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Theory of Operation
Table 1-1. Tag Bus Command Summary (Continued)
7925
TAG BUS
DISC
CONTROL BUS
BIT
DECODED
DRIVE
(REFER TO TABLE 1-2)
ACTION
FUNCTION
SELECTED
3
2
1
0
INFORMATION
DIRECTION
1
0
0
0
SEEK
YES
CYLINDER ADDRESS
FROM
The selected disc drive will check for a legal cylinder
CONTROLLER
address on the control bus, then clock this address
into its new cylinder address register (legal cylinder
addresses are 0 thru 822). A seek operation to that
address will then be initiated. When the heads are cor-
rectly positioned and settled over the specified cylinder,
the disc drive will make attention (status 7
=
1) avail-
able in its status word. If the cylinder address is illegal,
the disc drive will make seek check (status bit 2
=
1)
and attention (status bit 7
=
1) available in its status
word and it will not clock the illegal address into its new
cylinder address register.
1
0
0
1
ADDRESS
YES
HEAD-SECTOR ADDRESS
FROM
The selected disc drive will check for a legal head and
RECORD
CONTROLLER
sector address on the control bus, then clock these
(ADR)
addresses into its head address register and sector
address register, respectively (legal head addresses
are 0 thru 8and legal sector addresses are 0 thru 63).
If either address is illegal, the disc drive will make seek
check (status bit 2
=
1) available in its status word and
it will not clock the illegal address into the respective
register.
1
0
1
0
ADDRESS
NO
UNIT ADDRESS
FROM
Every disc drive connected to the controller will com-
UNIT
CONTROLLER
pare the unit address on the control bus with the unit
(ADU)
address set on its UNIT SELECT switch. If they com-
pare, only that disc drive will be set to its selected state
and the light-emitting diode (LED) in the upper left-
hand corner of the unit select display will light. In this
state, the selected disc drive can respond to all
fourteen tag bus commands.
1
0
1
1
RECALIBRATE
YES
- - -
- - -
The selected disc drive will clear its new cylinder ad-
(RCL)
dress register and present cylinder address counter,
then initiate a recalibrate (seek home) operation to
cylinder O. When the heads are correctly positioned
and settled over cylinder 0, the disc drive will make
attention (status bit 7
=
1) available in its sta lJs word.
1
1
0
0
TRANSMIT
YES
HEAD-SECTOR ADDRESS
FROM
The selected disc drive will check for a legal sector ad-
SECTOR
CONTROLLER
dress on the control bus, then clock this address into
(XMS)
its sector address register (legal sector addresses are
o
thru 63). If the address is illegal, the disc drive will
make seek check (status bit 2
=
1) available in its
status word and it will not clock the illegal address into
its sector address register. The head address on the
control bus is ignored and the contents of the head
address register will remain unchanged.
Note: Used to automatically increment the sector ad-
dress during cylinder and surface mode.
1
1
0
1
SET OFFSET
YES
OFFSET MAGNITUDE
FROM
The selected disc drive will clock the contents on the
(SOF)
AND SIGN
CONTROLLER
control bus into its offset magnitude and sign registers
and reposition the heads accordingly (valid offset
magnitudes are 0 thru 63 increments of 12.5 micro-
inches each in either a positive or negative direction
from track center).
Note: This function is designed to permi t marginal
data recovery.
1
1
1
0
CLEAR
YES
SELECT CLEAR
FROM
The selected disc drive will selectively clear either first
STATUS
CONTROLLER
status (status bit 3
=
0) or attention (status bit 7
=
0)
(CLS)
or both status bits depending upon the state of bits 0
and 1 on the control bus.
1
1
1
1
NOT USED
1-8
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