MPI_Floppy_Interfacing_Guide_Feb1980 MPI Floppy Interfacing Guide Feb1980

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FLEXIBLE

DISK

DRIVE

INTERFACING

GUIDE

MICRO

PERIPHERALS

INC.

REPRESENTED

BY

RSSDEIAIES

14431

MIRANDA

ct

LOS

ALTOS

HILLS

CALIFORNIA

94022

(415)941-4133

PROPRIETARY NOTICE

Information contained

in

this manual

is

copyrighted, and may

not

be duplicated

in

full or

in

part

by any person

without

prior

written approval from Micro Peripherals, Inc. (MP\)

The purpose

of

this manual

is

to

provide the user

of

MPI's Model

51/52

Flexible Disk Drives with adequately detailed documenta-

tion necessary for efficient installation, operation and interfacing

for

the equipment supplied.

Every

effort

has been made

to

keep the information contained

in

this manual

current

and accurate as

of

the date of publication

or

reVISion.

However, no guarantee

is

given or implied

that

the

manual

is

error free, or

that

it

is

accurate with regard

to

any

particular specification.

Published February 1980

MICRO

PERIPHERAlS

INC.

9754 Deering Avenue

Chatsworth, California 91311

(213) 709-4202

TWX: 910-494-1213

Section

Section

2

Section

3

Section

q

TABLE

OF

CONTENTS

Specifications.

. . . . .

Ins t a

11

at

ion • •

Interfacing

Requirements.

Data Encoding and

Recovery.

ii

Page

Page

Page 5

Page

10

SECTION

I

SPEC

I

FI

CAT

I

ONS

1.0

INTRODUCTION

This

section

provides

the

mechanical and

electrical

specifications

for

the

Model

51/52

Flexible

Disk

Drives.

1.1

SPECIFICATIONS

The

mechanical and

electrical

specifications

for

the

Model

51/52

Flexible

Disk

Drives

are

given

in

Table

1-2.

Data

capacity

for

the

Model

51/52

Flexible

Disk

Drives

is

given

in

Table

1-1.

TABLE

1-1

DATA

CAPACITY

UNFORMATTED

(K

BYTES)

PARAMETER

SINGLE

DENSITY

DOUBLE

D~NSITY

(FM)

(MFM.

M

FM)

51

52

51

52

Track

3.13 3.13

6.25

6.25

Disk

125

250 250

500

SECTI

ON

2

INSTALLATION

2.0

INTRODUCTION

This

section

provides

the

information

and

procedures

necessary

for

installing

the

Model

51/52

Flexible

Disk

Drives.

2.1

UNPACKING

During

unpacking,

care

must be

exercised

to

ensure

that

all

tools

are

nonmagnetic

and

do

not

inflict

damage

to

the

unit.

As

the

unit

is

unpacked,

inspect

it

for

possible

shipping

damage. All

claims

for

this

type

of

damage

should

be

filed

promptly

with

the

transporter

involved.

If

a

claim

is

filed

for

damages,

save

the

original

packing

material.

Most

packing

material

may

be

reuseable

if

reasonable

care

is

used in

unpacking.

Unpack

the

drive

as

follows:

A.

B.

C.

D.

Eo

F.

G.

Remove

external

packing

material

carefully.

~

Remove

the

drive

from

the

container.

Remove

internal

packing

materials,

following

instructions

provided

on

the

package.

Ensure

that

front

access

door opens and

closes,

and

that

the

head load

arm

raises

when

door

is

opened.

Ensure

that

bezel

is

secured.

Ensure

that

drive

hub

manually

rotates

freely.

Ensure

that

stepper

motor/head

carriage

assembly

is

not

binding

at

any

point,

by

manually moving

carriage

back and

forth.

TABLE

1-2

MECHANICAL

AND

ELECTRICAL

SPECIFICATIONS

PARAMETER

Media

Number

of

Tracks

Track

Density

Rotational

Speed

Average Latency

Head

loading

time

Access time

Head

settling

time

Head

life

Media

life

Recording method

Recording

density

Flux

density

Data-transfer

rate

Power-up Delay

Height

Width

Length

Weight

Power

Typical

Power

Dissipation

Operating

Temperature

Non

Operating

Temperature

Operating

Humidity

Non

Operating

Humidity

Operating

Altitude

Non

Operating

Altitude

Vibration

and Shock

(Operating)

MTBF

MTTR

CHARACTERISTICS

ANSI

standard

5

1/4

inch

diskette

40/51 80/52

48

TPI

300 rpm.!. 1

1/2%

100

msec

35

msec

5 msec,

track-to-track

15

msec

20,000 hours

3 x

10

6

passes

on

a

single

track

FM,

MFM,

M2FM

2938/5876 bpi max.

5876

fci

max.

125K/250K

bits/sec.

0.5

sec

3.25

inches

(8.255

cm)

5.75

inches

(14.605

cm)

7.6

inches

(19.30

cm)

3.0

pounds

(1.36

kg)

+12

VDC

!

5%,

0.8A (1.5A

surge),

.2

V

ripple

+5

VDC.!.

5%,

0.4A, .1 V

ripple

12W

Operation

6w

Standby

40°F

to

115°F

(4.4°c

to

46:1°C)

-40°F

to

160°F (-40°C

to

71°C)

20%

to

80%

(noncondensing)

5%

to

95%

(noncondensing)

-500

ft

to

10,000

ft

(-152.4m

to

3,048m)

-1000

ft

to

50,000

ft

(-304.8m

to

15,240m)

6

to

600

Hz,

0.5g

9,200

Hours

0.5

Hours

2

2.2

INSTALLATION

Due

to

its

small

size

and

light

weight,

the

Model

51/52

can be

installed

or

mounted

in

any

convenient

location

or

position.

However,

the

drive

must be

installed

in a

location

that

will

prevent

the

I/O

cable

from

exceeding

10

feet

in

length.

Refer

to

Figure

2-1

for

dimensions

and mounting

provisions.

2.3

HARDWARE

The

flexible

disk

drive

is

a

precision

device

in which

certain

critical

internal

alignments

must be

maintained.

Therefore,

in

keeping

with

rigid

disk

requirements,

it

is

important

that

the

mounting

hardware

does

not

introduce

significant

stress

on

the

drive.

Any

mounting scheme

in

which

the

drive

is

part

of

the

structural

integrity

of

the

enclosure

is

not

permitted.

Since

the

disk

drive

cannot

be

SUbjected

to

signiffcant

stress

when

it

is

slide

mounted,

this

type

of

mounting

generally

satisfies

the

foregoing

requirements.

Mounting schemes

should

allow

for

adjustable

brackets

or

incorporate

resilient

members

to

accommodate

tolerances.

Mounting schemes

involving

more

than

two

hard mounting

points

and a

third

point

should

be

avoided.

2.4

DUST

COVER

Since

the

flexible

disk

drive

is

not

provided

with

a

dust

cover,

the

design

of

an

enclosure

should

incorporate

a means

to

prevent

direct

ingress

of

loose

items,

e.g.,

dust,

paper

punch

waste,

etc.

2.5

COOLING

Heat

dissipation

from a

single

disk

drive

is

normally

12

watts

(32

Btu/Hr).

When

the

drive

is

mounted

so

that

the

components have

access

to

free

flow

of

air,

normal

convection

cooling

allows

operation

over

the

specified

temperature

range.

When

the

drive

is

mounted

in

a

confined

environment,

air

flow

may

have

to

be

provided

to

maintain

specified

air

temperatures

in

the

vicinity

of

the

motors,

PCBA,

and

the

diskette.

2.6

INPUT/OUTPUT

CABLE

Refer

to

Table

2-1

for

cable

connector

part

number and

attachment.

The

maximum

cable

length

from

connector

to

connector

is

10

feet.

All

inputs

and

outputs

are

paired,

one 1

ine

for

function,

one

for

ground.

Figure

3-9

provides

information

relative

to

the

connector

pin/signal

assignments

for

the

t/O

cable.

TYPE

OF

CABLE

Twisted

Pa

i

r,

26

Flat

Cable

TABLE

2-1

RECOMMENDED

CONNECTORS

-PI

CONNECTOR

MANUFACTURER

PIN

AMP

583717-S

3M

··Scotchflex"

3463-0001

3

CONTACT

PIN

1-583616-1

N.A.

~

1

,,-

...

Door

Open Position

'"

1.87

(4.74 cm)

I

~

I

.=\

I~

Bottom View

3.125

(7.94 cm)

\.:1;/

I

h

J=.

6-32 x

.31

deep

(4) mtg. holes

Door

Closed

Position

====-

3.38

(8.59

.06

(.15

n) L

t

IP

~

1.87

:m) (4.74 cm)

::t1

(.79 cm)

3;125

7.6 max (19

Side

View

.125

(.318 cm)

~

5.87

(14.9

o

7cm)

".

1

cm)

.....

L -

•

Top View

1-

l

.....

5.75

(14.6

-1-

frr

-

1 /

yp

(.15 cm) Single Interface

Connector

(J

1)

~

6-32

thru

mtg.

h(

- - .8n

C8

DC

Power Connector (J2)

es

(2

plcs.

each

side)

(2.22 cm)

t NOTE: Unless otherwise stated,

dimensions are in inches.

Tolerances:

.xxx

±.

0.010

.

xx

±.

0.020

Figure

2-1

Outline

and

Mounting Dimensions

1 cm)

2.7

DC

POWER

DC

power

to

the

drive

is

via

connector

J2,

which

is

located

on

the

non-component

side

of

the

PCBA

near

the

spindle

motor.

The

drive

uses

+12V

DC

and

+5V

DC.

Table

1-2.

outl

ines

the

voltage

and

current

requirements.

The

connector

is

an

AMP

Mate-N-Lock

Part

No.

1-480424-0.

SECTION

3

INTERFACING

REQUIREMENTS

3.0

I

NTRODUCT

ION

This

section

contains

the

interfacing

requirements

between

the

host

system

and

the

Model

51/52

Flexible

Disk

Drive.

Communication

is

established

via

two

connectors.

Connector

Jl

establishes

a communication

link

for

all

input/

output

signals.

These

signals

are

TTL

compatible.

Connector J2

provides

DC

power

to

the

device.

3.1

INPUT

LINES

The

input

control

lines

have

A.

B.

3.1.1

True,

Logical Zero

False,

Logical

One

Line

Termination

the

following

electrical

specifications:

=

OV

to

0.4v

(@

lin

=

48

ma

max).

= +2.5V

to

+5V

(open

collector

@

lout

= 250

~a

max).

The

signal

interface

used

by

the

Model

51/52

is

of

the

"bus"

or

"daisy-chain"

type.

Only one 51/52

unit

is

logically

connected

to

the

interface

at

any

given

time.

All

input

signals

are

terminated

directly

by

a

150

ohm

or

a 220/330

ohm

resistor

network.

In

a

daisy-chain

configuration,

only

the

last

device

in

the

daisy

chain

should

have

the

terminating

network;

while

in a

star

configuration,

every

device

should

be

terminated.

3. 1

.2

Programmable Shunt

The

main

function

of

this

device

js

to

assign

the

proper

address

t6

the

drive

in a

multidrive

configuration.

1n

addition,

it

also

determines

when

the

head

load

solenoid

should

be

activated

by

using

either

position

1-14

(with

Select)

or

7-8 (with Motor On).

If

position

5-10

is

left

shorted,

the

drive

is

essentially

always

selected,

but

the

activity

light

will

not

come

on,

and

the

solenoid

will

not

be

activated

until

the

drive

position

is

selected.

The

programmable

shunt

is

AMP

PIN

435704-6

(HPI

PIN

1-79600-0o~).

For

convenience,

the

programmable

shunt

could

be

replaced

by

a

dip

switch,

AMP

PIN

435626-J•

(MPI

PIN

1-7960J-001).

(If

dip

switch

is

used,

maximum

height

is

exceeded

by

;150.)

The

seven

lines

channeled

through

the

shunt

are:

Designator

Pins

a.

Head

Load

w/Select

Tl

1-14

b.

Drive

Select

1

T2

2-13

c.

Drive

Select

2

T3

3-12

d.

Drive

Select

3

T4

4-11

e.

MUX

T5

5-10

f.

Drive

Select

4

T6

6-9

g.

Head

Load

w/Motor

On

T7

7-8

5

3.

1

.3

Drive

Select

1

to

4

The

Drive

Select

lines

provide

a means

of

selecting

and

deselecting

one

of

up

to

four

disk

drives.

When

the

signal

logic

level

is

true,

the

disk

drive

electronics

are

activated,

the

head

is

loaded,

and

the

~rive

is

conditioned

to

respond

to

step

or

read/write

commands.

When

the

logic

level

is

false,

the

input

control

lines

and

output

status

lines

are

disabled.

A

select

line

must remain

stable

in

the

true

state

until

the

execution

of

a

step

or

read/write

command

is

completed.

After

the

desired

device

is

selected,

allow

a

35

msec

delay

before

initiating

a

read

(see

Figure

3-2)

or

write

(see

Figure

3-4).

3.1.4

Hotor

On

This

input

is

provided

to

extend

the

life

of

the

DC

spindle

motor.

The

motor

should

be

turned

off

if

no

activity

is

required

of

the

Hodel

51/52

after

10

revolutions

of

the

diskette.

A

true

level

on

this

line

turns

on

the

drive

motor.

A

minimum

of

0.5

second

is

required

before

performing

a

read

or

write

after

a

MOTOR

ON

command

is

transmitted

to

the

device

(see

Figures

3-2

and

3-4)

•

3.1

.5

Direction

Select

The

direction

of

motion

of

the

read/write

head

is

defined

by

the

state

of

this

input

line.

A

true

level

defines

direction

as

"IN"

(towards

center

of

the

disk);

a

false

level

defines

the

direction

as

"OUT"

(see

Figure

3-1).

3. 1

.6

Together

with

the

direction

line,

a

single

pulse

on

this

input

will

move

the

read/~rite

head one

track

in

or

out,

dependent

on

the

state

of

the

direction

line.

The

motion

of

the

head

is

initiated

on

the

trailing

edge

of

the

step

pulse.

A minimum

of

0.2

~s

pulse

width

at

a

maximum

frequency

of

200

Hz

should

be

maintained

to

assure

step

integrity

(see

Figure

3-1).

3.107

Write

Gate

When

true,

this

input

line

permits

writing

of

data.

When

false,

it

permits

transmitting

data

to

the

controller.

Write

gate

must be

high

for

10

ms

minimum

after

tornirig

off

DC

power

(see

Figure

3-6).

Allow a minimum

of

I msec

after

dropping

write

gate

before

expecting

val id Read Data

(see

Figures

3-2

and

3-6).

During

this

1 msec

period,

Side

Select

(see

paragraph

3.1.9)

must remain

stable.

3.

1.8

Write

Data

This

input,

in

conjunction

with

the

write

gate

input,

provides

data

to

be

written

on

the

diskette.

The

frequency

of

the

write

oscillator

shoul~be

held

within

0.1%

with

a

pulse

width

of

a

minimum

of

0.2

~sec

and

maximum

of

3.5

~sec.

The

frequency

is

dependent

upon

the

encoding

scheme used and

the

density

option

exercised

(see

Figures

3-4 and 3-5).

It

is

recommended

that

the

first

leading

edge

of

Write

Data

occurs

no

sooner

than

4

~sec

and no

later

than

8~sec

after

leading

edge

of

Write

Gate.

The same recommendation

exists

for

the

last

Write

Data and

trailing

edge

of

Write

Gate.

6

3.

1.9

Side

Select

This

input

is

used

to

select

either

the

upper.

or

lower

head.

A

true

level

selects

the

upper

head.

a

false

level

selects

the

lower

head.

A

35

~sec

delay

should

be allow.ed

for

the

read

amp

to

recover

after

a head

select

event

occurs.

Only

then

wi.ll

valid

data

be

present.

(For

Model

51,

this

line

should

always

be

high.)

3.1.10

In

Use

(optional

fe~ture)

This

line

is

connected

to

a

driver

which

could

be used

for

an

indicator

light

or

a

solenoid

for

locking

the

drive

door.

3.2

OUTPUT

LINES

The

control

output

signals

are

driven

with

an

open-collector

output

stage

capable

of

sinking

a

maximum

of

48

ma

at

logical

zero

with

maximum

voltage

of

0.4v

measured

at

the

driver.

When

the

output

Is

at

a

logical

one,

the

collector

cutoff

current

is

a

maximum

of

250

~a.

3.2.

1

Track

00

This

output,

when

true,

indicates

that

the

read/write

head(s)

are

located

at

Track

00.

3.2.2

Index/SeCtor

This

output,

when

true,

Indicates

that

an

index

or

sector

hole

in

the

diskette

is

present

at

the

·index

sensor

(see

Figure

3-7

and

3-8).

3.2.3

Write

Protect

This

output,

when

true,

indicates

that

a

write

protected

diskette

is

installed

in

the

drive.

When

an

unprotected

diskette

Is

installed,

this

output

is

false.

When

a

protected

diskette

is

installed,

the

write

and

erase

logic

on

the

PCBA

is

disabled.

By

making a

small

modification

to

the

PCBA

and

using

only

protected

diskettes,

this

output

can be used

as

a

Diskette

[nstalled

Indicator.

3.2.4

Read

Data

This

output

represents

digitized

data

as

detected

by

the

drive

electronfcs.

Information

transmitted

will

be

in

the

encoding

scheme

used.

Pulse

width

of

both

clock

and

data

bits

will

be I

usec

+ 350

nsec.

Maximum

bit

shift

from nominal

for

various

encoding

schemes

is

given

in

table

3-1

(see

Figure

3-3).

TABLE

3-1

BIT

SHIFT

Maximum

bit

shift

(ns)

FM

MFM

M2FM

clock

::.7

00

.!.7

00

.!.475

data

+400

.!.700

.!.7

00

-

7

Drive Select

~--------~,-------------

Out

Direction In

l--.-J

-II--O.l.,min.---!

to.1.,min.

0.3,1ls

min. L

---'

0.2,1.ts

min.

Step

------,suul~

Read

Data

0.2.,

min.

----I

I---

L .

--J

5msmm.

Figure

3-1

Track Access Timing

A

I

A I

A I

A

A - Leading

Edge

of

Bit May

Be

± 700

ns

From its Nominal Position

B - Leading

Edge

of

Bit

May

Be

± 400

ns

From its Nominal Position

DC

Power

Motor

On

Drive Select

Valid Trk. 00 and

Wrt. Prot.

Output

Valid

Index!

Sector

Output

Direction

Step

Write Gate

Write Data

Valid

Read

Data

Figure 3-3 Read Signal Timing

--.J

--1

r100m,min.

---,~~-------------

~0.5secmin.

----t-+---,

I---

O.5,1ls

max.

~~~lJ

-

~

8.us

max. I

10

ms

min.

~..;....-----

I--

20

m'

min.~

1--1

msec

---++-----~~

l--ru--

_____

0.-53!cm~~~~

Figure 3-6 General Control and Data

Timing Requirements (Head Load

Solenoid

is

Activated with Drive Select)

8

Motor

On

-----,L

___________________

_

Drive Select

Step

Write Gate

Valid

Read

Data

1S---

:P.:.min.

Figure 3-2 Read Initiate Timing

Motor

On

----,"'

____________________

_

Drive Select

Step 1.------------

I~----------l------------

J-35

ms

min._

-----1---

...

---J

1--20

m,

min.

i--.

0.5

sec

min.

-

Write Gate

1-1

---l..--J

Write Data

--1 !-s.OO,llS max.

----------~U1fUl~----

Figure 3-4 Write Initiate Timing

Write Data

(FM)

--l

1-1_..:

O

::.

2

:::.':.:m:.::i:.::n·:......·-II

__

..:

8

::.

OO

:::·::'_-l1

4.00.,

I--

3.5,us

max.

±.

40

ns

±.

20

ns

Figure 3-5 Write Data Timing

u U

~

200 ms ± 3.0

ms

- -

--I

Figure 3-7 Index Sector Timing (soft sector)

±.19ms ±.10ms

Figure 3-8 Index Sector Timing (hard sector)

J1

2 SPARE

4 IN USE*

6

DRIVE

SELECT 4

8 INDEX/SECTOR

10

DRIVE

SELECT 1

12

DRIVE

SELECT 2

14

DRIVE

SELECT 3

16 MOTOR ON

51/52

18

DIRECTION

CONTROLLER

20 STEP

22 WRITE

DATA

24

WRITE

GATE

26

TRACK

00

28

WRITE

PROTECT

30

READ

DATA

32 SIDE SELECT

34 RESERVED

ODD PINS

RETURN

(DC GROUND)

POWER

J2

1

+12V

51/52

2

12V

RETURN

CONTROLLER

3

5V

RETURN POWER SUPPLY

4

+5V

*IN

USE

may

be

configured

as

Door Lock or

Activity

Light.

Figure 3-9 Interface Signals -

51/52

Pin

1 +12V

DC

Pin

2 12V Return

Pin

3 5V Return

Pin

4 +5V

DC

BOARD THICKNESS

.06.2!

.007

Figure 3-10

DC

Power Connector,

J2

Figure 3-11

J1

Connector Dimensions

9

3.3

CONNECTOR

J2

The

DC

power

connector

is

located

on

the

non-component

side

of

the

printed

circuit

board.

The recommended mating

connector

is

AMP

PIN

1-480424-0

using

AMP

pins

PIN 60619-1

(see

Figure

3-10).

3.4

CONNECTOR

Jl

Connection

to

Jl

is

through

a

34-pin

PCBA

edge

connector.

Even

numbered

pins

are

located

on

the

component

side

while

odd numbered

pins

are

located

on

the

solder

side.

A key

slot

is

provided

between

pins

4 and

6.

The

recommended

connector

is

3M

Scotchflex

PIN

3463-001,

or

AMP

PIN

583717-5

using

AMP

contacts

PIN

1-583616-1

(see

figure

3-11).

SECTION

4

DATA

ENCODING

AND

RECOVERY

4.0

INTRODUCTION

This

section

provides

applications

information

relevant

to

the

recording

and

recovery

of

data

with

the

Model

51/52

Flexible

Disk

Drives.

4.1

ENCODING

METHODS

The

three

most

common

methods

for

encoding

data

are

described

in

the

following

paragraphs.

Table

4-1 shows a comparison

of

these

encoding

techniques.

4.

1 • 1

FM

FM

(frequency

modulation)

encoding has

the

following

rules.

A.

A

data

bit,

if

it

is

a

"1",

occurs

at

the

center

of

the

bit

cell.

B.

A

clock

bit

occurs

at

the

start

of

the

bit

cell.

1 o o o 1 o 1 1

BIT

CELL

=

8MS

Figure

4-1

FM Encoding

4.1.2

MFM

MFM

(modified

FM)

encoding

has

the

following

rules:

A.

B.

A

data

bit,

if

it

is

a

"1",

occurs

at

the

center

of

the

bit

cell.

A

clock

bit

occurs

at

the

start

of

the

bit

cell,

but

only

if

no

data

bit

occured

in

the

previous

bit

cell

and

no

data

bit

will

occur

in

the

current

bit

cell

•.

10

---_

...

-------------

I--

6J..Ls

-I

..

4J..LS--I

~8J..L5-1

U U U U U

Lr

1 0 I 0 I 0 1 0 1 1 I

BIT CELL =

4J..L5

Figure 4·2 MFM Encoding

4.1.3

M2FM

M2FM

(modified

MFM)

encoding

has

the

following

rules:

A.

A

data

bit,

if

it

is

a

"1",

occurs

at

the

center

of

the

bit

cell.

B.

A

clock

bit

occurs

at

the

start

of

the

bit

cell,

but

only

if

neither

a

data

bit

nor

a

clock

bit

occured

in

the

previous

bit

cell

and no

data

bit

will

occur

in

the

current

bit

cell.

/--6J..L5

-I"

10J..L5

-I"

8J..L5

LJ

U.

U

----

..

~14J..LS~

-------flU

if

I 1 0 0 0 I 1 0 I 1 1 I

BIT

CELL =

4J..L5

Figure 4·3

M2

FM

Encoding

4.2

DATA

RECOVERY

Data

recovery

refers

to

the

retrieving

of

data

off

of

the

flexible

diskette.

The

following

paragraphs

describe

the

problems

associated

with

data

recovery

and

methods

to

ensure

data

reliability.

4.2.

I

Bit

Shift

Bit

shift

refers

to

the

displacement

of

a

bit,

as

detected

by

the

drive,

from

its

nominal

position.

The

causes

of

bit

shift

are

manifold

--

R/W

head

resolution,

media

resolution,

diskette

speed

variation,

signal-to-noise

ratio

of

read

head

output.

Any

data

separation

technique

used must have a

tead

window

of

at

least

1500

ns

to

handle

this

bit

shift.

4.2.2

Write

Precompensation

Certain

data

patterns

cause

more

bit

shift

than

other

patterns.

This

bit

shift

is

predictable

and

can

thus

be

partially

conpensated

for.

For

example,

if

it

is

known

that

a

bit

will

be

shifted

by

500 ns

when

it

is

read

back,

then

the·

bit

11

can

be

deliberately

written

200 ns

early.

This

would

give

a

bit

shift

of

about

350 ns

when

it

is

read

back.

This

method

of

reducing

bit

shift

is

called

write

precompensation.

Bit

shift

is

greater

on

the

inner

tracks

of

the

diskette

than

on

the

outer

tracks,

making

write

precompensation

necessary

only

on

the

inner

tracks.

Write

precomp

of

250

to

300

ns

should

be used on

tracks

18

through

39.

If

due

to

controller

limitations,

write

precomp must be used

on

all

tracks,

then

125

to

150 ns

should

be

used.

Table

4-1 shows which

encoding

methods

re'quire;

the

use

of

write

precompensation.

4.2.3

Data

Separation

Data

separation

refers

to

the

separating

of

the

composite

data

coming from

the

drive

into

separate

clock

and

separate

data

bits.

For

FM

recording,

a

one-shot

data

separator

is

quite

sufficient.

For

double

density

recording,

a

phase-lock

oscillator

(PLO)

data

separator

should

be

used.

There

is

another

method

of

data

separation,

the

digital

counter

method.

This

method

is

a

very

poor

approxi,mation

of

a PLO.

It

has

a

theoretical

read

window

of

only

1000 ns

which,

as

per

paragraph

4.2.1,

is

not

sufficient

to

handle

bit

shift.

In

MFM

recording,

data

bits

and

clock

bits

are

subject

to

the

same amount

of

bit

shift.

A

PLO

separator

with

a

50%

data

window and a

50%

clock

window

should

be

used.

In

M2FM

recording,

data

bits

are

subject

to

more

bit

shift

than

clock

bits.

A

PLO

separator

with

a

60%

data

window and a

40%

clock

window

should

be

used

for

best

data

reliability.

It

should

be

noted

that

the

+700 ns

bit

shift

is

meaningful

only

when

associated

wi§h an

error

rate.

The

Model

51/92

drives

have

error

rates

of

1

error

in

10

bits

read.

Thus,

for

every

10

bits,

there

will

be

no

more

than

1

bit

shifted

more

than

~700

ns.

4.3

TRACK

FORMAT

When

determining

the

track

format

to

be

used,

the

following

timing

restraints

should

be

considered

(see

Figure

4-4).

4.3.1

Postamble

The

postamble

period

must be

at

least

3

ms

to

allow

for

spindle

speed

variation

of

+J-!%.

4.3.2

Data

Gap

The

data

gap

period

must be

at

least

1

ms

to

allow

for

tunnel

erase

turn

off

time.

12

TABLE

4-1

COMPARISON

OF

ENCODING

TECHNIQUES

Encoding

Technique

Bit

Cell Time

Possible

pulse

spacing

Frequency components

of

read

signal

Encoder

complexity

Write

precompensation

needed

Data

separator

recommended

Data

separator

complexity

FM

8

,us

4 ',IJ s

8us

125

KHz

62.5

KHz

Simple

No

One-shot

Simple

MFM

4

.us

4

.u

s

6

,us

8 .us

125

KHz

93.75

KHz

62.5

KHz

Moderate

Yes

PLO(50-50 window)

Moderate

M2FM

4

~S

4

)JS

6

)JS

8

IJS

10

.us

125

KHz

93.75

KHz

62.5

KHz

50

KHz

Moderate

Yes

PLo(60-40 window)

Moderate

INDEX

u~------------~$~~--------

TRACK

B E

A - Postamble

B - Preamble

C

-ID

Record

D -

ID

Gap

E - Data Record

F - Data

Gap

Figure 4-4 Typical Track Format

13

"