HOST SOFTWARE INTERFACE
6 – 5
Control Diagnostic Registers
These I/O port addresses reference three Control/Diagnostic registers:
T
R
O
P
O
/I
D
A
E
R
E
T
I
R
W
h
6
F
3
s
u
t
a
t
S
e
t
a
n
r
e
tl
A
l
o
rt
n
o
C
k
s
i
D
d
e
x
i
F
h
7
F
3
t
u
p
n
I
l
a
ti
g
i
D
d
e
s
u
t
o
N
Alternate Status Register
Contains the same information as the Status register in the Task File. However, this register may be read at
any time without clearing a pending interrupt.
Device Control Register
Contains the software Reset and Enable bit to enable interrupt requests to the host. Bit definitions follow:
7
6
5
4
3
2
1
0
0
0
0
0
0
T
S
R
S
N
E
I
0
t
e
s
e
R
e
l
b
a
n
E
Q
R
I
Reset
– Setting the software Reset bit holds the drive in the reset state. Clearing the bit re-enables the drive.
The software Reset bit must be held active for a minimum of 5 µsec.
IRQ Enable
– Setting the Interrupt Request Enable to 0 enables the IRQ 14 signal to the host. When this bit is set
to 1, IRQ14 is tri-stated, and interrupts to the host are disabled. Any pending interrupt occurs when the bit is set to 0.
The default state of this bit after power up is 0 (interrupt enabled).
Digital Input Register
Contains information about the state of the drive. Bit definitions follow:
7
6
5
4
3
2
1
0
x
G
W
-
3
S
H
-
2
S
H
-
1
S
H
-
0
S
H
-
1
S
D
-
0
S
D
d
e
v
r
e
s
e
R
e
t
a
G
d
a
e
H
3
t
c
e
l
e
S
d
a
e
H
2
t
c
e
l
e
S
d
a
e
H
1
t
c
e
l
e
S
d
a
e
H
0
t
c
e
l
e
S
e
v
ir
D
1
t
c
e
l
e
S
e
v
ir
D
0
t
c
e
l
e
S
Bit 7 of the host data bus is not driven when this register is read.
-Write Gate
– Reflects the state of the active low write gate signal on the drive.
-Head Select 3 through -Head Select 0
– Represents the ones complement of the currently selected head number.
-Drive Select 1
– Is 0 if drive 1 selected; 1 otherwise.
-Drive Select 0
– Is 0 if drive 0 selected; 1 otherwise.
