce only magnetic 
transitions of the sector N.sub.0 into corresponding electric signals. If 
the head 24a-h is off-center, the head 24a-h may begin to transduce 
transitions from data sectors of an adjacent track, resulting in an 
incorrect data read or write. 
During operation of the disk drive, each time a head 24a-h is over a pair 
of A and B bursts, the signals transduced by the head 24a-h are 
transmitted to the microprocessor 35, which sums all of the signals from 
the A burst 122, sums all of the signals from the B burst 124, and 
subtracts one of the sum values from the other to obtain a subtraction 
result comprising a difference value. When the head 24a-h is properly 
position over the centerline, the difference value is zero. When the 
difference value is non-zero, that indicates that the head 24a-h is 
off-center. 
For example, if the head 24a-h is completely off-center, above the 
centerline shown in FIG. 3c, the head 24c will transduce all of the 
transitions of the A burst 122, but none of the transitions of the B burst 
124. The difference value will equal the sum of the transitions of the A 
burst 122 since the B burst sum will have a zero value. This result 
signals the microprocessor 35 to control the actuator arm 28a-h, via the 
actuator control 40, to move the head 24a-h toward the centerline, until 
the difference value is again zero. 
FIGS. 4a and 4b illustrate the format and constituency of a representative 
data sector of the type used in connection with the present invention. The 
aspects of the format similar to the conventional format are indicated by 
like reference numerals, modified by a prime symbol "'", for convenience. 
The notable differences between the format shown in FIGS. 4a and 4b and 
the conventional format illustrated in FIGS. 3a-c are the absence of an 
address header field and the addition of a head identification information 
field 100" within the servo field 100' of FIGS. 4a and 4b. 
In the format of FIGS. 4a and 4b, the disk surface space ordinarily 
occupied by the address header field 104 (see FIGS. 3a-c) is now used to 
expand the circumferential length of the data field 108' for the storage 
of additional data. A small amount of the overhead imposed by the address 
header information, namely the head identification information (head 
identification field 100"), is relocated into the servo field 100' for use 
during certain failure conditions to identify the actual head 24a-h that 
is active, as will be explained in more detail below. 
As discussed above, the information usually contained within the address 
header field is stored, e.g. in the RAM 43. Monitoring clock signals from 
the time of detection of a sector mark 116 can be relied upon to associate 
a data field with a corresponding header in the RAM 43 to identify the 
data block stored in the data field being read or written. Whenever a 
failure condition exists resulting in the inability of the electronics to 
determine which head is active, the active head reads the head 
identification information now stored in the servo field 100' for 
confirmation of its identification. 
The servo fields 100, 100' are recorded onto the disk surfaces during the 
manufacture of the disk drive. For maximum efficiency, the heads 24a-h of 
the disk drive are used to complete a multi-head write operation to 
simultaneously write A and B burst servo patterns onto all of the disk 
surfaces in parallel. 
Referring now to FIG. 5, there is illustrated, in block diagram form, a 
write logic circuit 200 according to the present invention. The write 
logic circuit 200 is incorporated into the pre-amplifier 31 of the disk 
drive of FIG. 1 for control of either a multi-head write operation or a 
normal data write operation, as will appear. In FIG. 5, the heads 24a-h 
are represented schematically by coils 201. As well known, a coil 
generates a magnetic field when an electric current is conducted through 
the coil. This principle is utilized in a disk drive to write data as 
magnetic transitions within data tracks on the disk surfaces. Each coil 
acts as the transducer to convert an electric signal representing data to 
a magnetic field recorded onto a disk surface. 
As shown in FIG. 5, the coils 201 are coupled to respective write drivers 
202. Each write driver 202 includes an input that receives an electric 
signal containing encoded data, as is well known. The electric signal is 
transmitted by the pre-amplifier 31 to a selected write driver 202, 
causing the selected write driver 202 to energize the respective coil 201 
as a function of the encoded data to thereby magnetize the corresponding 
disk surface during rotation of the corresponding disk. The electric 
signal causes a series of magnetic transitions to be written onto the disk 
surface corresponding to the selected write driver/coil pair, with the 
series of magnetic transitions being representative of the data encoded in 
the electric signal energizing the particular coil 201 via the write 
driver 202. 
An address decode circuit 204 is coupled to address lines 206 that 
communicate a unique head identification number from the microprocessor 
35. During normal data write operation of the disk drive, the unique head 
identification number communicated by the microprocessor 35 at any one 
time corresponds to the head 24a-h flying above the specific disk surface 
where data to be written are located. As is well known, the address decode 
circuit 204 decodes the unique head identification number applied to the 
address lines 206 into a corresponding one of a set of select lines 208 
that is then activated by the address decode circuit 204. Each of the 
select lines 208 is coupled to an enable input of a respective one of the 
write drivers 202 that is coupled to a head 24a-h identified by the unique 
head identification number corresponding to the select line 208. When 
activated by the address decode circuit 204, a select line 208 enables the 
corresponding write driver 202 to write data, via the respective coil 201, 
to the corresponding disk surface, as will appear. 
In this manner, the microprocessor 35 can keep track of the disk surface 
location of each data block stored in the disk drive, and thereafter, 
during a normal data write operation, control the selection of a 
particular head 24a-h as a function of the disk surface location of a 
current data block to be written. The address decode circuit 204 also 
includes a BANK MODE input line 210 that can be activated during a servo 
write operation. When activated, the BANK MODE input line 210 causes the 
address decode circuit 204 to activate all of the select lines 208 at the 
same time for a multi-head write to the disk surfaces. 
Pursuant to the present invention, the write logic circuit 200 permits the 
writing of unique head identification information by each head 24a-h 
during a multi-head write operation, when the BANK MODE input line 210 is 
active. To that end, a memory device such as a shift register 212 is 
provided for each write driver 202. Each shift register 212 stores 
multiple bits having a unique value to identify the head 24 coupled to the 
corresponding write driver 202. An output of each shift register 212 is 
coupled to an input of a multiplexer 214. A second input of each 
multiplexer 214 is coupled to a common WR.sub.-- DATA line 216. The 
WR.sub.-- DATA line 216 is within the pre-amplifier 31 and transmits data 
to be written to a disk surface. 
As is well known, a multiplexer includes a select input that operates to 
select one input to the multiplexer for output by the multiplexer, as a 
function of a signal applied to the select input. A select input of each 
multiplexer 214 used in the present invention, is coupled to a WR.sub.-- 
ADR line 218. The WR.sub.-- ADR line 218 is controlled to select one of 
the inputs to the multiplexers 214 for output to a corresponding write 
driver 202. 
When the WR.sub.-- ADR line 218 is in a first preselected state, the 
WR.sub.-- DATA line 216 is selected for output by the respective 
multiplexer 214. When the WR.sub.-- ADR line 218 is in a second 
preselected state, the shift register 212 is selected for output by the 
respective multiplexer 214. In the example of FIG. 5, the first 
preselected state comprises the WR.sub.-- ADR line 218 asserted low and 
the second preselected state comprises the WR.sub.-- ADR line 218 asserted 
high. 
In accordance with an exemplary embodiment of the present invention, the 
most significant bit of each shift register 212 is coupled to a BIT.sub.-- 
3 line 220. This feature can be used to implement a multiple pre-amplifier 
arrangement. The most significant bit (bit 3 in this example for a two 
pre-amplifier arrangement) can be activated by the microprocessor 35 to 
designate which pre-amplifier is to be used at a given time, and provide a 
unique common identification number for each head within a set of heads 
coupled to that pre-amplifier. The remaining bits of the shift register 
212 (bits 0-2) are used to uniquely identify the particular head within 
the set of heads coupled to that pre-amplifier. 
The three bit arrangement of this example permits a unique identification 
of up to eight different heads coupled to a single pre-amplifier. With a 
single most significant bit, up to two pre-amplifiers can be uniquely 
identified, to support and uniquely identify up to a total of sixteen 
different heads. 
As in a conventional shift register, each of the shift registers 212 of the 
present invention includes a clock input to control the shifting of data 
bits to its output. As shown in FIG. 5, a clock input of each shift 
register 212 is coupled to an output of an AND gate 222. Each AND gate 222 
includes two inputs, one coupled to the WR.sub.-- ADR line 218, and the 
other coupled to the WR.sub.-- DATA line 216. Thus, the unique 
identification bits of a shift register 212 will be shifted to its output, 
one at a time, and thereby applied to an input of the respective 
multiplexer 214, when both the WR.sub.-- ADR and WR.sub.-- DATA lines are 
active high. As described above, the WR.sub.-- ADR line 218 is asserted 
high when selecting the contents of a shift register 212 for output by the 
respective multiplexer 214. In this manner, the selection of the shift 
register also acts as an enable signal for the corresponding AND gate 222. 
When the WR.sub.-- ADR line 218 is asserted high, each AND gate 222 will 
provide an output to the clock input of the corresponding shift register 
212 only when the signal on the WR.sub.-- DATA line 216 is also high. 
Pursuant to another feature of the present invention, the WR.sub.-- DATA 
line 216 is used as a clock signal during the time the WR.sub.-- ADR line 
218 is asserted high. To that end, high pulses are provided on the 
WR.sub.-- DATA line 218 at a preselected frequency, to shift bits out of 
each shift register 212 via the respective AND gate 222 (which provides an 
output pulse to the clock input of the corresponding shift register 212 
each time the WR.sub.-- DATA line 216 is pulsed high), at a rate suitable 
for application through the multiplexer 214 and write driver 202, to the 
head 24a-h such that transitions are properly recorded within the 
circumferential length of a head identification information field 100". 
During a normal write operation of the write logic circuit 200, the 
WR.sub.-- ADR line 218 is asserted low by the microprocessor 35 to select 
the common WR.sub.-- DATA line 216 for output by the multiplexers 214 to 
each respective write driver 202. The microprocessor 35 also provides a 
unique head identification number on address lines 206 to select a 
particular head 24a-h for writing data to a disk surface containing the 
data block to be written. The address decode circuit 204 is responsive to 
the unique head identification number to select the select line 208 
corresponding to the applied number for activation of the appropriate 
write driver 202. When activated, the selected write driver 202 energizes 
the corresponding coil 201 as a function of a data signal applied to the 
WR.sub.-- DATA line 216. Thus, any one of the heads 24a-h can be activated 
at any one time to write data to a particular disk surface of the disk 
drive. 
An example of a type of error condition that can occur in the disk drive is 
a malfunction of the address decode circuit 204. For example, the address 
decode circuit 204 can improperly decode a number applied by the 
microprocessor 35 to the address lines 206. Under such circumstances, the 
electronics of the disk drive expects data from a certain disk surface, 
when in fact a different head has been improperly selected by the address 
decode circuit 204. A recovery from this error condition can be achieved 
when using the headerless disk format of FIGS. 4a and 4b by having the 
active head 24a-h read the head identification number recorded in the head 
identification information fields 100" of the servo fields 100' of the 
disk surface. This information identifies the actual head activated by the 
address decode circuit 204. 
As discussed above, the most efficient scheme for writing the servo fields 
100' is to utilize the write logic circuit 200 in a multi-head write 
operation. During a servo write operation, the BANK MODE line 210 is 
asserted to cause the address decode circuit 204 to select all of the 
select lines 208 and thereby simultaneously activate all of the write 
drivers 202. The WR.sub.-- ADR line 218 is asserted low to select the 
WR.sub.-- DATA line 214 for output by the multiplexers 214. A servo 
pattern of A and B bursts is then applied to the WR.sub.-- DATA line 216 
to simultaneously write servo patterns to all of the disk surfaces. 
However, according to the present invention, when the heads 24a-h are over 
a head identification information field 100" of a servo field 100', the 
BANK MODE line 210 is still asserted, but the WR.sub.-- ADR line 218 is 
now asserted high to select the shift register 212 of each write driver 
202 for output by the multiplexer 214. At the same time, a clock signal is 
applied to the WR.sub.-- DATA line 216, as described above, to pulse the 
AND gates 222 and thereby shift the unique identification numbers stored 
in the shift registers 212 at an appropriate frequency, simultaneously 
through to the write drivers 202. Each write driver 202 will be activated 
by the assertion of the BANK MODE line 210 and, in addition, will be 
coupled to its corresponding shift register 212 through the respective 
multiplexer 214 due to the assertion high of the WR.sub.-- ADR line 218. 
Accordingly, all of the write drivers 202 are writing in parallel, but 
each of the write drivers 202 is writing a unique head identification 
number from its shift register 212 to the corresponding disk surface.