Tag Bus Logic - HP 7925D Service Manual

7925
Table 1-1. Tag Bus Command Summary
Theory of Operation
TAG BUS
DISC CONTROL BUS
BIT
DECODED
DRIVE (REFER TO TABLE 1-2)
ACTION
FUNCTION
SELECTED
3 2 1 0 INFORMATION
DIRECTION
0
0 0 0 READ YES CURRENT STATUS
FROM DISC The selected disc drive will gate its current status onto
DRIVE the control bus and keep it updated throughout the
entire read operation. It will then wait for the leading
edge of its internal sector compare signal before trans-
ferring sector compare (status bit 8
=
1) in its status
word on the control bus and the bit-encoded data from
the addressed sector to the controller on its data lines.
The transfer of data will continue until the end of the
addressed sector is reached or the read command is
dropped.
0 0 0 1 WRITE YES CURRENT STATUS FROM DISC
The selected disc drive will gate its current status onto
DRIVE the control bus and keep it updated throughout the
entire write operation. It will then wait for the leading
edge of its internal sector compare signal before trans-
ferring sector compare (status bit 8 = 1) in its status
word on the control bus and the bit-encoded data from
the controller to the addressed sector on its data lines.
The transfer of data will continue until the end of the
addressed sector is reached or the write command is
dropped.
0 0 1 0 REQUEST YES
CURRENT STATUS FROM DISC
The selected disc drive will gate its current status onto
STATUS DRIVE the control bus and keep it updated as long as the
(RQS)
strobe signal remains active.
0
0 1 1 REQUEST NO IDENTITY
FROM DISC
Every disc drive connected to the controller that has
IDENTITY
DRIVE
its attention bit set (status bit 7
=
1) will gate its identity
(RQI)
onto a line on the control bus that corresponds to the
unit number of that disc drive. The identity will remain
active until the attention bit is cleared or the request
identity command is dropped. The internal rotational
position sensing feature of each disc drive permits the
transfer of its identity up to 15 sectors before an actual
sector compare occurs. This feature and the amount
of look-ahead is jumper-selectable in each disc drive.
0
1
0 0
DISCONNECT NO
- - - - - -
Every disc drive connected to the controller will be
(DCN)
reset to its unselected state and the light-emitting
diode (LED) in the upper left-hand corner of each unit
select display will go out. In this state, each disc drive
can only respond to four tag bus commands (ADU,
CPS, DCN, or RQI).
0 1 0 1 CONTROLLER NO
- - -
- - - Same as DISCONNECT. In addition, each disc drive
PRESET
connected to the controller will clear any nondestruc-
(CPS)
tive read/write faults (W • AR and R • W); an AGC
fault; its head and sector address registers; its illegal
head and sector address flip-flops; and the seek check,
first status, drive fault, and attention status bits.
0 1 1 0 REQUEST YES HEAD-SECTOR ADDRESS FROM DISC The selected disc drive will gate the contents of its
SECTOR DRIVE head address register and its present sector address
(RQP) counter onto the control bus. This information will
remain on the control bus as long as the strobe
signal remains active and the present sector address
will be continually updated by the sector counter.
0 1 1 1
NOT USED
1-7