HOST SOFTWARE INTERFACE
6 – 5
Control Diagnostic Registers
These I/O port addresses reference three Control/Diagnostic registers:
I/O PORT
R E A D
WRITE
3 F 6 h
Alternate Status
Fixed Disk Control
3 F 7 h
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
- W G
-HS3
-HS2
-HS1
-HS0
- D S 1
DS0
Reserved
Write
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.
