Flowcharts; Read/Write System - HP 7936 Support Manual

(SPR-L)
is low. These errors are detected by the
servo controller when detailed status is requested.
5-84. ERROR REPORTING. Servo system error
reporting is performed by 2~byte immediate status
and
2~byte
detailed status information exchanged
across the Servo Control Bus.
Immediate status is returned by the servo
control1~
er in response to all request servo function com~
mands issued by controller PCA-A6. Immediate
status reported can be: no error (normal response),
soft error, hard error, or interface error. Also
reported is the servo function command for which
the status is being returned, and a detailed descrip-
tion of the bad status associated with the iden-
tified command.
Detailed status is returned by the servo controller
in response to a report status command from con-
troller PCA-A6. The
2~byte
detailed status contains
a bit array in which one of the elements represents
the current servo system fault. The fault is not re~
lated to any servo command. Detailed status also
contains a description of the bad immediate status
associated with the function command indicated
by immediate status.
5-85.
FLOWCHARTS
A number of flowcharts are provided which chart
the various operations of the servo system. Figures
5-6 through
5~
11 describe servo controller power-
on, recalibrate, seek, servo commands, and con-
troller power-on, respectively. These flowcharts
also show the activity which the drive goes
through to generate certain errors. The status
numbers shown in the flowcharts are error codes
generated by the servo controller. These codes are
sent to controller
PCA~A6
where they are convert-
ed to a different error code. Refer to chapter 8
for additional details.
5-86.
READ/WRITE SYSTEM
The principal component in the read/write system
is a
custom~designed
read/write controller in-
tegrated circuit with associated read chain, write
chain, and head select encoder electronics. See
figure 5-16.
Functional Description
7936 and 7937
To perform the task of writing or recovering data
from the disc media, the read/write controller
receives commands from the microprocessor in
controller PCA-A6 (passed over the Data Path
Control bus), transforms byte~parallel data to or
from the DMA/ECC electronics in PCA-A6
(passed over the Disc Data Bus), and obtains timing
information from the servo system. Read and
write operations are commanded on a sector by
sector basis and the operation for every sector is
completely independent. When not reading a sec-
tor or writing a sector, the read/write system is in
an idle state.
When the microprocessor in PCA-A6 sends a write
command to the read/write controller, the servo
system identifies the start of sector and a driver in
the write chain will be enabled and a constant
frequency sync field will override any previous
pattern on the disc. Following the sync field, a
framing word will be written, and then the data to
be written will start being clocked from the
DMA/ECC in PCA-A6 into the encoder portion of
the read/write controller. The encoder takes the
byte~parallel
data and maps it to serial VLFM
code. This code drives the write drivers in the
read preamplifier/write driver ICs in head-disc as~
sembly A3. After the second CRC byte is clocked
into the encoder, two bytes are clocked in as
dummies, allowing the ECC to switch paths and
become operable before supplying the ECC bytes.
A counter within the encoder will stop clocking
the data at the end of the ECC field. Write current
will be turned off at the end of the sector. All
timing in the read/write system during a write is
based on a write clock from the servo system.
When
PCA~A6
sends a read sector command to
the read/write controller, the controller waits for
the start of sector, as defined by the servo system.
After a fixed delay following start of sector, a
PLL and data separator in the read chain is en-
abled to lock to the sync field. (The time delay is
to allow for start up delays when the sector was
written, propagation time through the read cir-
cuitry, and time error between the servo head and
the data head.) After a time interval adequate for
the PLL to obtain lock, the read/write controller
switches the PLL from a locking mode to a track-
ing mode. Timing up to this point is based on the
write clock from the servo system. Now, the PLL
becomes the timing ref erence for the read
5~27