PC87338 datasheet(72/221 Pages) NSC

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FIGURE 44. Perpendicular Recording Drive

Read/Write Head and Pre-Erase Head

Unlike conventional disk drives which have only a

read/write head, the 2.88 MB drive has both a pre-

erase head and read/write head. With conventional

disk drives, the read/write head, itself, can rewrite the

disk without problems. 2.88 MB drives need a pre-

erase head to erase the magnetic flux on the disk sur-

face before the read/write head can write to the disk

surface. The pre-erase head is activated during disk

write operations only, i.e. FORMAT and WRITE

DATA commands.

In 2.88 MB drives, the pre-erase head leads the

read/write head by 200

µm, which translates to 38

bytes at 1 Mbps (19 bytes at 500 Kbps).

For both conventional and perpendicular drives,

WGATE is asserted with respect to the position of the

read/write head. With conventional drives, this means

that WGATE is asserted when the read/write head is

located at the beginning of the preamble to the data

field.

With 2.88 MB drives, since the preamble must be

erased before it is rewritten, WGATE should be as-

serted when the pre-erase head is located at the be-

ginning of the preamble to the data field. This means

that WGATE should be asserted when the read/write

head is at least 38 bytes (at 1 Mbps) before the pre-

amble. Tables 49 and 50 on page 104 show how the

perpendicular format affects gap 2 and, consequent-

ly, WGATE timing, for different data rates.

Because of the 38-byte spacing between the

read/write head and the pre-erase head at 1 Mbps,

the gap 2 length of 22 bytes used in the standard IBM

disk format is not long enough. The format standard

for 2.88 MB drives at 1 Mbps called the Perpendicular

Format, increases the length of gap 2 to 41 bytes. See

Figure 58 on page 99.

The PERPENDICULAR MODE command puts the

Floppy Disk Controller (FDC) into perpendicular re-

cording mode, which allows it to read and write per-

pendicular media. Once this command is invoked, the

read, write and format commands can be executed in

the normal manner. The perpendicular mode of the

FDC will work at all data rates, adjusting the format

and write data parameters accordingly. See “The

PERPENDICULAR MODE Command” on page 104

for more details.

3.2.4

Data Rate Selection

The FDC sets the data rate in two ways. For PC com-

patible software, the Configuration Control Register

(CCR) at address 3F7h programs the data rate for the

FDC. The lower bits D1 and D0 in the CCR set the

data rate. The other bits should be set to zero. See

Table 40 on page 82 to see how to encode the de-

sired data rate.

The lower two bits of the Data rate Select Register

(DSR) at address 4 can also set the data rate. These

bits are encoded like the corresponding bits in the

CCR. The remainder of the bits in the DSR have other

functions. See the description of the DSR in Section

3.3.7 on page 82 for more details.

The data rate is determined by the last value written

to either the CCR or the DSR. Either the CCR or the

DSR can override the data rate selection of the other

gine and data separator clocks are scaled appropri-

ately. Also, the DRATE0 and DRATE1 output signals

will reflect the state of the data selection bits that were

last written to either the CCR or the DSR.

3.2.5

Write Precompensation

Write precompensation is a way of preconditioning

the WDATA output signal to adjust for the effects of bit

shift on the data as it is written to the disk surface.

Data that is subject to bit shift is much harder to read

by a data separator, and can cause soft read errors.

Bit shift is caused by the magnetic interaction of data

bits as they are written to the disk surface. It shifts

these data bits away from their nominal position in the

serial MFM (MFM or FM in the PC98338) data pattern.

Write precompensation predicts where bit shift could

occur within a data pattern. It then shifts the individual

data bits early, late, or not at all so that when they are

written to the disk, the shifted data bits will be back in

their nominal position.

The FDC supports software programmable write pre-

compensation. Upon power up, the default write pre-

compensation values shown in Table 42 on page 82,

will be used. In addition, the default starting track

number for write precompensation is track zero

You can use the DSR to change the write precompen-

sation using any of the values in Table 41 on page 82.

Also, the starting track number for write precompen-

sation can be changed with the CONFIGURE com-

mand.

200 mm

(38 bytes @ 1 Mbps)

End of

ID Field

Data Field

Preamble

Intersector

Read/

Pre-

Head

Write

Gap 2

= 41 x 4Eh

Erase

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