DISC DRIVE

General Information-8002A: Flexible Disc Unit Service

Sector Interleaving

Sectors are written sequentially in the following manner: sector 00, sector 16, sector 01, sector 17 ... This pattern is called sector i nterleavi ng, and is produced by the Flexible Disc Controller.

Interleaving increases system performance during multiple sector record transfers.

DISC DRIVE

The Disc Drive rotates a flexible disc at a constant 360 rpm, mechanically positions and engages (loads) the read/write head, and electrically writes or reads data. In the 8002AJlProcessor Lab systems, two or four Disc Drives can be used. Only one drive may be accessed at any time. The accessed drive is determined by which SEL DR signal is activated; the Flexible Disc Controller activates one of the SEL DR signals in response to a JlProcessor Lab command.

Drive Selection

Before a Disc Drive may be accessed, all drives must be up to speed, have flexible discs installed, and dc power steady. The status of all drives is determined by the Flexible Disc Controller from the wire-ORed READY signal, which enables drive selection. Following application of primary power to a Disc Drive, approximately 2.5 seconds are required for the drive to come up to speed, as shown in Fig. 1-6. The SEL DR signal, which enables the input and output gates of the selected Disc Drive, is supplied by the Flexible Disc Controller in response to a command from the JlProcessor Lab.

1-9

General Information-8002A: Flexible Disc Unit Service

J ^{~ S}

AC&DC POWER ON

~ 2.5s ^max.

r-- ^~ f--l

READY

~ S

-{~

SELECT

~ ~0.1J.LSmin. ( S

~ ~

(~N~f I L

DIRECTION (IN)

~ ~0.1J.LSmin. I_

-I

STEP

~f-lJ ^{~ f}

~6ms~ ~ ^~200

min. _I

I I (S

TRACK 00 I

f f

I

f S

I I

HEAD LOAD

1--0

-1

J {

min.

WRTGATE

~I~S

~f ~ ~4J.LSmin.

WRTDATA

U1JH

~250J.LS~

f i

~f-SEP DATA

4 30 ms min.

~I

Remarks:

1. Step Settling Time = 6 ms 2. Disc Speed = 360 rpm 3. Stepping Rate = 167 tracks/s 4. NRZ/Raw Data = fixed; A6 plug 5. Data Decoder w/PLO Board = not used

2587-7

Fig. 1-6. Disc Drive Interface Timing.

1-10 ^@

@

Generallnformation-8002A: Flexible Disc Unit Service

Track Selection

One of the 77 recording tracks is accessed by the head carriage. The carriage is mechanically positioned by the Disc Drive in response to the DIRECTION (IN) and STEP commands from the Flexible Disc Controller. The DIRECTION (IN) signal (Iow=in; high=out), which must precede the first STEP pulse by at least 0.1 J1s, determines whether the head carriage moves in (toward track 76) or out (toward track 00). The head carriage moves one track position for each STEP pulse received; STEP pulses occur at a rate of 167 pps (6 ms per track).

Track positioning is relative totrack 00 position. When the head carriage is aligned with track 00, the Disc Drive sends an active TRK 00 signal to the F,lexible Disc Controller. The controller uses this signal as a reference for STEP-pulse counting. If an access (position) error occurs, the Flexible Disc Controller may recover either by commanding the head carriage out to track 00 or by reading the header (ID) of a sector in the present, unknown track.

Head loading

The Flexible Disc Controller readies the selected Disc Drive for reading or writing by sending the appropriate HD LD command, which causes the read/write head to engage the flexible disc. This is called head loading. Only one Disc Drive is loaded at a given time; the head may remain loaded while changing tracks. As shown in Fig. 1-6, a 30-ms delay for settling is required after initial head loading before data may be written or read; only 10 ms of settling time is required after changing tracks with the head loaded.

Sector Selection

As shown in Fig. 1-5, the SECTOR pulse issued by the Disc Drive marks the start of a disc sector.

The number of the sector equals the number of SECTOR pulses counted by the Flexible Disc Controller following an INDEX pulse from the Disc Drive. Thus, data is not sent during writing and is not normally accepted by the Flexible Disc Controller during reading until the correct sector count has been reached. This enables the Disc Drive to read data continuously, which is useful when sector IDs must be read by the Controller before a read or write operation.

Reading or Writing Data

A write or read operation is commanded by the WRT GATE signal from the Flexible Disc Controller after head positioning and loading. Writing is selected if the signal is low (active);

reading is selected if the signal is high. Writing is inhibited if the WRITE PROTECT status signal from the Disc Drive is active, signifyi ng that the flexible disc is prerecorded and has the write-protect hole uncovered.

For writing, data is transferred serially from the Flexible Disc Controller one sector at a time via the WRT DATA line. The signal presentedtothe Disc Drive is an encoded composite of clock and data pulses. Write data timing is shown in Fig. 1-7. (The >TRK 43 command from Flexible Disc Controller, which reduces the current drive tothe read/write head, is inactive in present B002A J1Processor Lab system.)

1-11

General Information-B002A: Flexible Disc Unit Service

1-12

DFFM (Double-Frequency FM)

DATA

~4.2J.LS READ

~4.0J.LS WRITE

o

o

2587-8

Fig. 1-7. Read and Write Data Timing.

Duri ng a read operation, the raw data from a disc is separated by the Disc Drive into the clock (SEP ClK) and data (SEP DATA) components. The Disc Drive transfers a full sector of information serially to the Flexible Disc Controller as reading occurs. Read data timing is shown in Fig. 1-7.