Commodore 1541 Maintenance Manual page 6

Section 2-THEORY OF OPERATION
2-1. Overall Theory
The VIC-1541 cons is ts mainly of three sub-assemblies:
controller PC Board, drive unit and frame.
disk
The frame sub-assembly provides the power supply on the drive
controller PC 13oard with +9 Vrms and +16 Vrms, which are
derived from the 120 VAC power input. (Refer to Section
3
for selection of input power.)
The drive unit sub-assembly is capable of reading or writing
to or from a floppy disk. The drive unit is also capable of
rotating the floppy disk, changing read/write head location,
detecting write protect status, and magnetically reading or
writing data.
The disk controller PC Board sub-assembly includes the power
supply, read/write circuitry, track select circuitry, timing
circuitry, and a 6502-based computer. The computer operates
the drive unit as well as managing the floppy disk. The
program which this computer uses is cal led the DOS ( Disk
Operating System).
The floppy disk consists of a mylar disk with a magnetic
coating inside a jacket (Refer to Figure 2-1). The jacket
has several cutouts in it. The index cutout (Item 1) is not
used in the VIC-1541 since sector information is written on
each block. The slot (Item 2) allows the head to touch the
magnetic coating. A similar slot is cut on the opposite side
of the floppy disk. The write protect slot (Item 3) is
provided as a means of protecting a disk against accidental
erasure or overwriting. If the write protect slot is left
uncovered, writing to the floppy disk is permitted. If the
write protect slot is covered with opaque tape, writing is
disabled.
The recording principles involved are identical to those of
magnetic tape. The shape of the disc is more convenient than
magnetic tape since each piece of information passes the
vicinity of the head three hundred times per minute. This
allows random access of the information on the disk. Data is
physically stored in rings on the face of the disk. These
rings are called tracks. There are 35 tracks on each disk.
Each tracK is further divided into sectors. Each sector
contains sync, ID, track, sector and checksum information
along with 254 bytes of data. Track #18 is used for
housekeeping purposes (i.e., the directory and the block
availability map). Track #18 is automatically managed by the
DOS. Any information to be written or read is received and
transmitted by the disk controller to the VIC-20/COMMODORE 64
over the serial bus.
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