PC87309VLJ datasheet(69/192 Pages) NSC

69 / 192 page

The Floppy Disk Controller (FDC) (Logical Device 0)

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TABLE 3-19. Maximum RECALIBRATE Step Pulses for

Values of R255 and ETR

The pulses actually occur while the controller is in the drive

polling phase. See Section 3.4.5 on page 48.

An interrupt is generated after the TRK0 signal is asserted,

or after the maximum number of RECALIBRATE step puls-

es is issued.

Software should ensure that the RECALIBRATE command

is issued for only one drive at a time. This is because the

drives are actually selected via the Digital Output Register

(DOR), which can only select one drive at a time.

No command, except a SENSE INTERRUPT command,

should be issued while a RECALIBRATE command is in

progress.

Result Phase

None.

3.7.15

The RELATIVE SEEK Command

The RELATIVE SEEK command issues STEP pulses that

make the head of the selected drive step in or out a pro-

grammable number of tracks.

Command Phase

First Command Phase Byte, Opcode, Bit - 6 Step Direction

DIR

This bit defines the step direction.

0: Step head out.

1: Step head in.

Second Command Phase Byte

See “Second Command Phase Byte” on page 57 for a

description of the Drive Select (DS1,0) and Head Select

(HD) bits.

Third Command Phase Byte - Relative Track Number

(RTN)

This value specifies how many tracks the head should

step in or out from the current track.

Execution Phase

After the last command byte is issued, the Drive Busy bit for

the selected drive is set in the Main Status Register (MSR).

See bits 3-0 in Section 3.3.5 on page 42.

The controller waits the Delay Before Processing time (see

TABLE 3-24 on page 74) for the selected drive., and then

becomes idle. See Section 3.4.4 on page 47.

Then, the controller enters the idle phase and issues RTN

STEP pulses until the TRK0 disk interface input signal be-

comes active or until the specified number (RTN) of STEP

pulses have been issued. After the RELATIVE SEEK oper-

ation is complete, the controller generates an interrupt.

Software should ensure that the RELATIVE SEEK com-

mand is issued for only one drive at a time. This is because

the drives are actually selected via the Digital Output Reg-

ister (DOR), which can only select one drive at a time.

No command, except the SENSE INTERRUPT command,

should be issued while a RELATIVE SEEK command is in

progress.

Result Phase

None.

3.7.16

The SCAN EQUAL, the SCAN LOW OR EQUAL

and the SCAN HIGH OR EQUAL Commands

The scan commands compare data read from the disk with

data sent from the microprocessor. This comparison pro-

duces a match for each scan command, as follows, and as

shown in TABLE 3-20 on page 70:

SCAN EQUAL - Disk data equals microprocessor da-

ta.

SCAN LOW OR EQUAL - Disk data is less than or

equal to microprocessor data.

SCAN HIGH OR EQUAL - Disk data is greater than or

equal to microprocessor data.

Command Phase

SCAN EQUAL

R255

ETR

Maximum Number of

RECALIBRATE Step Pulses

85 (default)

255

3925

4095

76543210

DIR

001111

XXXXX

DS1

DS0

Relative Track Number (RTN)

76543210

MFM

10001

IPS

XXXX

DS1

DS0

Track Number

Head Number

Sector Number

Bytes-Per-Sector Code

End of Track (EOT) Sector Number

Bytes Between Sectors - Gap 3

Sector Step Size

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