Neer the end of the command, afler
aU
the steps have been
issued,
an
inlernai 15ms delay occurs and lhe
1791
lhen walls
for
Hl
T la occur.
For Type
Il
and
lU
commands with E lIag off.
HLO
is
made
active and
HlT
is sampled until true. Wilh E
f1ag
on,
Hlo
is
made
aClive,
an
internai 15ms delay accurs and then
HL
T is
sampled untillrue.
Ollk Read Operations
Sector lenglhs of
128,256.
512 or 1024 are oblainable
in
eilher
FM
or MFM formats. For
FM,
BOEN
should be placed
10
logical 1. For MFM formats.
DoEN
should be placed
10
a
togical0.
Sector Iengths are determlned al format time by a special byte
in
the
10
field.
Ilthis
Seclor Length byte
in
lhe
10
field lS zero.
then the sector length is 128 bytes.
Ir 01, then 256 bytes. Il
tJ2.
then 512 bytes.
JI
03, then lhe sector length
is
1024 bytes.
The number
of
SectOfS
per Irack.
as
lar
as
the
1791
is
concemed, can be Irom 1
to
255 seclors. The number
01
tracks,
as
lar as the
1791
is concerned. is from
0to
255 Iracks.
For IBM
3740 compatibiHly, sector lenglhs are
128
bytes wilh
26 sectors per track. For System
34
compalibitlty (MFM),
sector lengths are
256 bytesisector wilh
26
seetorSllrack;
or
lengths
01
1024 byteS/sector wilh eight seclors/track.
For read operation, the F01791 request a RAW REAo DaIa
(Pin 27) signal which
is a 250 ns pulse per lIux transition (at 2
MHz
dock)
and
a Read dock
(RClKI
signal
to
indicate flux
transition spacings. The RCLK (Pin 26) signal is provided
by
a
phase-locked
toop
or counler techniques.
ln addition, a Read Gate Signal
is provided
as
an
output (Pin
25) which informs sorne phase-Iocked loops when
10
acquire
synchronization. However,
pin
25 is not used
in
lhis design.
O\lk
Wrlle
Operations
When writing
Îs
to
lake place
on
lhe diskelle. lhe Write Ga'e
(WG) output is activated. This allows currenl
to
lIow inlo
t~e
Read/Wrile head. As a precaution to erroneous writing. the
fir;t
dala byte must be loaded inlo the Data Aeglsler
in
response .0
a Daia Request trom the
1791
belore the Wrile Gale sign
11
can be activated.
Wriling is Inhibited when the Write Protecl
Inpul is a togic
10~1.
in which case. any Wrile command is immedialely terminalet!.
an
interrupt is generated. and the Write Proteet stalus bit is
se:.
The Write Fault inpul. when activaled. signifies a writing 'aLlt
condition delecled ln disk-drive electronics such
as
failure 1;)
detect Wnte current flow when the Write Gate is activated.
Cn
detection of this fault, the Fo1791 terminales the currellt
command, and sets the Write Faull
bit (bit 5) in the Stall.S
Word. The Write
Faull inpul should be made mactive when
lh~
Wrile Gate output becomes inactive.
For Write operation, the W01791 provides a Wrile Gate
(PI1
3(3)
and Write DaIa (Pin 31) oulputS. Write Daia consists
01
a
series
01500
ns pulses in
FM
(oOEN
'=
1)
and 250 ns
PUiSES
in MFM (OoEN =
0)
for 1 MHz clock. Write Data provides lhg
unique address marks in bath formats.
Aise during Write.
IWO
additlonal signais are pravided lor Wnt
~
precompensation. These are EARLY /pin
17)
and
LA
TE
(Pi'
18). EARLY is active lrue when the
WO
pulse appearing 0 l
(Pin 30) is
to
be wrillen early. EARLY is valid for the duration
(f
the pulse.
lATE
is active true when Ihe
WO
pulse
IS
to
bl
written late.
If
balh are law when a
WO
pulS&
is present.
th
~
WO
is wrillen at nominal.
The Write precompensation
signaIS
EARLY and
lA
TE
ar
~
valid
in
bath FM and MFM 'ormats. Howevsr. the
1691
will
ignore lhese signaIs unless
TG
43
and OoEN are both active
Whenever
a Read
or
Write
command
(Type
Il
or
III) i:;
received. the F01791 samples the Ready
input
If
this input b
logic low, the command is not executed and
an
interrupt
i:;
generaled. This aise applies la Type 1 commands.
Re<:ordlng Codes
Information is stored
on
a disk using a code that takes
lhl'
desired information and conver1s il
10
a pulse tha\ the recordin! 1
system can write and recover trom the disk. The idea! systen,
requires thal
aU
lhe pulses
wriUen
on
the disk be informational
9
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