HOST SOFTWARE INTERFACE
6 – 5
Control Diagnostic Registers
These I/O port addresses reference three Control/Diagnostic registers:
I/O PORT
READ
WRITE
3F6h
Alternate Status
Fixed Disk Control
3F7h
Digital Input
Not used
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
SRST
IEN
0
Reset
IRQ Enable
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
-WG
-HS3
-HS2
-HS1
-HS0
-DS1
DS0
Reserved
Gate
Head
Select 3
Head
Select 2
Head
Select 1
Head
Select 0
Drive
Select 1
Drive
Select 0
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.
