ACT apricot - Head Positioning

FLOPPY DISK INTERFACE
The time taken for the standard disk drive (the version
using 70 track single-sided disks), to engage the read/write
head after the head load signal is set active, is of the order of
60 ms. A
60ms delay should therefore be implemented after
the head load signal is set active, to ensure the head is
engaged before performing
disk data transfer operations. All
command operations can be performed with the head loaded
without any detrimental effect to the disk.
Head Positioning
Positioning the read/write head of the selected disk drive
is achieved by issuing one of the five Type 1 commands to
the Command Register of the FDC. Termination of a
command (successful or otherwise) is signified by an
interrupt request to the Interrupt Controller (PIC). The
format of each of the head posi tioning commands is detailed
below.
D7
0
0
0 0
0 0
0
1
0 0
1
T
0 1 0 T
0 1
1 T
Type 1 Commands
1 V
1
V
1
V
1 V
1 V
DO Command
rl rO Restore
rl
rO Seek
rl
rO Step
rl
rO Step-in
rl rO Step-out
T = Track Update Flag
V
= Verify Flag
rl,rO = Stepping motor rate
Each of the commands contain a Verify Flag bit and
stepping motor rate bits. The stepping rate bits have to be
set according to the track -to-track access time of the disk
drive.
The combination of bits allow four different stepping
motor rates to be selected as detailed below. The track-to-
track access time of the standard disk drives is 15 ms.
rl rO Rate
0 0
3 ms
0
1
6
ms
1 0
10 ms
1 1
15 ms