IBM 3274 51C Maintenance Information page 257

F.2 Device Theory of Operation
The Diskette Drive is an I/O device that relies on the using
system for power, commands, and control. The drive can
read from, and write to either side of a diskette. This sec-
tion contains theory information about the device interface,
data flow, and operation of the diskette drive.
F.2.1 Control Card Interface
Cylinder access is shown in Figure F-10; the interface lines
at connector A 1 are shown in Figure F-1L Following is a
description of the interface lines at connector A 1 :
Write Data: For each change of this signal, the current
switches in the read/write head. This process records the
data on the diskett~ surface.
+Inner Tracks: This line is active from track 43 through
track 76. When this line is active, the write current through
the data is decreased because the bit density increases
toward the center tracks and, therefore, less write current is
needed. This line.is also used to increase the read amplifier
gain from tracks 43 through 76.
+Select Head 1: This line, when active, selects head 1.
+Write Gate: This line activates the write circuits and deac-
tivates the read circuits for a write operation.
+Erase Gate: This line activates the tunnel erase circuits
during a write operation to erase the edges of the track just
recorded. This erasing prevents crosstalk between tracks
during later read operations.
Access 0
i
\ i
Access 1
\ t
I I
I
I
I
I I
I I
I
01
2
I
3
141
I I
I
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+Switch Filter: This line iS,used with the Inner Tracks line
to make corrections for bit shift on those tracks greater
than cylinder 60 (for MFM encoding). The Switch Filter
line is used only during a read operation.
+Write/Erase Enabled: When this line is active, either write
or erase current has been enabled on the card.
File Data: This line is a series of clock and data pulses that
represent the data read from the diskette surface. The VFO
circuits supplied by the using system separate the clock
pulses from the data pulses.
+Index: This line indicates the start of a track. This 1.5- to
3.0-ms pulse occurs every 166.7 ms.
Diskette Sense: When this line is active, it indicates that a
diskette 2 or 2D is being used. This line is not activated by
a diskette 1.
Access Lines 0 and 1: Sequentially activating the access
signal lines causes the read/write head to move from one
cylinder to the next. Note, in Figure F-l0, that the
sequence is repeated every four cylinders.
These two access signal lines, 0 and 1, are sequentially acti-
vated to cause the head to move in (toward the drive hub)
or out (away from the drive hub).
+Head Engage: When it is active, this line loads the read/
write head.
1
I \
Cylinder 0
~~~~4
Cylinder 5 •
~
J..--
5 ms Cylinder-to-Cylinder Seek Time
Figure F-l0. Cylinder Access
Appendix F. IBM 5lTD Diskette Drive Maintenance F-13