Graf Elektronik FLO3 Handbuch Seite 49

Stilft Register, and requires a new data byte. It is reset
when the Data Register is loaded with new data by the
processor. H new data is not loaded at the time the next
serlal byte is required by the Floppy Dlsk, a byte of zeroes
Is written on the diskette and the Losi Data blt is set in the
Status Register.
At the completlon of every command an INTRQ is
generatsd. INTRQ is reset by either reading the Status
register or by loading the command register with a new
command. In addition, INTRQ is generated if a Force
Interrupt command condition is meL
The 179X has two modes of Operation according to the
State of ÖDEN (Pin 37). When ÖDEN = 1, Single density is
selected. In either case, Iris CLK input (Pin 24) is at 2 MHz.
However, when interiacing with tha minl-lloppy, the CLK
input is set at 1 MHz for both Single density and double
density.
GENERAL DISK READ OPERATIONS
Sector lengths of 128, 256, 512 or 1024 arej>btainab!e in
either FM or MFM formats. For FM, ODEN should be
placed to logical "1." For MFM formats, DDEN should be
placed to a logical "0." Sector lengths are determined at
formal time by the tourth byte in the "ID" fleld.
Sector Length Table*
Sector Length
Fleld (hex)
Numberof Bytes
in Sector (d8cimal)
00
01
02
03
128
256
512
1024
'1796)97 may vary — see command summary.
Tha number of sectors per track as far as the FD179X is
concemed can be trom 1 to 255 sectors. The number of
tracks as far as the FD179X is concerned Is trom 0 to 255
tracka. For IBM 3740 compatlbility, sector lengths are 128
bytss with 26 sectors per track. For System 34 com-
patiNllty (MFM), sector lengths are 256 bytes/sector with 26
sectorartrack; or lengths of 1024 bytes/sector with 8
sectors/track. (See Sector Length Table)
For read operatlons in 8" double density the FD179X
requires RAW READ Data (Pin 27) Signal which is a 200 ns
pulse per flux transltlon and a Read dock (RCLK) Signal to
Indlcate flux transltlon spacings, Tha RCLK (Pin 26) Signal
Is provlded by some drives but If not It may be derived
extemally by Phase lock loops, one shots, or counter
technlques. In addition, a Read Gate Signal Is provided as
an Output (Pin 25) on 1791/92/93)94 which can be used to
Inform phase lock loops when to acqulre synchronization.
When reading from the media in FM. RG Is made true when
2 bytes of zeroes are detected. The FD179X must find an
address mark wlthln the next 10 bytes; otherwise RG is
reset and the search for 2 bytes of zeroes begins all over
agaln. If an address mark Is found withln 10 bytes, RG
remalns true as long as the FD179X Is derlvlng any useful
Information from the data stream. Simllarly for MFM, RG is
made actlve when 4 bytes of "00" or "FF" are detected. The
FD179X must find an address mark wlthln the next 16
bytes, otherwise RG Is reset and search resumes.
Durlng read operatlons (WG = 0), the VFÖT- (Pin 33) is
provlded for phasa lock loop synchronization. VFOE will go
actlve Iow when:
a) Both HLT and HLD are True
b) SettiingTime, ilprogrammed, hasexpired
c) The 179X is inspecting data off the disk
If WF/VFOE is not used, leave open or tie to a 10K resistor
to+5.
GENERAL DISK WH1TE OPERATION
When writing is to take place on 1ha diskette the Wrlte Gate
(WG) Output is activated, allowlng current to flow into the
Read/Write head. As a precaution to erroneous writing the
first data byte must be loaded into the Data Register in
response to a Data Request from the FD179X before the
Write Gate signal can be activated.
Writing is inhibited when the Wrlte Protect input is a loglc
Iow, in which case any Wrlte command is immedlately
terminated, an interrupt Is generated and the Wrlte Protect
Status bit is set. The Write Fault Input, when activated,
signlfies a writing fault condition dstected in disk drive
eleclronics such as failure to detect write current flow
when the Write Gate is activated. On detection of this fault
the FD179X terminates the current command, and sets the
Write Fault bit (bit 5) In the Status Word. The Write Fault
input should be made inactlve when the Write Gate Output
becomes inactive.
For write operations, Ihe FD179X provides Write Gate (Pin
30) and Write Data (Pin 31) outputa.Wnte data conslsts of a
series of 500 ns pulses in FM (DOEN = 1) and 200 ns
pulsss in MFM (DDEN = 0). Write Data provides the unlque
address marks in both formats.
Also during write, two additional Signals are provided for
write precompensation. These are EARLY (Pin 17) and
LATE (Pin 18). EARLY is active true when the WD pulse
appearing on (Pin 30) is to be written EARLY. LATE is acilve
true when the WD pulse is to be written LATE. If both
EARLY and LATE are Iow when the WD pulse Is present,
the WD pulse is to be written at nominal. Since write
precompensation values vary from disk manufacturer to
disk manufacturer, ihe actual value is determined by
several one shots or delay lines which are located external
to the FD179X. The write precompensation Signals EARLY
and LATE are valid for the duration of WD in both FM and
MFM formats.
READY
Whenever a Read or Write command (Type II or III) is
recelved ihe FD179X samples the Ready Input If this input
Is logic Iow the command Is not executed and an Interrupt
Is generated. All Type I commands are performed re-
gardless of the State of the Ready input Also, whenever a
Type II or III command is received, the TG43 Signal Output
is updated.
COMMAND DESCRIPTION
The FD179X will accept eleven commands. Command
words should only be loaded In the Command Register
when the Busy Status bit Is off (Status bit 0). The one
exception is the Force Interrupt command. Whenever a
command is being executed, the Busy Status bit Is set.
When a command is comploted, an Interrupt is generated
and the Busy Status blt is reset The Status Register
indicates whether the completed command encountered
an error or was fault free. For ease of discussion,
commands are divlded into four types. Commands and
types are summarized In Tabla 1.
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