IEEE TRANSACTIONS ON MAGNETICS,VOL. MAG-17, NO. 4, JULY 1981

THE MD 122 DUAL 3 MBYTE FLEXIBLE DISK DRIVE

A. Stewart

p r o v i d e s y t e Abstract -

oackaoe desianed

The dual 3Mbyte flexible disk drive of formatted data storage in a compact for in-built use. The two disks may be

'insertid or removed independently and share a common double ended spindle. A common actuator is used to position the four recording heads. High areal recording density is achieved using advanced technology recording heads and media in association with a microprocesor controlled environmentally compensated servo positioner. The drive incorporates an intelligent controller which provides sophisticated data handling functions and controls the drive in such a manner that the interface to the host is free of drive dependant parameters.

INTRODUCTION

The evolution of the flexible disk drive over the past ten years spans three main generations starting from the original basic design to the advanced Dual 3 Mbyte Drive designed at the Burroughs Corporation Glenrothes Plant.

A flexible disk drive is characterised by unlimited low cost off-line data storage and low hardware cost. Despite the considerable increase in drive complexity to achieve greater data storage, drive cost per Mbyte has considerably reduced while the intelligence necessary to provide increased data density can be used to provide other features at small additional cost.

The evolution from first generation to second generation flexible disk drives may best be illustrated by the following table of parameters, as shown in Table I.

TABLE I

.~ ~

Introduction Bits per Tracks per Capacity Inch Inch

First Generation 1971

4,775 64 1 Mbyte Second Generation 1975

3,300 48 250Kbyte

- The low track density of the first generation drives is

relat ively undemanding in mechanical tolerancing and remains an industry standard being used on the majority of present day drives.

The Burroughs 1 Mbyte Super Mini Disk Drive which is used above to illustrate the parameters of a second generation drive was the front runner in.the so-called double sided double density drives. In addition to the increased capacity the new generation of drives had inbuilt data decode and clock separation circuitry thus placing the data integrity of the drive under the control of the drive designer. The interface between host and drive, however, still contained several drive dependent parameters and data transfers to and from the drive were totally t ied to drive seek and disk rotat ion times.

The MD122 Dual 3 Mbyte Disk Drive may justifiably c la im to be the first of the third generation flexible disk drives. Al l dr ive dependent parameters are under

Manuscript received January 30, 1981. The author is w i th Burroughs Machines Ltd.,

Glenrothes, Scotland.

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the control of the drive itself and a high level interface protocol is used between the host and the drive. As can be seen from the drive parameters listed in Table 11, a substantial advance in capacity and performance has been achieved.

TABLE 11

Capacity Number of disks Capacity per disk Rotational speed Average seek time (including settling) Average latency Average access time to data Track density

B i t density Power consumption (average)

6 Mbyte 2 3 Mbyte 524 rpm 81ms 57ms 138ms 150 tracks per inch 7203 bits per inch lOOW

The mechanical layout of the drive may be seen in Fig. 1 and Fig. 2. An extremely compact configuration allows two disks to be accommodated within the dimensions previously required for a single disk drive. The disks may be inserted and removed independently but share a common double-ended spindle and a common actuator positions the heads for both disks.

Fig. 1. Layout of r ight hand side of drive

Three printed wiring boards (PWB) are mounted directly on the diecast aluminium chassis. The large board contains the servo electronics while the two small boards provide respectively data pre-amplification and motor speed control.

The cage a t the rear of the drive contains the 6 PWB which comprise the inbuilt drive controller.

A "third generation" flexible disk drive such as the MD 122 has pararneters which overlap those of the smaller rigid disk drives. It has however one distinct advantage in that no separate load-dump device is required and this is extremely important when total cost-of-ownership is calculated.

0018-9464/81/0700-1403$00.75 0 1981 IEEE

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Fig. 2. Layout of l e f t hand side of drive.

DISK TECHNOLOGY

Disks are provided to the customer certified and pre- formatted wi th address headers. Disk flaws are accommo- dated by shuffling sectors within a track and relocation of larger area defects to a relocation track on the disk. Access to these relocated sectors is achieved via a table also recorded on the disk.

Although the basic disk technology is very similar to that of the early floppy disks, considerable advances in manufacturing processes were necessary to achieve the present recording density.

Reduction in track widths meant increased drive sensitivity to disk flaws and a reduction in signal output. Considerable advances in coating technology were required to provide a more uniform defect free coating while providing proportionally increased signal output.

Increase in bit density per track required a commensurate decrease in coating thickness. This, however, tended to lower the signal output, again requiring a compensatory improvement in oxide formulation. Thinner disk coatings required careful study for wear characteristics and are also more affected by irregularities in the disk substrate.

It is a considerable tribute to disk technology that all these sometimes contradictory constraints were overcome t o provide recording media' of suitable magnetic properties wi th durabi l i ty unsurpassed by any other flexible disk.

To achieve specified performance in any flexible disk drive it is important that the disk surface be protected from damage and contamination. Conventional disk packaging leaves this very much to the care of the.user. The disks used in the MD122 use an envelope similar to industry standards but the disk and envelope are encased in a r igid transparent plastic sleeve and retained within this sleeve at all times except when actually loaded in the drive. This affords a high degree of protection to the media surface. This sleeve is used to insert and to remove disks from the drive and a simple latch arrangement ensures that the disk is fully inserted before the door can be closed to clamp the disk. Centre hole damage due to incorrect clamping is thus avoided.

Disk wear is affected by two factors: the wear caused by the heads on the disk and the wear caused by the disk rotat ing wi th in i ts envelope. The MD 122 copes wi th these effects in the following manner. When the drive is idle, after a short delay the drive causes the heads to cruise over

the disk surface and thus avoid any concentration of head induced wear. Af ter a longer delay, i f the drive is s t i l l idle, the drive wil l completely retract the heads from the disk surface and stop the drive motor, thus eliminating both wear mechanisms. The disk wi l l restar t and the heads load automatically when the next disk access command is received.

RECORDING HEADS

A major step forward in recording head technology was required to achieve the recording density of the MD122. Both the magnetic and the mechanical design of the head were produced by Burroughs Heads and Media Engineering Group, Westlake, Ca.

Original designs of flexible disk recording heads embedded the ceramic and ferr i te assembly of the head in a ~ n ~ u l d e d plastic button. The disk was engaged against the head using a soft fabric pressure pad. This simple head design was very suitable for single-side recording but double sided recording using this technique involved a loss o f usable disk area since heads could not be directly opposed.

To overcome this disadvantage head designers looked to the flexure techniques used in r igid disk drives. Such recording heads were gimbaIIed on a very fIexibIe flat metal spring and held against the disk surface using a separate load spring pivoting on a point contact at the back of the head. Contact between the head and the disk surface was prevented by the air bearing between the head and disk formed by the air f i lm dragged round on the rotat ing disk. This technique is very attractive for flexible disk recording since a head pair can be directly opposed, one head providing effectively the "pressure pad" for the other. Several problems, however, had to be overcome in adopting flexure designs for flexible disk usage.

Since surface velocities on flexible disks are much lower than on rigid disks, considerably larger air bearing surface areas must be provided for equivalent load spring force. The surface location and stabil ity of f lexible disks are poorer so that the flexure assembly must be more compliant. Since the disk is replaceable, some mechanism must be provided for separation of the heads for loading and unloading.

The head is designed to f ly at a height of 8 micro inches above the disk surface. To achieve this design, rigorous computer simulation studies and practical evaluation were required. Any instability of the head would cause severe distortion of the readback signal and consequent poor data integrity in addition to media wear problems such as those which beset early versions of other flexure-mounted head designs.

The core structures of the head are fabricated from hot pressed manganese zinc ferrite which provides higher output voltage than alternative materials. Due to the br i t t le nature of ferr i te and the very slim structures demanded by the narrow tracks, head assembly and bonding processes must be very highly controlled.

DRIVE MECHANICS

The MD122 records data a t 150 tracks per inch using track widths of 4.4 m i l and erased guard bands between tracks of 2.2 mil. These dimensions mean that several effects, insignificant a t 48 tpi must either be eliminated or compensated for using new design techniques.

The design philosophy for replaceable media drives requires world-wide interchange o f media over a wide band of temperatures and humidities. The base material of the flexible disks changes dimension with both temperature and humidity and w i l l also exhibit anisotropic expansion

2

coefficients. The drive accommodates disk expansion in the following manner. The microprocessor centres each head in turn over servo tracks which are located at the inner and outer edges of the data bands on the disks. The offset from nominal track centre is stored for each centring operation and an interpolation routine is used to provide compensated position demand for individual data tracks. To ensure that calibration is maintained during operation of the drive, recalibration is performed at pre-determined intervals and, as required, during the read retry routine. This calibration eliminates much of the precise manual alignment required by earlier drive designs in addition to giving much improved positional accuracy. This radial alignrnent process, however, cannot accommodate eccentricity in the data tracks so that great attention must be paid to the specification and manufacturing processes of the recording disks and to the centring mechanisms within the drive.

All flexible disk drives use some variant of a cone and spindle technique to locate, clamp and rotate the disk. TO achieve the high centring accuracy required by the MD 122 over all temperature ranges and for the full life of the disk very careful design of the cone, disk and spindle geometry was necessary.

Rotation of the disk is provided by a dc motor servo under the control of an optical encoder, mounted on the drive spindle itself. The dc motor avoids the dependence on local ac line standards exhibited by the more common ac drive motors while requiring reduced power dissipation and mounting space.

Access t o the disks is provided by two independently operated doors. Each door may be opened by i t s own push button which also contains two indicator lights; Write Enable, and Disk Ready. The doors may be locked under host control thus preventing the operator from opening a door when, for example, data is being written to a disk.

DATA CHANNEL AND POSITIONER

The read preamplifier, write driver, and head selection circuitry are mounted on a small printed circuit card in close proximity to the heads. MFM Readback data is amplif ied and converted to TTL levels on this card, clock separation and NRZ decode is performed on the large PWB, mounted on the other side of the chassis.

Position and velocity feedback for the actuator servo are derived from an optical grating mounted on the head carriage. This system is configured to provide track counting to seek between tracks while accurate positioning for track centre is achieved using a linear interpolative servo. Servo control during seeks and fine offsetting to sett le on track centre are performed under microprocessor control. Circuitry is also provided on this PWB which detects accurately the centre of the disk servo tracks. This detector output is sampled by the microprocessor and used to check that disks are centred properly. I f not, a warning is sent to the user and the disk may be reseated.

CONTROLLER

The inbuilt controller interfaces with the host system and translates the commands f rom the host interface into the appropriate sequence of dr ive operations. The inter- face used is a Burroughs Standard Disk Interface which i s designed to have no host or drive dependent parameters. The interface between drive and host uses two undirectional data highways with associated control lines. Commands and data from the host use one nine-bit bus while status and data from the drive is transmitted along the other nine-bit bus.

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from host along the two buses is controlled by the host Selection of status, command data to host and data

system via two status lines.

Data is transferred as eight bit words plus a ninth b i t proriding vertical parity protection. Transfers across the interface may be of variable length, each transfer being terminated by a longitudinal parity byte. Presence of the longitudinal parity byte or indication of a detected parity error, longitudinal or vertical is indicated by parity control lines, one driven by the host and one by the drive. Signals along the interface are strobed and controlled by a clock line and two strobe lines, one host generated and one drive generated.

TABLE I11

Command Summary

Command Function

----- -----_1_

Read

Read Statistics

Read Relocation Map

Write

Search

Read Search Result

Read Device Attribute Record

Read Status

Host Receive MTR

Abort

Set/Reset Write Protect

Lock/unlock door

Set Motor Stop

Read data from Disk

Provides details of drive act iv i ty

Provides location of re- located sectors

Writes data t o disk

Searches predefined areas of disk for particular data pattern

Responds with block of data found during search and location of key data pattern

Provides preset details of drive configuration

Provides status data on drive

Provides extended status data following drive mal function

Terminates current command

Prevents host writing to specified drive

Controls door lock mechanism

Allows motor to stop automatically when drive idle.

l--~---ll--l-----y-I.---I

Each command as 1iste.d in Table 111 is defined by a single byte op-code followed as required by a number of bytes defining particular parameters relating to that command.

The sequence of functions required to implement such high level commands may be illustrated by the following example of the read and write commands. The read command is an 8 byte transfer specifying which of the two disks is to be accessed, start address of data and the length of data transfer required. The drive wi l l decode the address and drive selection to obtain head, track and sector data, wil l position the heads to the appropriate track, and will read back and decode the address headers on that track.

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By pretriggering one sector in advance of the required sector the drive provides the host with an interrupt so that the host can perform other tasks and return prepared to service the drive when data is imminent. When the actual address required is detected data from that sector is read from disk into an internal drive buffer and, providing the error detection circuitry shows good data read from disk, the host is flagged that data is available. Two buffers are contained within the drive so that their data can be trans- ferred continuously to the host for the duration of a multi- sector transfer operating the two buffers in a push-pull configuration. If an error in read-back data is detected the drive automatically enters a retry routine and only flags data available to the host on the completion of the retries.

The retry routine is terminated on:

(a) error free data

(b) correction of corrupted data

(c) failure of both retry and correction

The correction algorithm can correct up t o 11 bits and the data is transrnitted to the host with a corrected status flag. If correction cannot be accomplished the actual data read back from the disk is transmitted t o the host with an error status flag. Since inability to read back data error free by retry alone implies a 'hard' error on the disk surface, that sector and the data contained therein is relocated to the relocation track with an appropriate 'corrected' or 'error' flag set. This f lag wi l l be reset when fresh data is written to the sector. Changes in t rack or head selection, or access to the relocation track, are performed automatically under the control of the microprocessor and so are transparent to the user.

The write operation is substantially the inverse process to read. The one sector pretrigger allows the drive to request data from the host to be loaded into the internal drive buffer so that the write operation may be init iated when a successful address match is obtained. The drive generates preamble and synchronising data which precedes the actual written data and also generates a cyclic redundancy check code which is wr i t ten appended to the data. If the data to be written is less than one sector in length the drive will 'end-fill' the remainder of the sector wi th zero and append CRC on the resultant full length sector.

If the data to be wr i t ten is greater than one sector in length the drive will automatically set up to detect the next sector header and wil l load a second internal buffer with the data for the second sector while the first sector i s being wri t ten to the disk. These two buffers wi l l again thus operate in a push-pull mode f o r the duration of a multisector transfer.

Drive responses to the host are achieved by means of status transfers listed in Table IV. These may be of one or three byte duration. Three byte transfers in general denote some exception condition.

MAINTENANCE FEATURES

The maintenance plan for any product should be directed to providing rapid fault repair in a cost-effective manner, Factors to be considered in this equation are:

(a) failure rates of individual components or modules

(b) minimisation o f special test tools

(c) minimisation of time and ski l l level to effect repair

TABLE I V

Status Summary

Status Function

._.-l___------__-l-_I__

Drive address

N sectors before read

N sectors before write

Operation complete

Interrupt

Error

Corrected

Search unsuccessful

Command not accepted

Command error

Address error

Address not found

Not ready

W r i t e protected

New disk

Disk expiring

Temporarily not available

Confidence t e s t complete

Danger

Mandatory interrupt

Identifies which disk is respond- ing

Read pre-warning

W r i t e pre-warning

Command execution complete

Extension bit, requires transfer of further status bytes

Error detected in data transfer

Corrected data contained in transfer

Key data not found in specified area

Valid command which cannot be executed by drive, e.g. attempt to wr i te on write protected disk

Invalid command

Address specified in excess of drive capacity

Drive fai led to recognise specified address on disk

Drive not available for transfer, e.g. disk not inserted

Disk hardware or software write protected

New disk inserted and on-line

fu l l Relocation table approaching

Disk on-line but unavailable for transfer, e.g. when other disk being inserted

Successful confidence test following POR

Drive requires operator attention, e.g. relocation table full on a disk

Drive malfunction. Host should receive MTR transfer for further identification of problem

--I_------.-- 1_11--_ _I_ _---_-__-_-- (d) minimisation of spares holding

(e , maximisation of confidence in success of repair

(f) provision of early warning of onset of problem

The M D 122 approaches these factors in the following way.

Using the microprocessor, diagnosis of faults in the logic cards i s performed automatically under the control of the field engineer and repair effected by board replacement. Fault diagnosis in the analogue circuitry and electro- mechanical components is achieved using the microprocess- or t o exercise the drive and monitoring response using a general purpose digital meter. High bandwidth low

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amplitude circuitry is notoriously difficult to test using simple test equipment so that segregation of such pre- amplif ier circuitry on a small low-cost board considerably simplifies diagnosis in this area.

Several interface features of the drive provide un- precedented advantages in maintenance. Consider for example failure of the drive motor. The resultant loss of disk speed is detected by the microprocessor and reported to the host via Mandatory' Interrupt and Host Receive MTR. Statistics logging provides individual retry rates on a l l heads so that degradation of performance can be diagnosed t o individual heads or to a general data channel failure.

CONCLUSIONS

Advances in semiconductor technology have completely transformed the relationship between a computer mainframe and i ts peripherals. The size, performance and cost of a computer system will be determined to a large extent by the peripheral devices rather than the mainframe i tse l f .

By distributing intell igence from the mainframe to the peripheral, mainframe performance will be improved by allowing the processor to perform other tasks while the peripheral is busy. In addition the performance of the peripheral itself is enhanced by increased capacity and optimisation of drive parameters under the control of the microprocessor.

One feature which wil l emerge with intell igent peripheral devices is a change in the role of peripheral control circuitry within the host. Where once this module was fundamental to the performance of the peripheral, distributed intelligence means that it wi l l act more as a simple adaptor between the host processor and the peripheral.

By the combination of inbuilt intelligence and state- of-the-art hardware there is no question that flexible disk technology will continue to advance and play an important par t in the computer systems of the future.