I/O Control System - HP 7925D Service Manual

Theory of Operation
7925
1-8.
I/O CONTROL SYSTEM
controller and the selected disc drive via bidirectional data
lines unique to that disc drive.
The I/O control system (see figure 4-24) consists of circuits
on I/O sector PCA-A2 and drive control PCA-A4, although
all communication between these two PCA's occurs via
motherboard PCA-A7. Included are the tag bus, control
bus, select, unit identity, status, and attention logic. The
purpose of the I/O control system is to provide the com-
munication link between the controller and the disc drive.
As can be seen in figure 1-7, communication between the
controller and all connected disc drives takes place via a
4-bit unidirectional tag bus and a 12-bit bidirectional con-
trol bus. A command is placed on the tag bus by the
controller to specify what function the disc drive should
perform. The command is validated by an active strobe
signal from the controller. Upon receipt of certain com-
mands, the disc drive will transfer information to the
controller via the control bus. Other tag bus commands
require that the controller place supplemental informa-
tion on the control bus in order for the disc drive to execute
the commanded function. Data is transferred between the
1-9. TAG BUS LOGIC. Each disc drive connected
to the controller can respond to fourteen individual tag
bus commands. Table 1-1 provides a summary of the tag
bus commands. Included are the 4-bit codes that must be
placed on the tag bus by the controller, the function that
will be decoded, an indication of which functions require
previous selection of the disc drive, the type of information
that will be placed on the control bus, its direction of flow,
and the action that will take place. As can be seen, tag bus
bit 3 determines whether the disc drive or the controller
will transfer information on the control bus. If bit 3 is
active (bit 3
=
0), the disc drive will send information to
the controller. If bit 3 is inactive (bit 3 = 1), the controller
will send information to the disc drive. All commands
placed on the tag bus by the controller are ground-true.
They will remain valid as long as the strobe signal from
the controller remains active (see figure 1-8). All valida-
tions occur on the leading edge of the strobe signal and
terminate on its trailing edge.
HP 13037
HP 7925
DISC
-
DISC DRIVE
CONTROLLER
(UNITO)
•
......
•
L
•
HP 7925
DISC DRIVE
(UNIT 1·7)
HP 13037
DISC
CONTROLLER
HP 7925
DISC
DRIVE
7311-38A
DATA
Figure 1-7. Disc Drive Interface
1-6