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TYPE I COMMAND FLOW
,"
NO
NO
os
COMMAND
A
STEP· IN
os
COMM"ND
A
STEP
,
os
COMMAND
A
STEP·OUT
RESTORE (SEEK TRACK 0)
Upon receipt of this command the Track 00 (TROO) input is
sampled.
If
TROO is active low indicating the Read-Write
head is positioned over track
0,
the Track Register is loaded
with zeroes and an interrupt is generated. If TROO is not
active low, stepping pulses (pins 15 to 16) at a rate specified
by the r1 r
o
field are issued until the TROO input is activated.
At this time the Track Register is loaded with zeroes and an
interrupt is generated. If the TROO input does not go active
iow after 255 stepping pulses, the 279X terminates
operation, interrupts, and sets the Seek error status bit. A
verification operation takes place if the V flag is set. The h
bit allows the head to be loaded at the start of command.
Note that the Restore command is executed when MR
goes from an active to an inactive state.
SEEK
This command assumes that the Track Register contains
the track number of the current position of the Read-Write
head and the Data Register contains the desired track
number. The WD279X will update the Track register and
issue stepping pulses in the appropriate direction until the
HEAD LOAD TIMING
When both HLO and HLT are true, the 279X will'then read
from or write to the media. The "and" of HLO and HLT ap-
pears as status Bit 5 in Type I status.
In summary for the Type I commands: if h
=
0 and V
=
0,
HLD is reset. If h
=
1 and V
=
0, HLO is set at the
beginning of the command and HLT is not sampled nor is
there an internal 15 ms delay. If h
=
0 and V
=
1, HLO is set
near the end of the command, an internal 15 ms occurs,
and the 279X waits for HLT to be true. If h
=
1 and V
=
1,
HLO is set at the beginning of the command. Near the end
of the command, after all the steps have been issued, an
internal 15 ms delay occurs and the 279X then waits for HLT
to occur.
For Type II and III commands with E flag off, HLO is made
active and HLT is sampled until true. With E flag on, HLD is
made active, an intemal15 ms delay occurs and then HLT is
sampled until true.
HW~
1---50 TO l00mS--1:-
_
11----;
I
HL T (FROM ONE SHOTl
a 1 MHz clock. There is also a 15 ms head settling time if
the E flag is set in any Type II or III command.
When a Seek, Step or Restore command is executed an
optional verification of Read·Write head position can be
performed by setting bit 2 (V
=
1) in the command word to
a logic 1. The verification operation begins at the end of the
15 millisecond settling time after the head is loaded against
the media. The track number from the first encountered 10
Field is compared against the contents of the Track
Register. If the track numbers compare and the 10 Field
Cyclic Redundancy Check (CRG) is correct, the verify
operation is complete and an INTRQ is generated with no
errors. If there is a match but not a valid CRC, the CRC error
status bit is set (Status bit 3), and the next encountered 10
field is read from the disk for the verification operation.
The W0279X must find an 10 field with correct track
number and correct CRC within 5 revolutions of the media'
otherwise the seek error is set and an INTRQ is generated:
If V
=
0, no verification is performed.
The Head Load (HLO) output controls the movement of the
read/write head against the media. HLO is activated at the
beginning of a Type I command if the h flag is set (h
=
1), at
the end of the Type I command if the verify flag (V
=
1), or
upon receipt of any Type II or III command. Once HLO is
active it remains active until either a Type I command is
received with (h
=
0 and V
=
0); or if the 279X is in an idle
state (non-busy) and 15 index pulses have occurred.
Head Load timing (HLl) is an input to the 279X which is
used for the head engage time. When HLT
=
1, the 279X
assumes the head is completely engaged. The head
engage time is typically 30 to 100 ms depending on drive.
The low to high transition on HLO is typically used to fire a
one shot. The output of the one shot is then used for HLT
and supplied as an input to the 279X.
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