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FLOPPY DISK INTERFACE

logic

state

of

the

Side Select

Output

(SSO),

supplied to

the

disk drive.

When

U

is

set

to logic low,

the

SSG is updated

to

logic

low

(side

0).

When

U is

set

to logic high, SSO is

updated

to

logic

high

(side

1).

The

Multiple

Record Flag

bit

selects

whether

data

transfers are

from/to

a single

sector

or

multiple

sectors

within

a track. Single sector transfers are

initiated

when

the

m Flag is

set

to logic low,

multiple

sector

transfers

when

the

m Flag is

set

to logic high.

Write Sector.

On

receipt of

the

Write

Sector command,

the

FDC begins

the

process of searching

the

sector ID fields of

the

track

for

the

desired

destination

for

the

data.

When

an

ID field is found

with

the

correct

track

number

(as

specified

by

the

Track

Register),

the

correct side

number

(as

specified

by

the

U

bit

in

the

command),

the

correct sector

number

(as

specified

by

the

Sector Register),

and

correct CRC character;

the

FDC generates a

DMA

request

via

the

DRQ

output,

to

inform

the

lOP

to

write

the

first

data

byte

into

the

Data

Register.

If

an

ID field

is

not

found

containing

the

correct

information

within

five revolutions of

the

disk,

the

command

is aborted

and

the

Record

Not

Found

bit

in

the

Status Register set.

If

any

of

the

ID fields encountered,

contain

an

incorrect

CRC

character,

this

is also recorded

in

the

Status Register.

On

receipt of

the

first data byte,

the

FDC activates

the

Write

Gate

output

and

on

detecting

the

start

of

the

data

field,

writes

the

data

byte

to

the

disk.

The

process

then

continues

with

the

FDC generating

DMA

requests every

time

a

new

byte

of data is required. After

the

512th data

byte is

written

to

the

disk, a two-byte CR C character is

automatically

generated

and

written

onto

the

disk.

If

the

data

written

to

the

sector

only

fills a

part

of

the

data field,

the

remaining

area

should

be

programmed

with

a series of

zeroes,

to

complete

the

whole

data

field.

If

a single

sector

write

operation

was

specified

by

the

Write Sector

command,

the

Write

Gate

output

is

then

deactivated

and

the

command

operation

terminated.

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Service Manual305 pages