Control Bus Logic; Select Logic; Tag Bus Timing - HP 7925D Service Manual

7925 Theory of Operation
CONTROL BUS (CBUS 0-111
INFORMATION FROM DISC DRIVE
(TBUS 3=0)
\
/
CONTROL BUS (CBUS 0-11)
I
INFORMATION FROM CONTROLLER
(TBUS 3=11
1
\
I
/
I
I
I
I
I
I
I
TAG BUS (TBUS 0-3)
I
I
V
~
I
I
I
I
I
I
I
STROBE (STB)
I
I
I
I
\1
V
I
I
I
I
I
I I
I
Tl
~
600
ns T2 ~ 600 ns T3;;' 600
ns
1
I
I
NOTE: Tl
=
TAG BUS SET·UP TIME
T2 = STROBE WIDTH
T3
=
TAG BUS HOLD TIME
7311-39
Figure 1-8. Tag Bus Timing
1-10. CONTROL BUS LOGIC. Each disc drive
connected to the controller can transmit information to
the controller or receive information from the controller
via the 16-bit bidirectional control bus (the HP 7925 Disc
Drive uses only 12 bits of the control bus, see figure 1-7).
Upon receipt of certain tag bus commands, the disc drive
will transmit its current status (READ,' WRITE, or RQS),
its identity (RQD, or its stored head address and present
sector address (RQP) to the controller via the control bus.
Other tag bus commands require that the controller place
supplemental information on the control bus in order for
the disc drive to execute the commanded function. This
information includes the cylinder address (SEEK), head
and sector addresses (ADR or XMS), unit address (ADU),
offset magnitude and sign (SOF), or the information to
selectively clear the attention and/or first status, status
bits (CLS). Table 1-2 provides a summary of the control
bus bit assignments for each decoded tag bus function.
The control bus receivers within each disc drive are al-
ways enabled to receive the supplemental information
from the controller. The ground-true signals from the con-
troller are converted to positive-true signals by the control
bus receivers for use throughout the disc drive circuitry.
The control bus drivers are only enabled by four tag bus
commands, i.e., READ, WRITE, RQS, or RQP. These
commands require that the particular disc drive be
selected. Once enabled, the control bus drivers convert the
positive-true signals from the disc drive into ground-true
signals for transfer to the controller. The identity of each
disc drive connected to the controller that has its attention
bit set (status bit 7
=
1) will be placed directly onto the
control bus in response to a decoded RQI comma!1d. This
information will bypass the control bus drivers, but it will
still be transferred as a ground-true signal.
1-11. SELECT LOGIC. A disc drive must be
selected by the controller in order for it to respond to ten of
the fourteen tag bus commands. If not selected, it can only
respond to four commands, i.e., ADU, CPS, DCN, or RQI.
The controller selects a disc drive by placing the unit
address of the desired disc drive on the control bus and an
ADU command on the tag bus. Each disc drive connected
to the controller will compare its unit address, established
by the setting of its UNIT SELECT switch, with the unit
address on the control bus. If they compare, only that disc
drive will set its select flip-flop once the ADU command is
decoded. When set, the SEL signal will become active
(SEL
=
1) to enable the remaining ten tag bus commands
to be decoded by that disc drive. It will also enable the
read/write logic in that disc drive. The SELL signal will
become active (SELL
=
0) to light the light-emitting diode
(LED), located in the upper left-hand corner of the unit
select display. The disc drive will be reset to its unselected
state whenever the controller issues either a CPS or DCN
command on the tag bus, or the RUN/STOP switch is set to
RUN, or the power-on sequence is initiated.
1-9