ACT apricot - Disk Drive Control

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FLOPPY

DISK

INTERFACE

compared

with

the

track

number

stored

in

the

Track

Register. Providing

the

two

numbers

are identical

and

the

ID field

eRe

bytes are correct,

the

track

position is

deemed

true,

and

the

INTRQ

output

is activated.

If

the

track

numbers

match,

but

the

eRe

is incorrect,

the

eRe

error

bit

within

the

Status

Register is

set

and

the

next

encountered

ID field is analysed.

If

the

FDC fails

to

detect

an

ID field

with

a

matching

track

number

and

correct

CRC

within

5

revolutions

of

the

disk,

the

operation is aborted,

INTRQ

is activated,

and

the

Seek Error

bit

within

the

Status

Register is set.

The

FDC

can

be also

issued

with

a

command

(Force

Interrupt),

which

sets

the

Status

Register to

monitor

the

state

of

the

input

status

lines

from

the

drive, and causes

the

INTRQ

output

to

be

activated

for

any

of

the

conditions

detailed below:

(a)

Immediately

the

command

is received.

(b)

Every

time

an

Index

Pulse

is detected.

(c)

On

detecting

a

transition

on

the

Ready

input.

Disk

Drive Control

The

disk drive

controller

circuitry

acts as

the

interface

between

the

disk

drives

and

the

other

areas

of

circuitry

within

the

FDC

and

consists

of

the

disk

data encoding

and

decoding sections,

the

head

positioning

control

section

and

a

status

monitoring

section.

Connections

to

and

from

the

FDC

disk

controller

circuitry

are detailed

in

the

table

"Disk

Drive

Connections"

at

the

end

of

the

chapter.

At

the

start

of a data

transfer

operation

to

the

disk (write

operation),

the

Write

Gate

output

is

activated

and

the

lOP

begins

the

process of transferring

data

bytes

to

the

Data

Register

in

parallel format,

under

DMA

control.

The

FDC

transfers

each

byte

from

the

Data

Register

to

the

encoding

circuit.

The

encoding

circuit

converts

the

bytes

into

MFM

double

density

encoded data,

which

is

then

supplied

to

the

disk

via

the

Write

Data

output.

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Service Manual305 pages