PC87306 datasheet(61/110 Pages) NSC

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50 FDC Functional Description (Continued)

The 288M drive has unique format and write data timing

requirements due to its readwrite head and pre-erase head

design (see

Figure 5-4 ) Unlike conventional disk drives

which have only a readwrite head the 288M drive has

both a pre-erase head and readwrite head With conven-

tional disk drives the readwrite head by itself is able to

rewrite the disk without problems For 288M drives a pre-

erase head is needed to erase the magnetic flux on the disk

surface before the readwrite can write to the disk surface

The pre-erase head is activated during disk write operations

only ie Format and Write Data commands

In 288M drives the pre-erase head leads the readwrite

head by 200 mm which translates to 38 bytes at 1 Mbps

(19 bytes at 500 kbps) For both conventional and perpen-

dicular drives WGATE is asserted with respect to the posi-

tion of the readwrite head With conventional drives this

means that WGATE is asserted when the readwrite head is

located at the beginning of the Data Field preamble With the

288M drives since the preamble must be pre-erased be-

fore it is rewritten WGATE should be asserted when the

pre-erase head is located at the beginning of the Data Field

preamble This means that WGATE should be asserted

when the readwrite head is at least 38 bytes (at 1 Mbps)

before the preamble See Table 4-5 for a description of the

WGATE timing for perpendicular drives at the various data

rates

Because of the 38 byte spacing between the readwrite

head and the pre-erase head at 1 Mbps the GAP2 length of

22 bytes used in the standard IBM disk format is not long

enough There is a new format standard for 288M drives at

1 Mbps called the Perpendicular Format which increases

the GAP2 length to 41 bytes (see

Figure 4-1 ) The Perpen-

dicular Mode command will put the floppy controller into

perpendicular recording mode which allows it to read and

write perpendicular media Once this command is invoked

the read write and format commands can be executed in

the normal manner The perpendicular mode of the floppy

controller will work at all data rates adjusting the format and

write data parameters accordingly See Section 426 for

more details

TLC12379 – 35

FIGURE 5-3 Read Data AlgorithmState Diagram

TLC12379 – 36

FIGURE 5-4 Perpendicular Recording Drive RW Head and Pre-Erase Head

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