Commodore Computers 1581 Service Manual page 13

1581 SERVICE MANUAL
DATA OUT
BUFFERS
DATA
REG.
COMMAND
REG.
DATA
SHIFT
REG.
SECTOR
REG.
AM DETECTOR
TRACK
REG.
STATUS
REG.
WRITE
PRECOMP
DATA
SEPERATOR
^ DRO
^ INTRO
MR
CS
R/W
AO
A1
CLK(8 MHZ)
DDEN
w
COMPUTER
INTERFACE
CONTROL
CONTROL
PLA
CONTROL
(240 X 19)
CONTROL
DISK
INTERFACE
CONTROL
WG
WPRT
IP
TROO
STEP
niRr
MOTOR ON
-►
-►
ARCHITECTURE
The Floppy Disk Formatter block diagram is illustrated on page 11. The primary sections include the parallel processor
interface and the Floppy Disk Interface.
Data Shift Register — This 8-bit register assembles serial data from the Read Data input (RD) during Read
operations and transfers serial data to the Write Data output during Write operations.
Data Register — This 8-bit register is used as a holding register during Disk Read and Write operations. In Disk Read
operations, the assembled data byte is transferred in parallel to the Data Register from the Data Shift Register. In Disk
Write operations, information is transferred in parallel from the Data Register to the Data Shift Register.
When executing the Seek command, the Data Register holds the address of the desired Track position. This register
is loaded from the DAL and gated onto the DAL under processor control.
Track Register — This 8-bit register holds the track number of the current Read/Write head position. It is incremented
by one every time the head is stepped in and decremented by one when the head is stepped out (towards track 00).
The contents of the register are compared with the recorded track number in the ID field during disk Read, Write, and
Verify operations. The Track Register can be loaded from or transferred to the DAL. This Register should not be loaded
when the device is busy.
Sector Register (SR) — This 8-bit register holds the address of the desired sector position. The contents of the register
are compared with the recorded sector number in the ID field during disk Read or Write operations. The Sector Register
contents can be loaded from or transferred to the DAL. This register should not be loaded when the device is busy.
Command Register (CR) — This 8-bit register holds the command presently being executed. This register should not
be loaded when the device is busy unless the new command is a force interrupt. The command register can be loaded
from the DAL, but not read onto the DAL.
Status Register (STR) — This 8-bit register holds device Status information. The meaning of the Status bits is a func
tion of the type of command previously executed. This register can be read onto the DAL, but not loaded from the DAL.
CRC Logic — This logic is used to check or to generate the 16-bit Cyclic Redundancy Check (CRC). The polynomial is:
G(X) = X16 + X12 + X5 + 1.
The CRC includes all information starting with the address mark and up to the CRC characters. The CRC register is
preset to ones prior to data being shifted through the circuit.
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